Today, you will not surprise anyone with electronic preventive warning devices of various purposes and effectiveness, which notify people or turn on a burglar alarm long before the direct contact of an unwanted guest with a protected border (territory). Many of these nodes described in the literature, for example, in, according to the author, are interesting, but complicated.
In contrast, they developed a simple electronic circuit for a non-contact capacitive sensor (Fig. 2.2), which even a novice radio amateur can assemble. The device has a high input sensitivity, which allows it to be used to warn of a person approaching the E1 sensor.
The principle of operation of the device is based on a change in capacitance between the sensor-antenna E1 and the "ground" (common wire: everything that corresponds to the ground loop - in this case, this is the floor and walls of the room). When a person approaches, this capacitance changes significantly, which is sufficient to trigger the K561TL1 chip.
Rice. 2.2. Wiring diagram non-contact capacitive sensor
The design is based on two elements of the K561TL1 (DD1) microcircuit, included as inverters. This microcircuit incorporates four elements of the same type with the 2I-NOT function with Schmitt triggers with hysteresis (delay) at the input and inversion at the output.
The use of the K561TL1 microcircuit is due to low current consumption, high noise immunity (up to 45% of the supply voltage level), operation in a wide range of supply voltage (in the range of 3–15 V), input protection from static electricity and short-term excess of input levels, and many others. advantages that allow the microcircuit to be widely used in amateur radio structures without requiring any special measures precautions and protection.
In addition, the K561TL1 microcircuit allows you to connect its independent logic elements in parallel, as buffer elements, as a result of which the output signal power increases proportionally. Schmitt triggers are bistable circuits that can handle slowly increasing input signals, including noise. At the same time, the steep fronts of the pulses that provide output can be transmitted to subsequent nodes of the circuit for docking with other key elements and microcircuits. The K561TL chip (as well as the K561TL2, by the way) can allocate a control signal (including digital) for other devices from an analog or fuzzy input pulse.
Foreign analogue of K561TL1 - CD4093B.
The inverter switching circuit is a classic one, it is described in reference books. The peculiarity of the presented development is in the design nuances. After turning on the power at the input of the element DD1.1 there is an undefined state close to a low logic level. At the output of DD1.1 - high level, at the output of DD1.2 - again low. Transistor VT1 is closed. Piezoelectric capsule HAI (with internal generator 34) is not active.
An antenna is connected to the E1 sensor - a telescopic car is suitable. When a person is near the antenna, the capacitance between the antenna pin and the floor changes. From this switch elements DD1.1, DD1.2 in the opposite state. To switch the node, a person of average height must be (pass) next to an antenna 35 cm long at a distance of up to 1.5 m. A high voltage level appears at pin 4 of the microcircuit, as a result of which the transistor VT1 opens and the capsule HA1 sounds.
By selecting the capacitance of the capacitor C1, you can change the mode of operation of the microcircuit elements. So, when the capacitance C1 decreases to 82-120 pF, the node works differently. Now the sound signal sounds only while the DD1.1 input is affected by AC voltage interference - a human touch.
The electrical circuit (fig. 2.2) can also be used as the basis for a trigger sensor. To do this, exclude a constant resistor R1, a shielded wire, and the sensor is the contacts of the microcircuit 1 and 2.
A shielded wire is connected in series with R1 (cable RK-50, RK-75, shielded wire for AF signals - all types are suitable) 1-1.5 m long, the shield is connected to a common wire, the central core at the end is connected to the antenna pin.
Subject to these recommendations and the use of the types and ratings of the elements indicated in the diagram, the node generates a sound signal with a frequency of about 1 kHz (depending on the type of capsule HA1) when a person approaches the antenna pin at a distance of 1.5–1 m. There is no trigger effect. As soon as the object moves away from the antenna, the sensor switches to the armed (standby) mode.
The experiment was also carried out with animals - a cat and a dog: the node does not react to their approach to the sensor-antenna.
The capabilities of the device can hardly be overestimated. In the author's version, it is mounted next to door frame; the front door is metal.
The volume of the AF signal emitted by the HA1 capsule is sufficient to hear it on a closed loggia (it is comparable to the volume of a house bell).
The power supply is stabilized, with a voltage of 9-15 V, with good filtering of the ripple voltage at the output. The current consumption is negligible in the standby mode (several microamperes) and increases to 22–28 mA when the emitter HA1 is actively operating. A transformerless source cannot be used due to the likelihood of damage electric shock. The oxide capacitor C2 acts as an additional power filter, its type is K50-35 or similar, for an operating voltage not lower than the power supply voltage.
During the operation of the node revealed interesting features. The supply voltage of the node affects its operation: when the supply voltage increases to 15 V, only an ordinary stranded unshielded electrical copper wire with a cross section of 1-2 mm and a length of 1 m is used as a sensor-antenna; in this case, no screen and resistor R1 are needed, the electric copper wire is connected directly to terminals 1 and 2 of the DD1.1 element. The effect is similar. When the phasing of the mains plug of the power supply is changed, the node catastrophically loses sensitivity and is able to work only as a sensor (reacts to touching E1). This is true for any value of the power supply voltage in the range of 9-15 V. Obviously, the second purpose of this circuit is an ordinary sensor (or sensor-trigger).
These nuances should be taken into account when repeating the device. However, in the case correct connection described here, an important component is obtained burglar alarm, ensuring the safety of the home, warning the owners even before the occurrence of an emergency.
The elements are mounted compactly on a fiberglass board. The housing for the device is any dielectric (non-conductive) material. To control the power on, the device can be equipped with an indicator LED connected in parallel with the power source.
Adjustment with strict adherence to the recommendations is not required. If you experiment with the length of the shielding cable, the length and area of the E1 sensor-antenna, and changing the supply voltage, you may need to adjust the resistance of the resistor R1 over a wide range - from 0.1 to 100 MΩ. To reduce the sensitivity, increase the capacitance of the capacitor C1. If this does not bring results, a constant resistor with a resistance of 5-10 MΩ is connected in parallel with C1.
Rice. 2.3. capacitive sensor
Non-polar capacitor C1 - type KM6. Fixed resistor R2 - MLT-0.25. Resistor R1 - type VS-0.5, VS-1. Transistor VT1 is needed to amplify the signal from the output of the element DD1.2. Without this transistor, the HA1 capsule sounds soft. Transistor VT1 can be replaced with KT503, KT940, KT603, KT801 with any letter index.
Capsule emitter HA1 can be replaced by a similar one with a built-in generator 34 and an operating current of not more than 50 mA, for example, FMQ-2015B, KRX-1212V and the like.
Thanks to the use of a capsule with a built-in generator, the unit exhibits an interesting effect: when a person approaches the E1 sensor-antenna, the sound of the capsule is monotonous, and when a person moves away (or approaches a person, starting from a distance of 1.5 m to E1), the capsule emits an intermittent sound that is stable in nature in accordance with the change in the potential level at the output of the element DD1.2. (A similar effect formed the basis of the first electronic musical instrument- "Theremin".)
For a more complete picture of the properties of a capacitive sensor, the author recommends reading the material.
If a capsule with a built-in AF generator, for example, KRI-4332-12, is used as HA1, then at a relatively large distance from the sensor-antenna, the sound will resemble a siren, and at maximum approximation - an intermittent signal.
Some disadvantage of the device can be considered the lack of selectivity (of the “friend / foe” recognition system), so the node will signal the approach of any person to E1, including the owner of the apartment who went out “for bread”. The basis of the operation of the device is electrical pickups and capacitance changes that are most useful when operating in large residential areas with a developed network of electrical communications; obviously, the device will be useless in the forest, in the field and wherever there are no electrical communications.
Kashkarov A.P. 500 schemes for radio amateurs. Electronic sensors.
What tricks do the owners not resort to, protecting their property! Starting from the simplest padlocks the size of good brick(in the North, even ... wolf traps were used!) to modern signaling with sophisticated electronics. Electronic security is often based on the fact that the offender will give himself away in some way, send information about his appearance. It can be the sound of steps - electronic "ears" will instantly react and give a signal of danger. There are security systems that respond to human radiation, the spectral composition of which differs sharply from the main background. But the criminal does not sleep, trying to go unnoticed while doing his dirty deeds - there are special camouflage suits, all sorts of ingenious devices.
Meanwhile, there is an absolutely reliable protection system. It is tuned to such a physical field of a person, for which nature itself excludes the possibility of any barriers. This is the gravitational field that every object that has mass has. Gravity is gravitation (attraction), a universal interaction between any kind of physical matter (ordinary matter, any physical fields), as the third law of Isaac Newton says.
This principle formed the basis of the device of the famous inventor S. Lifshits. Gravitational forces are negligible. Say, the mutual attraction between two bodies located at a distance of one meter from each other and with a mass of one ton each is only about 0.006 g. They can be observed only with the help of bulky devices that are used only in planetariums. Sh. Lifshitz's device is small, compact, extremely simple to manufacture and witty, like everything ingenious. Its basis is a transparent vessel glued from plexiglass. Inside - a partition that symmetrically divides it up to half the height and goes out. On both sides of the partition, two tubes with a cross section of 1 sq. mm. On the sides of the vessel there are two short tubes with taps. All connections of the device are sealed.
The vessel is installed on a table or on a fixed platform. A drop of tinted liquid is introduced into the small tubes. Both drops should be at the same level. After that, the vessel is filled with water through short tubes to a level at which the lower part of the partition is completely immersed in the liquid, and a layer of air of 2–3 mm remains up to the lid of the vessel. The taps are closed, and the device is ready for operation. If now a person approaches one of its ends, part of the liquid under the action of gravitational force from one half of the vessel will pass into the other - into the one to which he approached. And since the movement of the liquid in the separated parts of the vessel is associated with the movement of the air layer, the tinted drops in small tubes will also move. Removing a person from the device will cause the opposite effect - a reverse displacement of the drops. There is a demonstration of the effect of gravity.
If a weight is brought to the device, then the drop in the left capillary will rise, and in the right one it will fall.
Now guess what we're getting at? It is only necessary to slightly improve our apparatus in such a way that it automatically gives a signal when a person approaches it. There are many options here. Moving, tinted droplets can block the beam of light and cause the photocell to work, turn on the siren.
Look at the picture and you will better understand the mechanism of action of such a watchman. The device works if it is strengthened behind the armored door of the safe or behind a thick concrete wall- there are no obstacles for gravity. In other words, such a security device is the most reliable.
Such a device will automatically give a signal when a person approaches it.
A capacitive sensor is one of the types of non-contact sensors, the principle of operation of which is based on a change in the dielectric constant of the medium between two capacitor plates. One plate is a circuit sensor in the form of a metal plate or wire, and the second is an electrically conductive substance, such as metal, water, or the human body.
When developing a system automatic start supplying water to the bidet toilet, it became necessary to use a capacitive presence sensor and a switch with high reliability, resistance to changes in external temperature, humidity, dust and supply voltage. I also wanted to eliminate the need for a person to touch the controls of the system. The requirements could only be provided by sensor circuits operating on the principle of capacitance change. Finished scheme satisfying necessary requirements I couldn't find it so I had to develop it myself.
The result was a universal capacitive touch sensor that does not require adjustment and responds to approaching electrically conductive objects, including a person, at a distance of up to 5 cm. The scope of the proposed touch sensor is not limited. It can be used, for example, to turn on lighting, alarm systems, detect water levels and in many other cases.
Electrical circuit diagrams
Two capacitive touch sensors were needed to control the flow of water in the toilet bidet. One sensor had to be installed directly on the toilet, it had to give a logical zero signal in the presence of a person, and in the absence of a logical unit signal. The second capacitive sensor was supposed to serve as a water switch and be in one of two logical states.
When a hand was brought to the sensor, the sensor had to change the logical state at the output - from the initial single state to go to the state of logical zero, when the hand was touched again from the zero state to go to the state of logical one. And so on ad infinitum, until the sensor switch receives a logical zero enable signal from the presence sensor.
Capacitive touch sensor circuit
The basis of the capacitive touch presence sensor circuit is a master square-wave generator, made according to classical pattern on two logic elements of the chip D1.1 and D1.2. The oscillator frequency is determined by the values of the elements R1 and C1 and is chosen at about 50 kHz. The frequency value has practically no effect on the operation of the capacitive sensor. I changed the frequency from 20 to 200 kHz and visually did not notice any effect on the operation of the device.
From the 4 outputs of the D1.2 chip, a rectangular signal is fed through the resistor R2 to the inputs 8, 9 of the D1.3 chip and through the variable resistor R3 to the inputs 12.13 D1.4. The signal arrives at the input of the D1.3 chip with a slight change in the slope of the pulse front due to installed sensor, which is a piece of wire or a metal plate. At the input D1.4, due to the capacitor C2, the front changes for the time required to recharge it. Due to the presence of a tuning resistor R3, it is possible to set the pulse fronts at input D1.4 equal to the pulse front at input D1.3.
If you bring a hand or a metal object closer to the antenna (sensor), then the capacitance at the input of the DD1.3 microcircuit will increase and the front of the incoming pulse will be delayed in time relative to the front of the pulse coming to the input of DD1.4. to "catch" this delay, about inverted pulses are fed to the DD2.1 chip, which is a D flip-flop that works as follows. On the positive edge of the pulse arriving at the input of the microcircuit C, the signal that was at the input D at that moment is transmitted to the output of the trigger. Therefore, if the signal at the input D does not change, the incoming pulses at the counting input C do not affect the output signal level. This property of the D trigger made it possible to make a simple capacitive touch sensor.
When the capacitance of the antenna, due to the approach of the human body to it, at the input of DD1.3 increases, the pulse is delayed and this is fixed by the D trigger, changing its output state. The HL1 LED serves to indicate the presence of the supply voltage, and HL2 to indicate the proximity to the touch sensor.
Touch switch circuit
The capacitive touch sensor circuit can also be used to operate the touch switch, but with a little refinement, since it needs not only to respond to the approach of the human body, but also to remain in a steady state after the removal of the hand. To solve this problem, it was necessary to add another D trigger, DD2.2, to the output of the touch sensor, connected according to the divider-by-two circuit.
The capacitive sensor circuit has been slightly modified. To eliminate false positives, since a person can bring and remove his hand slowly, due to the presence of interference, the sensor can output several pulses to the counting input D of the trigger, violating the necessary switch operation algorithm. Therefore, an RC chain of elements R4 and C5 was added, which for a short time blocked the possibility of switching the D trigger.
The trigger DD2.2 works in the same way as DD2.1, but the signal to the input D is not supplied from other elements, but from the inverse output of DD2.2. As a result, on the positive edge of the pulse arriving at input C, the signal at input D changes to the opposite. For example, if in the initial state there was a logical zero at pin 13, then by bringing your hand to the sensor once, the trigger will switch and a logical unit will be set at pin 13. The next time the sensor is acted upon, a logical zero will again be set at pin 13.
To block the switch in the absence of a person on the toilet, a logical unit is supplied from the sensor to the input R (setting zero at the trigger output, regardless of the signals at all its other inputs) of the DD2.2 microcircuit. At the output of the capacitive switch, a logical zero is set, which is fed through the harness to the base of the switching switching transistor solenoid valve in the power supply and switching unit.
Resistor R6, in the absence of a blocking signal from the capacitive sensor in the event of its failure or a break in the control wire, blocks the trigger at input R, thereby eliminating the possibility of spontaneous water supply to the bidet. Capacitor C6 protects input R from interference. LED HL3 serves to indicate the water supply in the bidet.
Structure and details of capacitive touch sensors
When I started designing a bidet sensor system, the most difficult task for me seemed to be the development of a capacitive presence sensor. This was due to a number of restrictions on installation and operation. I did not want the sensor to be mechanically connected to the toilet lid, since it must be removed periodically for washing, and did not interfere with sanitization the toilet itself. Therefore, I chose the capacitance as the reacting element.
Presence sensor
According to the above published scheme did prototype. Capacitive sensor parts are assembled on printed circuit board, the board is placed in a plastic box and closed with a lid. To connect the antenna, a single-pin connector is installed in the housing; a four-pin RSh2N connector is installed to supply power and signal. The printed circuit board is connected to the connectors by soldering with copper conductors in fluoroplastic insulation.
The touch capacitive sensor is assembled on two microcircuits of the KR561 series, LE5 and TM2. Instead of the KR561LE5 chip, you can use the KR561LA7. Chips of the 176 series, imported analogues, are also suitable. Resistors, capacitors and LEDs will fit any type. Capacitor C2, for stable operation of the capacitive sensor when operating under conditions of large fluctuations in ambient temperature, must be taken with a small TKE.
The sensor is installed under the platform of the toilet, on which it is installed cistern in a place where, in the event of a leak from the tank, water cannot enter. The sensor body is glued to the toilet bowl using double-sided adhesive tape.
The antenna sensor of the capacitive sensor is a piece of copper stranded wire 35 cm long in PTFE insulation, glued with transparent adhesive tape to the outer wall of the toilet bowl a centimeter below the plane of the glasses. The sensor is clearly visible in the photo.
To adjust the sensitivity of the touch sensor, it is necessary, after installing it on the toilet, by changing the resistance of the tuning resistor R3 to make the HL2 LED go out. Next, put your hand on the toilet lid above the location of the sensor, the HL2 LED should light up, if you remove your hand, go out. Since the human thigh by mass more hands, then during operation the touch sensor, after such a setting, will work guaranteed.
The design and details of the capacitive touch switch
The capacitive touch switch circuit has more details and a housing was needed to accommodate them bigger size and for aesthetic reasons, appearance the housing in which the presence sensor was placed was not very suitable for installation in a conspicuous place. The rj-11 wall socket for connecting the phone drew attention to itself. It fit true to size and looks good. Having removed everything superfluous from the outlet, I placed the printed circuit board of the capacitive touch switch in it.
To fix the printed circuit board, a short post was installed at the base of the case, and a printed circuit board with touch switch parts was screwed to it with a screw.
The capacitive sensor sensor was made by gluing a sheet of brass to the bottom of the socket cover with Moment glue, after cutting out a window for the LEDs in them. When closing the lid, the spring (taken from a flint lighter) comes into contact with the brass sheet and thus ensures electrical contact between circuit and sensor.
The capacitive touch switch is attached to the wall using one self-tapping screw. For this, a hole is provided in the body. Next, the board, connector is installed and the cover is fixed with latches.
The setting of the capacitive switch is practically the same as the setting of the presence sensor described above. To configure, you need to apply the supply voltage and adjust the resistor so that the HL2 LED lights up when a hand is brought to the sensor, and goes out when it is removed. Next, you need to activate the touch sensor and bring and remove your hand to the switch sensor. The HL2 LED should blink and the red HL3 LED should light up. When the hand is removed, the red LED should remain lit. When the hand is brought up again or the body is removed from the sensor, the HL3 LED should go out, that is, turn off the water supply in the bidet.
Universal PCB
The capacitive sensors presented above are assembled on printed circuit boards, which are slightly different from the printed circuit board shown in the photo below. This is due to the combination of both printed circuit boards into one universal one. If you assemble the touch switch, then you only need to cut track number 2. If you assemble the presence sensor, then track number 1 is removed and not all elements are installed.
The elements necessary for the operation of the touch switch, but interfering with the operation of the presence sensor, R4, C5, R6, C6, HL2 and R4, are not installed. Instead of R4 and C6, wire jumpers are soldered. The chain R4, C5 can be left. It will not affect work.
Below is a drawing of a printed circuit board for knurling using the thermal method of applying tracks to the foil.
It is enough to print the drawing on glossy paper or tracing paper and the template is ready for the manufacture of a printed circuit board.
Trouble-free operation of capacitive sensors for the touch control system of water supply in the bidet has been proven in practice for three years of continuous operation. No failures were recorded.
However, I want to note that the circuit is sensitive to powerful impulse noise. I received an email asking for help with setup. It turned out that during the debugging of the circuit there was a soldering iron with a thyristor temperature controller nearby. After turning off the soldering iron, the circuit worked.
There was another case. The capacitive sensor was installed in the lamp, which was connected to the same outlet as the refrigerator. When you turn it on, the light turns on and when you turn it off again. The issue was resolved by connecting the lamp to another outlet.
A letter came about the successful application of the described capacitive sensor circuit for adjusting the water level in storage tank from plastic. In the lower and upper parts, it was glued with silicone along the sensor, which controlled the on and off of the electric pump.
The motion sensor is most often used to turn on the lights when you pass or are close to it. With it, you can save electricity well and save yourself from having to flip the switch. This device is also used in alarm systems to detect unwanted intrusions. In addition, they can also be found on production lines, they are needed there for the automated execution of any technological tasks. Motion sensors are sometimes referred to as presence sensors.
Types of motion sensors
Motion sensors are distinguished according to the principle of operation, their operation, accuracy of operation and features of use depend on this. Each of them has strengths and weak sides. The final price of such a sensor also depends on the design and type of the element used.
The motion sensor can be made in the same housing and in different housings (the control unit is separate from the sensor).
Contact
The easiest motion sensor option is to use or. A reed switch (sealed contact) is a switch that operates when a magnetic field. The essence of the work is to install a limit switch with normally open contacts or a reed switch on the door, when you open it and enter the room, the contacts will close, turn on the relay, and it will turn on the lighting. Such a scheme is shown below.
infrared
They are triggered by thermal radiation, react to temperature changes. When you enter the field of view of such a sensor, it is triggered by thermal radiation from your body. The disadvantage of this method of determination are false positives. Thermal radiation is inherent in everything that is around. Here are some examples:
1. stands in a room with an electric heater, which periodically turns on and off by a timer or thermostat. When the heater is turned on, false alarms are possible. You can try to avoid this by a long and painstaking sensitivity adjustment, as well as an attempt to direct it so that there is no heater in the line of sight.
2. When installed outdoors, triggering from gusts of warm wind is possible.
In general, these sensors work fine, while this is the cheapest option. A PIR sensor is used as a sensitive element, it creates electric field proportional to thermal radiation.
But the sensor itself does not have a wide directivity; a Fresnel lens is installed on top of it.
It would be more correct to say - a multi-segment lens, or a multilens. Pay attention to the window of such a sensor, it is divided into sections, these are the lens segments, they focus the incoming radiation into a narrow beam and direct it to the sensitive area of the sensor. As a result, beams of radiation from different directions fall on the small receiving window of the pyroelectric sensor.
To increase the efficiency of motion detection, dual or quarter sensors or several separate ones can be installed. Thus, the field of view of the device is expanded.
Based on the foregoing, it should be noted that the sensor should not be exposed to light from the lamp, and there should not be incandescent lamps in its field of view, this is also a strong source of IR radiation, then the operation of the system as a whole will be unstable and unforeseen. IR doesn't pass through glass well, so it won't work if you walk behind a window or glass door.
This is the most common type of sensor; you can buy it, or you can assemble it yourself on the basis, so let's consider its design in detail.
How to assemble an IR motion sensor with your own hands?
The most common option is the HC-SR501. It can be bought at a radio parts store, on ali-express, it is often supplied in Arduino kits. It can be used both in tandem with a microcontroller, and independently. It is a printed circuit board with a microcircuit, a strapping and one PIR sensor. The latter is covered with a lens, there are two potentiometers on the board, one of them regulates the sensitivity, and the second is the time that a signal is present at the sensor output. When motion is detected, a signal appears at the output and the set time is kept.
It is powered by a voltage of 5 to 20 volts, it works at a distance of 3 to 7 meters, and the output signal keeps from 5 to 300 seconds, you can extend this period if you use a microcontroller or a time delay relay. The viewing angle is about 120 degrees.
The photo shows the sensor assembly (left), lens (bottom right), reverse side boards (top right).
Let's take a closer look at the board. On its front side there is a sensitive element. On the back is a microcircuit, its strapping, on the right are two tuning resistors, where the top one is the signal delay time, and the bottom one is the sensitivity. In the lower right part there is a jumper for switching between H and L modes. In L mode, the sensor produces an output signal only for the period of time set by the potentiometer. Mode H gives a signal while you are in the range of the sensor, and when you leave it, the signal will disappear after a time set by the upper potentiometer.
If you want to use the sensor without microcontrollers, then assemble this circuit, all elements are signed. The circuit is powered through a quenching capacitor, the supply voltage is limited to 12V using a zener diode. When a positive signal appears at the sensor output, the relay P turns on through the NPN transistor (for example, BC547, mje13001-9, KT815, KT817 and others). You can use a car relay or any other with a 12V coil.
If you need to implement some other functions, you can use it in tandem with a microcontroller, for example. Below is the connection diagram and program code.
Ultrasonic
The emitter operates at high frequencies - from 20 kHz to 60 kHz. This leads to one trouble - animals, such as dogs, are sensitive to these frequencies, moreover, they are used to scare them away and train them. Such sensors can annoy them and cause problems.
The ultrasonic motion sensor works on the Doppler effect. The radiated wave, reflected from a moving object, returns and is received by the receiver, while the wavelength (frequency) changes slightly. This is detected and the sensor outputs a signal that is used to control the relay or triac and switch the load.
The sensor works out movements well, but if the movements are very slow, it may not work. The advantage is that they are not sensitive to changes in environmental conditions.
Laser or photosensors
They have an emitter (for example, an IR LED) and a receiver (a photodiode of a similar spectrum). This is a simple sensor, it can be implemented in two versions:
1. The emitter and photodiode are mounted in the passage (controlled area) opposite each other. When you pass through it, you block the radiation and it does not reach the receiver, then the sensor is triggered and the relay is turned on. This can also be used in alarm systems.
2. The emitter and the photodiode are next to each other, when you are in the area of the sensor, the radiation is reflected from you and hits the photodiode. This is also called an obstacle sensor, and is successfully used in robotics.
Microwave
It also consists of a transmitter and a receiver. The first generates a high frequency signal, the second receives them. As you pass by, the frequency changes. The receiver is configured in such a way that when the frequency changes, the signal is amplified and transmitted to the executive body, such as a relay, and the load is turned on.
Microwave motion sensors are very sensitive, they allow you to “see” an object even behind a door or behind glass, but this also causes problems. false positive when the object is out of the intended field of view.
These are quite expensive sensors, but they respond to even the smallest movements.
Capacitive devices work in a similar way. Such a scheme is shown below.
How to connect a motion sensor?
You can come up with countless options and schemes for connecting a motion sensor, depending on your needs, sometimes you need the system to work when moving in different places, For example street lighting on the way from the house to the gate and vice versa, in other cases it is necessary to force the lights to turn on or off, etc. We will look at several options.
Typically, a motion sensor has three wires or three terminals for connection:
1. Incoming phase.
2. Phase outgoing to power the load.
If you do not have enough sensor power - use an intermediate relay and. To do this, instead of a light bulb in the diagrams below, the coil leads are connected.
The photo below shows the terminals to which the power wires are connected.
Conclusion
Using motion sensors, no matter how it sounds, is a step. Firstly, it will help save energy and lamp life. Secondly, it will eliminate the need to flip the switch every time. For outdoor lighting, with the right settings, you can make the light turn on when you approach the gate of the house.
If the distance from the gate to the house is 7-10 - you can get by with one sensor, then you don’t have to lay a cable to the second sensor or assemble a circuit with a pass-through switch.
As already mentioned, IR sensors are most common, they are enough for simple tasks, if you need more sensitivity or accuracy, look at other types of sensors.
The application of an alternating current voltage to adjacent conductors contributes to the remote accumulation of positive and negative charges on them. They create a variable electromagnetic field that is sensitive to many external factors, primarily to the distance between the conductors. This property can be used to create appropriate capacitive sensors that are able to control the operation various systems control and tracking.
Voltage Applications different sign, according to Ampère's law, causes the movement of conductors on which electrical particles are located. This gives rise to alternating current, which can be detected. The amount of current flowing is determined by the capacitance, which, in turn, depends on the area of the conductors and the distance between them. Larger, closer objects induce more current than smaller, more distant ones.
The capacity is determined by the following parameters:
- The nature of a non-conductive dielectric medium located between the conductors.
- Conductor sizes.
- The strength of the current.
A pair of such surfaces forms the plates of the simplest capacitor, the capacitance of which is directly proportional to the area and dielectric constant of the working medium, and inversely proportional to the distance between the plates. With the constant dimensions of the plates and the composition of the working medium between them, any change in capacitance will be the result of a change in the distance between two objects: the probe (sensor) and the tracked target. It only suffices to convert capacitance changes to focused values. electrical voltage, which will control further actions of the device. These devices, therefore, are designed to determine the changing distance between objects, as well as to clarify the nature and quality of the surface of the measured products.
The principle of operation of the capacitive sensor
Structurally, such a device includes:
- Reference voltage generation source.
- The primary circuit is a probe, the surface and dimensions of which are determined by the measurement purposes.
- A secondary circuit that generates the necessary electrical signal.
- A protective circuit that ensures the stability of the sensor readings regardless of external disturbing factors.
- An electronic amplifier, the driver of which generates a strong control signal to the actuating elements, and ensures the accuracy of operation.
Capacitive sensors are divided into single and multi-channel. In the latter case, the device may include several of the above circuits with different probe shapes.
The electronics driver can be configured as master or slave. In the first version, it provides synchronization of control signals, therefore it is used mainly in multichannel systems. All devices are touch-sensitive, reacting exclusively to non-contact parameters.
The main characteristics of the considered devices are:
- Dimensions and nature of the target - the object of sounding. In particular, the electric field created by it should have the shape of a cone, for which dimensions must be at least 30% larger than the corresponding dimensions of the primary circuit;
- Measuring range. The maximum clearance at which the readings of the device give the required accuracy is about 40% of the usable area of the primary circuit;
- Accuracy of measurements. Reading calibration usually reduces the range but increases the accuracy. Therefore, the smaller the sensor, the closer it should be installed to the controlled object.
The characteristics of the sensors do not depend on the material of the object, as well as its thickness
How does a capacitor become a sensor?
In this case, cause and effect are reversed. When a voltage is applied to a conductor, an electric field is formed at each surface. In a capacitive sensor, the measuring voltage is applied to the sensitive zone of the probe, and for accurate measurements, the electric field from the probed area must be contained precisely in the space between the probe and the target.
Unlike a conventional capacitor, when capacitive sensors operate, the electric field can spread to other objects (or to separate areas of them). The result will be that the system will recognize such a composite field as multiple targets. To prevent this from happening, the back and sides of the sensitive area are surrounded by another conductor, which is maintained at the same voltage as the sensitive area itself.
When the reference supply voltage is applied, a separate circuit applies exactly the same voltage to the sensor protection. If there is no difference in voltage values between the sensitivity zone and protective zone, there is no electric field between them. Thus, the original signal can only come from the unprotected edge of the primary circuit.
Unlike a capacitor, the action of a capacitive sensor will be affected by the density of the material of the object, since this will disturb the uniformity of the generated electric field.
Measurement problems
For objects of complex configuration, the achievement of the required accuracy is possible if a number of conditions are met. For example, in multi-channel probing, the excitation voltage for each probe must be synchronized, otherwise the probes will interfere with each other: one probe will try to increase the electric field, while the other will tend to decrease it, thereby giving false readings. Therefore, a significant limiting condition is the requirement that the measurements be carried out under the same conditions in which the sensor was calibrated at the factory. If we evaluate the signal by changing the distance between the probe and the target, then all other parameters should have constant values.
These difficulties are overcome using the following methods:
- Optimizing the size of the measured object: the smaller the target, the more likely the field sensitivity will spread to the sides, resulting in an increase in the measurement error.
- Carrying out calibration only on a target with flat dimensions.
- Decreasing the speed of target scanning, as a result of which a change in the nature of the surface will not affect the final readings.
- During calibration, the probe must be positioned equidistant from the target surface (parallel for flat surfaces); this is important for high sensitivity sensors.
- The state of the environment: most capacitive sensors of the touch type operate stably in the temperature range of 22 ... 35 0 С: in this case, the errors are minimal
ny, and do not exceed 0.5% of the full measuring scale.
However, there are problems that cannot be fixed. Among them is the factor of thermal expansion / contraction of the material, both the sensor and the controlled object. The second factor is the electrical noise of the sensor, which is caused by the voltage drift of the device driver.
Operation Block Diagram
Being non-directional, the capacitive sensor measures some capacitance from objects that are constantly present in the environment. Therefore, unknown objects are detected by him as an increase in this background capacity. It is much larger than the capacity of the object, and is constantly changing in magnitude. Therefore, the devices in question are used to detect changes in the environment, and not to detect the absolute presence or absence of an unknown object.
When the target approaches the probe, the value of the electric charge or capacitance changes, which is recorded by the electronic part of the sensor. The result can be displayed on the screen or touch panel.
For measurement, the device is connected to a printed circuit board with a touch controller. Sensors are equipped with control buttons. With which you can turn on several probes at the same time.
Touch screens use sensors with electrodes arranged in rows and columns. They are located either on opposite sides of the main panel, or on separate panels that are separated from each other by dielectric elements. The controller cycles through the different probes to first determine which row is touched (Y direction) and then which column (X direction). Probes are often made of transparent plastic, which increases the information content of the measurement result.
Using LC filters
A dedicated analog interface converts the signal from the capacitive sensor into a digital value suitable for further processing. This periodically measures the output of the sensor and generates a drive signal to charge the sensor plate. The sample rate at the output of the sensor is relatively low, less than 500 samples per second, but the resolution of the A/D conversion is needed to capture small differences in capacitance.
In a capacitive meter, the stepped excitation waveform charges the sensor electrode. Subsequently, the charge is transferred to the circuit and measured by an analog-to-digital converter.
One of the problems with capacitive sensing (as already mentioned) is the presence of extraneous noise. Effective way Improving noise immunity is to modify the sensor by connecting a frequency-sensitive component. In addition to the variable capacitor element, an additional capacitor and inductor are added to the sensor to form a resonant circuit. The narrowband response allows it to suppress electrical noise. Despite the simplicity of the LC circuit, its presence provides a number of operational advantages. First, due to its inherent narrow band characteristics, the LC resonator provides excellent immunity to electromagnetic interference. Second, if the frequency range where noise exists is known, then the sensor's operating frequency offset can filter out these noise sources without the use of external circuitry.
LC filters are more commonly used in multichannel sensors.
Applications
These devices are used for the following purposes:
- For detecting plastics and other insulators.
- In alarm systems, when establishing the fact of movements in a controlled area.
- As a component of car security devices.
- To determine the surface finish of materials after machining.
- In order to determine the level of liquid or gaseous media in closed tanks.
- When installing systems for automatically turning on / off lamps.
In all cases, capacitive sensors are subject to mandatory calibration in the factory or other specialized conditions.
Do-it-yourself schemes
To organize touch control, it is easy to create a capacitive sensor based on a capacitor and a pair of resistors. When touching the wires, an electric charge is accumulated, by adjusting the value of which, you can change the charging / discharging time. This scheme can be used to control table lamp or other lamp. The circuit must contain an electronic comparator that will compare the charging time of the capacitor with a reference (threshold) value, and issue an appropriate control signal.
Touch-controlled electronic circuits are more interactive to the user than traditional ones, so they can be effectively used for switching power. The capacitance of the capacitor determines the sensitivity level: as the capacitance increases, the sensitivity increases, but more power and less response time are required to power the device. For indication, you can use a conventional LED.
