Non-contact presence sensor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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I propose a device from the category of "electric tricks", which is based on the sensitivity of field-effect transistors to static electricity and network pickups. A simple field-effect transistor hidden wiring finder circuit was taken as a basis, however, after conducting a series of experiments and reworking it a little, we managed to get a completely independent and reliable machine that responds to a change in the electric field.

The "blue dream" of a lazy person is to automate life in the house to such an extent that all lighting, heating, ionizing and other devices turn on automatically, without the influence of familiar and non-original electrical switches. So, you come home after a tiring and “difficult” day, when you don’t even feel like eating, but just want to lie down, slowly approach your favorite sofa, lie down, and lo and behold! Immediately, a sconce lights up with a soft light. It remains to pick up a book, turn on the TV or music center using the remote control.

To translate such thoughts into reality, you need quite a bit - to assemble the simple diagram shown in Fig. 1.

Fig.1. Schematic diagram of the sensor

The load is connected to the contacts of the executive relay. The phase insulated wire "Ф" with a diameter of 0,8 ... 1 mm is located in a "snail" of 5 ... 6 turns and is fixed on the inner lining (side) of the sofa seat (Fig. 2).

Fig.2. Sensor Installation Diagram

At a distance of 5 ... 6 cm from it, slightly lower relative to the plane of the chair (sofa), closer to the floor, the WA1 antenna of the described device is located, and the device itself is attached nearby in an unshielded case. When someone sits down (leans, lies down) on a sofa, under the weight of a person, he bends, and the electric field around the "Ф" wire approaches the sensor-antenna and is captured by it. As a result, the actuator is turned on. When the impact on the sofa stops, the antenna is outside the zone of influence of the electric field, the relay turns off, the sconce goes out.

The antenna is a piece of a metal knitting needle or a similar conductive wire with a total length of up to 20 cm, bent with a "question mark".

The principle of operation of the device. When there is no electrical (mains) interference in the sensitivity zone of the WA1 antenna, the field-effect transistor VT1 (good results are obtained with KP1OZD) is open and shunts the rest of the circuit, not giving enough potential to the VT2 base to open. Relay K1 is de-energized.

When an electric field is applied to the VT1 gate, it closes, rectified by a diode chain and filtered by C1, the positive potential enters the VT2 base, opens the electronic key, turns on the relay and the load. Resistor R2 should be selected for stable opening of the key. As experiments have shown, the value of R1 can be in the range of 100 kOhm ... 5 MΩ. Diodes VD1 ... VD3 - necessarily germanium (type D2, D9). Capacitor C1 - type K50-6, with a capacity of 50 ... 200 microfarads. Resistor R3 limits the base current VT2. Instead of KT312B, you can use KT315 with any letter index. Diode VD4 eliminates the bounce of the relay contacts and smoothes the reverse current surges through K1. If the contact bounce cannot be completely eliminated, it is necessary to connect an electrolytic capacitor in parallel with the relay with a capacity of not more than 50 microfarads in the appropriate polarity.

Setting up the device includes selecting the distance between the antenna and the phase wire, and selecting R2 within the specified limits. When tuning, it must be replaced with a 2,2 MΩ trimmer with a 100 kΩ fixed resistor in series.

In the manufacture of the device, electrical safety measures (when installing and connecting the phase wire) and protection measures against static electricity when working with a field effect transistor should be observed. It should be installed and configured by grounding a low-power soldering iron (no more than 25 W) and putting an antistatic grounded bracelet on your hand.

The circuit is operable at a supply voltage of 9 ... 12 V. The relay should be selected based on these parameters.

Good results are achieved when using this FET assembly as a troubleshooting device in high voltage automotive circuits.

Such a scheme can be used as a non-contact door open sensor (the antenna moves away from the phase wire) or as a kind of device for restricting access to a room. In this case, a magnetic or metal card is inserted into a thin cell between the masked source of network interference and the antenna of the device. In the latter case, the principle of operation of the electronics must be reversed (relay operation when the effect of pickups ceases).

Author: A.Kashkarov, St. Petersburg; Publication: cxem.net

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