Electronic watchman. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling
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The electronic watchman performs the functions of blocking the ignition system, turning on an audible alarm signal when trying to find the system code or when trying to turn on the ignition. A schematic diagram of an electronic watchman with connection circuits on a car is shown in fig. 1.
The return of the watchman to its original state after operation is carried out by briefly breaking circuit 2 with the secret button SB1.
Main technical characteristics:
- Supply voltage: for car watchman, V.......12
- for motorcycle guard, V.......6
- Current consumption in armed mode.......none
- Ambient temperature, °С....... from -40 to +60
- Overall dimensions, mm......120x49x35
The device consists of the following nodes:
- thyristor VS3 blocking the ignition system;
- thyristor VS1, which includes a sound signal;
- a code device consisting of a socket X2 and a plug X1 (key);
- blocking thyristors VS4, VS2;
- protection elements VD1, VD2, R1, R3, R4, C1.
All the nodes and elements listed above are located in the housing of the electronic unit, the diagram of which is circled in Fig. 1 by a dotted line. The remaining elements of the circuit, except for the SB1 button, are standard equipment of the car: SA1 - ignition switch, Pr - breaker, C2 - breaker capacitor.
When the ignition is switched on with switch SA1 and the key (socket X1) of the code device is correctly installed, as shown in the diagram, the supply voltage is supplied only to the thyristor VS3. The thyristor opens and connects power to the ignition coil. The serial connection of the KU201A thyristor with an ignition coil causes a voltage drop on it of about 2 V, which is undesirable, since this can make it difficult to start a cold engine in the winter season.
When the key (plug) of the code device is removed, power is not supplied to the VS3 thyristor and the ignition coil and the engine cannot be started.
When trying to select a code without a regular key, plus power is supplied to one of the slots 2-5 of the socket of the code device connected to the control electrode of the thyristor VS4. As a result, the thyristor VS4 opens and a positive voltage is applied to the control electrodes of the thyristors, VS1 and VS2. Thyristor VS1 switches and turns on the sound signal VA of the car. Thyristor VS2 remains in the off state, since there is no positive voltage on its anode. If, as a result of continued attempts to select the code, a positive voltage enters sockets 6 or 7, the thyristor VS2 will switch and shunt the control electrode of the thyristor VS3, while the latter remains in the off state. As a result, the ignition coil remains de-energized. After that, even if the "master key" is used, the horn will continue to work and the ignition coil will remain de-energized.
To turn off the sound signal, it is necessary to briefly break the circuit between contact 1 of the device and ground using the SB2 secret push-button switch. When this circuit breaks, thyristors VS1. VS2 and VS4 turn off and thyristor VS3 turns on. The sound signal stops working, and the supply voltage is supplied to the ignition coil.
The electronic watchdog circuit for a motorcycle differs in +6 V supply voltage and resistor values.
See other articles Section Security devices and object signaling.
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