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Simple car caretaker

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Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms

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What car enthusiast does not care how to securely protect their car from theft or intrusion by unauthorized persons? Here, as a rule, electronics comes to the rescue. However, most electronic devices designed to protect vehicles are often complex and expensive, and they often require limit switches isolated from the body. All this limits the widespread use of such "watchmen" by motorists.

We offer several options for such devices that are devoid of these disadvantages. Even an inexperienced radio amateur can assemble an electronic "watchman".

For a watchdog assembled on two transistors (Fig. 1), the first stage VT1 limits the current flowing through the contact system of the sensor. These can be limit or mercury switches, tilt sensors, etc. A time relay is assembled on the transistor VT2, which provides a delay in turning on the "watchman". It allows you to get out of the car after turning on the security device. The duration of the delay within 6-12 s is set by selecting the capacitance of the capacitor C1.

A simple car caretaker. Dual transistor watchdog

Relay K3 and reed switch K4 serve as the key to the "watchman". The reed switch is hidden under some plastic (non-magnetic) part of the outer lining of the car body in a place known only to the driver. Before getting out of the car, the driver switches the SA1 switch to the opposite position according to the scheme, and the device is connected to a power source. Capacitor C1 starts to discharge through resistor R3 and the base-emitter junction of transistor VT2, and opens. Relay K2 is activated and with its contact plates K2.1 breaks the power circuit of the key stage on VT1. At this time, the driver gets out of the car, closes the door. The state of the sensors does not yet affect the work of the "watchman". After 6-12 s, the voltage on C1 decreases to a level at which the semiconductor triode VT2 closes. The power circuit of the first cascade is restored, the "watchman" switches to the armed mode. Since the current consumption from the power source is very small, the electronic device can work for a long time not only from the car battery, but also from three 3336L batteries connected in series.

If you now open, for example, a door, the limit switch installed in the doorway will close. The base VT1 through the resistor R1 will be connected to a common wire, and the transistor will open. Relay K1 with its contact system will block the sensor circuit and turn on the alarm. If the door is then closed, the alarm will not stop. You can turn it off only with the SA1 switch.

To enter the car without disturbing the peace of others, the alarm is turned off for a while. Unnoticed by others, they bring a small permanent magnet disguised as some object (candy, matchbox) to the place where the reed switch is hidden. Under the influence of a magnetic field, its contact plates will close. Relay K3 will operate, breaking the power supply circuit of the "watchman" due to a violation of contact K3.1 and self-locking by closing contact KZ.2. Now the driver can safely get into the car and finally turn off the alarm with the SA1 switch.

In the electronic security device in the first stage, you can also use transistors KT501B (V, D, E, I, M, and in the second - KT503B (G). Diodes D1, D2 with any letter index can also be used as VD7 and VD226. Resistors MLT-0,25 or MLT-0,5 Capacitor - K50-3, K50-6 Relays RES-15 (passport PC4.591.003) and RES-22 (passport RF4.500.129) are used in the device. RES-6 or RES-9 with actuation voltage 12 V. Reed switch - any type (KEM-3, KEM-1, KEM-2A).

Please note that the "watchman" will work in conditions of increased vibration, dust, humidity. Therefore, the installation must be rigid, soldering reliable. Such requirements are provided by printed wiring (Fig. 2).

Simple car caretaker. Security device circuit board

After assembling and checking the operation of the watchdog, glue the installed radio elements with epoxy to the board. When the glue hardens, cover the board on both sides with a transparent varnish, dry it, and then place it in a case of suitable dimensions, taking into account the relays K1, KZ, which are fixed inside the case with the help of corners. "Watchman" install in the car, hiding in a secluded place.

Simple car caretaker. Simplified watchdog diagram

If there are no strict requirements for the amount of current flowing through the sensors, the transistor VT1 (Fig. 1) in the signaling circuit can be excluded (Fig. 3). The circuit board of a simplified version of the watchdog is shown in Figure 4.

Simple car caretaker. Printed circuit board

When using a "watchman" to protect premises, such as a garage, a summer house, etc., it may be necessary to increase the delay for turning on the device up to 1 minute. In this case, we recommend assembling the time relay on a composite transistor (Fig. 5), which has a high input resistance. Therefore, the discharge current of the capacitor C1 is significantly reduced compared to the first circuit (Fig. 1), and the discharge time increases accordingly.

Simple car caretaker. "Watchman" for the protection of premises: garages, cottages, etc.

The duration of the "watchman" turn-on delay (Fig. 5) depends on the capacitance C1 and the resistance of the resistor R4. Due to the large spread of transistor parameters, the exact values of C1 and R4 are selected experimentally in each specific case. The maximum capacitance C1 should not exceed 500 microfarads, and the resistance R4 can be in the range of 30-100 kOhm. Sometimes it happens that the current flowing through the collector-emitter junction VT2 is sufficient to open the transistor VT3. In this case, a resistor with a resistance of 2-1 kOhm is included in the collector circuit VT2. If it is not enough, then through a resistor with a nominal value of 20-50 kOhm, the VT2 base is connected to a common wire. The watchdog circuit board with a composite transistor time relay is shown in Figure 6.

Often there is a need for the "watchman" to work to open the contact circuit. A diagram of such a security device is shown in Figure 7.

Simple car caretaker. "Watchman" to open the contact circuit

It differs from the first signaling device by the presence of an executive stage on the transistor VT2. At its base, there is always a positive bias sufficient to open the transistor. But since the resistance of the sensor connected between the VT2 base and the common wire is many times less than the value of the resistor R2, a negative blocking potential appears on the base: the transistor is closed. If now the sensor circuit is broken, VT2 opens, relay K2 is activated and contact K2.1 activates the cascade on transistor VT1. If necessary, the additional device can be turned off with the toggle switch SA1). An electronic "watchman" with an additional device can protect the glass of a car or the windows of a room. To do this, glue strips of foil 5–8 mm wide around the perimeter of the windshield and rear windows and connect them with a thin stranded wire to socket X2.

If the glass is broken or removed, the foil will break, setting off an alarm. The rear window defroster foil can also be used as a sensor.

Simple car caretaker. Universal watchdog circuit board

(click to enlarge)

When installing the signaling device, please note that the car signal consumes a current of about 6 A, and it cannot be switched using small-sized relays used in the "watchman". Therefore, the free contact plates of relay K1 are connected to the standard signal relay. If there is no such relay in the car (for cars of old brands), it is installed additionally by using a relay from Zhiguli or Moskvich cars. Instead of an electromagnetic relay, it is better to use a KU202 thyristor with any letter index, including a 750 Ohm resistor with a dissipation power of at least 1 W in the control electrode circuit. The circuit board of the universal "watchman" is shown in Figure 8.

Author: Yu. Shurchkov, "Model designer", No. 9 1985; Publication: cxem.net

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