Switch for additional brake lights. Scheme, description

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Switch for additional brake lights. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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As you know, the rear brake lights of domestic cars shine constantly while the brake pedal is pressed. Recently, more and more often you can find cars in which, when braking, along with the brake lights, additional signal lights of an intermittent glow are turned on, moreover, sometimes they are mobile in shape and place.

Traffic police specialists consider it expedient from the point of view of increasing traffic safety to install duplicate flashing brake lights, one should only avoid "illumination" excesses. A few years ago, the domestic industry produced additional brake lights equipped with a simple electronic current switch. They had to be mounted behind the rear window of the passenger compartment and connected in parallel to the circuit of the main lamps. When you press the brake pedal, the additional lights flashed red.

A simple current interrupter for additional lights is described below (see the diagram in Fig. 1), which operates according to a specific algorithm. The device is assembled on two popular and inexpensive CMOS chips and two transistors. The device can only be connected in parallel with the stop lamps. Any other option for connecting additional lights is dangerous because in the event of an accidental malfunction of the switch, the main brake lights may turn out to be de-energized, due to which the risk of a rear collision, especially at night, increases dramatically.

Switch for additional brake lights

On the elements DD1.1, DD1.2, a rectangular pulse generator operating at a frequency of 4 Hz is assembled. The pulses are fed to the counting input of the decimal counter DD2. Since a low level is first applied to the counting enable input CP, the counting occurs on the positive voltage drop at the counting input.

When the supply voltage is applied, a high-level pulse from the resistor R2 sets the counter to its initial state. Then a high-level pulse alternately appears at the outputs of the counter. As soon as a high level occurs at output 8, further counting stops, and the counter remains fixed in this state until the brake pedal is released and the unit is de-energized.

The outputs of the counter are connected to the base of the composite output transistor VT1VT2 by an OR node assembled on diodes VD1-VD5 and resistor R3. The load of the output transistor is two car lamp lamps HL1, HL2.

While high-level pulses alternately appear at the outputs of the counter, opening the composite transistor, the lamps flash synchronously with the pulses. Since the counter does not use all the outputs, but only every second, there are pauses of the same duration between the flashes of the lamps.

Therefore, each time you press the brake pedal, the lamps flash four times at a frequency of 4 Hz, after which they turn on constantly until the switch is de-energized. The flash frequency can be changed within certain limits by a tuning resistor R1.

Zener diode VD6 and capacitor C1 together with resistor R5 protect the device from voltage surges in the on-board network and impulse noise in the power circuit. In normal mode, the zener diode is closed and does not take part in the work.

The device is assembled on a printed circuit board made of foil fiberglass with a thickness of 1 mm. The drawing of the board is shown in fig. 2. Powerful transistor VT2 does not need a heat sink. The board is installed in a durable plastic box, which is mounted in a convenient place in the trunk.

Switch for additional brake lights

Capacitor C3 - K53-4; any other oxide capacitor with minimal capacitance versus temperature will do. Capacitor C1 - K53-18, C2 - low-voltage K73-17a. Trimmer resistor R1 - SPZ-6a.

The switch does not require adjustment. If you do not need to change the flashing frequency of additional lights, it is better to set a constant of the same resistance instead of a tuning resistor.

The switch and additional lights can also be installed on cars with an on-board network voltage of 24 V. In this case, a stabilizer chip KR5EN1B or KR2EN142D must be installed in the gap of the positive wire between the resistor R8 and the connection point of the lamps HL142, HL8, it has an input - output 17 (1 ), output - 2 (3), total - 8(2). For the output current amplifier, transistors with a collector-emitter voltage of at least 50 V should be selected. Lamps for this version of the switch will require twenty-four volts.

Too powerful lamps in additional lamps should not be used so as not to overload the contacts of the brake light switch.

The switch will work more reliably if, instead of the polar capacitor C3, a non-polar one is installed for the same capacitance and voltage.

Author: A.Kashkarov, St. Petersburg

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