Smooth turning off the light in the car. Encyclopedia of radio electronics and electrical engineering

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

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Many foreign cars have a function for smoothly turning off the interior lights. I also wanted to have such convenience in my car. To do this, I assembled a device using two transistors, three resistors, one diode and an oxide capacitor. Its diagram is shown in Fig. 1.

Fig. 1

At the moment the standard door limit switch SF1 of the car opens when the doors are closed, capacitor C1 is discharged, so current begins to flow through the +12 V circuit, the interior lamp EL1, C1, R1, the emitter junctions of transistors VT1, VT2 and the common wire. Transistors VT1, VT2 open. Due to the action of the feedback loop on the voltage formed by the circuit R1C1, a voltage of 1,4...1,5 V is established on the transistors, equal to the total at their emitter junctions (capacitor C1 is discharged, and the resistance of resistor R1 is low). The EL1 lamp maintains the on-board voltage (+12 V) minus the specified voltage drop across the transistors. The lamp shines brightly.

Capacitor C1 begins to charge, and the current through it decreases. This leads to a decrease in the base and collector currents of transistors VT1, VT2. The current through lamp EL1 and the voltage across it drop, and it goes out smoothly. The complete shutdown time depends on the power of the lamp EL1, the capacitance of the capacitor C1, the resistance of the resistors and the current transfer coefficients of the transistors VT1, VT2. In the author's version it is approximately equal to 5 s. To quickly discharge the capacitor when opening any door, a VD1 diode is installed.

The device can use medium (VT1) and high (VT2) power transistors of any type. In the case of using transistors of the pnp structure, it is necessary to change the polarity of connecting capacitor C1 and the polarity of connecting the device to the standard switch SF1 of the car. When assembling the structure, I used mounted mounting of the elements, placing the transistors on a small heat sink (Fig. 2). Since the transistors are in active mode for a short time (5 s), installing them on a heat sink is not necessary.

Fig. 2

A correctly assembled device does not require adjustment. If you need to change the time to turn off the lights in the cabin, you should select the capacitance of capacitor C1. The larger it is, the longer the delay in turning off the light, and vice versa. You can install the assembled device in any convenient place; I placed it in the center pillar of the car, next to the light switch. Due to the low current consumption when switched off, it does not affect the operation of the security alarm, also connected to the door switches.

Author: N. Kamenev

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