Device for automatically turning on the headlights of a car. Encyclopedia of radio electronics and electrical engineering

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

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The law on the mandatory inclusion of dipped headlights when driving a car during daylight hours came into force a long time ago, but it happens that even professional drivers forget to turn on the headlights and start driving without light. Dipped beam headlights can be replaced with fog lights (PF) or daytime running lights (DRL), but this does not exclude the possibility of forgetting about the need to turn them on.

The proposed device is designed to automatically turn on headlights, DRLs or other light sources on a car. Its peculiarity is that the "polite light" function is implemented, which is used on some expensive cars. The device can be connected to any car and configured to turn on any light source.

The Polite Light feature has three main features:

- a delay in turning on the headlights for the period of starting the engine;
- delay of turning off the light after the engine stops;
- forced switching off of headlights at the moment of engine start.

The headlight turn-on delay is needed to facilitate starting the engine in the cold season, while the headlights turn on 15 seconds after starting, allowing the engine to stabilize its operation. The light off delay is convenient for those who leave the car in the parking lot or in the garage without an additional light source. After turning off the engine, the headlights burn for about 2 minutes, and then go out, allowing you to slowly get out of the car, pick up things and leave the parking lot. Forced turning off the headlights at the time of starting also makes it easier to start the engine, while conserving headlight bulbs, especially xenon ones. Additionally, the device is equipped with an LED headlight indicator.

Fig. 1

The scheme of the device is shown in fig. 1. Pin 6 of the XP1 connector of the device is supplied with +12 V of the car's on-board network, and pins 1 and 2 are connected to a common wire ("ground") of the car. Pin 4 is connected to the ignition switch so that voltage appears on it only at the moment the engine is started. Pin 5 is connected to any wire that has voltage after starting the engine. On some cars, this may be the control output of the generator, if there is no such output, you can connect pin 5 to the turn-on wire of an audio device, such as a radio. Then the headlights will turn on 15 seconds after the key is turned in the ignition, regardless of whether the engine is running or not. Pins 7 and 8 are connected in parallel with the car headlight switch.

After power is applied, the transistor VT2 is closed, the relay K1 is de-energized and the headlights are off, as indicated by the HL1 LED, starting to glow. After starting the engine, +5 V appears on pin 12 and capacitor C1 begins to slowly charge. The charging time, and therefore the time after which the headlights turn on, depends on the resistance of the resistor R3 and the capacitance of the capacitor. After charging the capacitor C1 to the threshold voltage of the transistor VT2, the latter opens, the relay K1 is activated and the headlights turn on, and the HL1 LED stops shining.

Since +5 V is present on pin 12 when the engine is running, the headlights are always on. After the engine stops, the voltage on it disappears and the capacitor C1 begins to slowly discharge through the resistor R4. The time that the headlights stay on after the engine is stopped depends on the time constant of circuit R4C1. After the capacitor is discharged to a voltage below the opening threshold of transistor VT2, the latter closes, de-energizing relay K1, and the headlights go out. If, after stopping the engine, it becomes necessary to start it again, then at the moment the starter is turned on, +4 V is applied to pin 12 and transistor VT1 opens, quickly discharging capacitor C1 and blocking relay K1 from turning on, i.e., the headlights are forced to turn off at the moment the engine starts .

Fig. 2

The device is assembled on a printed circuit board made of foil fiberglass with dimensions of 45x25 mm (Fig. 2). The VT2 field effect transistor can be replaced with another powerful one with a low open channel resistance (no more than 0,15 ohms), for example, IRF540, IRFZ44. Installing it on a heat sink is not required. Transistor VT1 - any low-power npn structure. The HL1 LED can be of any glow color, it is useful to use a flashing LED to attract the driver's attention. Mount it in any place convenient for the driver. Relay K1 - ELZET LR-T78-12VDC or other low-power relay with a winding voltage of 12 V. The parameters of the timing circuits R3C1 and R4C1 are selected individually. No deviation of their parameters during the operation of the car in the cold season at low ambient temperature was observed. The appearance of the device is shown in fig. 3.

Fig. 3

In addition, for the convenience of controlling the device, a three-position switch can be included in the executive circuit of relay K1 by connecting it to the vehicle's on-board network so that in one extreme position the device turns on fog lights or daytime running lights, in the other extreme position the dipped beam headlights are turned on, and in the middle (neutral) position, the device had no effect on vehicle lighting.

Author: D. Zakharov

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