Automatic Bike Headlight Switch

Improved bike headlight switch circuit presented here has been designed to switch bike headlight on (or similar loads) at a presettable ambient light level. The circuit is based on the renowned timer chip LM555 (IC1), here triggered by a decrement in ambient light level. Here is a 220V automatic light switch, just in case you need one.

how does the automatic headlight switch works?

When 12VDC supply is applied to the circuit, the voltage at pin 2 of IC1 is set by the voltage divider built around the Light Dependent Resistor (LDR) and trimpot P1 (100K). If the ambient light level (presetted by P1) is sufficient, pin 2 of IC1 is at a high potential and as a result IC1 is disabled. Capacitor C1 (100uF/25V) added across the LDR eliminates possible unwanted circuit functioning which may caused by sudden fluctuations in the light level.

However, when ambient light level drops, the resistance of LDR increases and the voltage level at pin 2 of IC1 drops to trigger the monostable circuit. Simultaneously, the base of transistor T1 (BC557) is pulled low and discharges the timing capacitor C3 (100uF/25V). Next, output (pin 3) of IC1 goes high to turn on the electromagnetic relay (12V/SPDT). The RC time constants (R1 and C3) sets this on time for a period of near 5 seconds. But if the light level stays low, T1 holds C3 in the discharged state and the relay stay on. Common (C) and Normally-Opened (N/O) contacts of the relay can be used to switch the headlight.

The timeout of five seconds is introduced deliberately to avoid erratic switching of the headlight due to other strong light reflections. This also allows the headlight to stay on for a finite time after the ambient light level restoration.

Automatic Light Switch Circuit Schematic

Automatic Bike Headlight Switch Schematic

Bike Headlight Switch Notes

  • The whole circuit should be powered from the 12V bike battery
  • Slight intervention in the existing electrical wiring of the vehicle may become necessary to add this circuit
  • Use of a medium-size encased LDR is recommended for reliable operation. Fit the LDR at the front side of the bike in an appropriate position
  • The relay used in the prototype draws near 37.5 mA. 555 IC output can switch up to 100mA, which is adequate for driving the relay
  • 555 IC will only rise to near 10v for the 12V input supply and the diode D1 will drop 0.7Volt. As a result the 12V relay will only get about 9.3Volt. However, pick-up voltage of a standard 12V relay is 9V, this is not a problem. D1 is added intentionally to avoid episodic timer latch up

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