Push-bike Light

Automatic switch-on when it gets dark

6V or 3V battery operation

Circuit diagram:

Push-bike Light


R1_____________Photo resistor (any type)
R2______________22K  1/2W Trimmer Cermet or Carbon type
R3_______________1K  1/4W Resistor
R4_______________2K7 1/4W Resistor
R5_____________330R  1/4W Resistor (See Notes)
R6_______________1R5   1W Resistor (See Notes)

D1____________1N4148  75V 150mA Diode

Q1_____________BC547  45V 200mA NPN Transistor
Q2_____________BD438  45V 4A PNP Transistor

LP1____________Filament Lamp(s) (See Notes)

SW1_____________SPST  Toggle or Slider Switch

B1______________6V or 3V Battery (See Notes)


This design was primarily intended to allow automatic switch-on of push-bike lights when it gets dark. Obviously, it can be used for any other purpose involving one or more lamps to be switched on and off depending of light intensity.
Power can be supplied by any type of battery suitable to be fitted in your bike and having a voltage in the 3 to 6 Volts range.
The Photo resistor R1 should be fitted into the box containing the complete circuit, but a hole should be made in a convenient side of the box to allow the light hitting the sensor.
Trim R2 until the desired switching threshold is reached. The setup will require some experimenting, but it should not be difficult.