Build Your Motor Bike Headlight Controller Circuit

Description

This circuit automatically activates a motorcycle’s headlight and deactivates it, irrespective of the illumination and ignition controls, provided the battery is completely charged. The initial stage incorporates a 220-ohm resistor and ZD1 to maintain transistor Q1 in an off state when the motorcycle is not in operation; it consumes approximately 2mA. Upon the battery voltage surpassing 7.0V during the charging process, Q1 begins to switch on.

The concluding stage utilizes a 220-ohm resistor and ZD2 to enable transistor Q2, which pulls the base of Q1 low, thus switching it fully on. In conjunction with the Vbe drop of Q2, ZD2 will deactivate Q2 at a battery voltage around 6.7V. Consequently, the headlight remains illuminated for the majority of the time while the motor is operating and recharging the battery. Heatsinks are necessary for both transistors. The circuit can be situated near the battery with a single connection to the headlight power supply.

Circuit diagram:

Parts:

Transistor Details

Transistors, such as the BD139 and MJ4502, are fundamental semiconductor devices used as electronic switches and amplifiers. The BD139 is a general-purpose NPN transistor commonly used in switching and amplification applications due to its good switching characteristics and moderate gain. The MJ4502 is another NPN transistor that provides a good balance between gain and switching speed, often employed in similar applications as the BD139. These transistors operate based on the flow of current between their three terminals: the collector, emitter, and base, controlled by a small current applied to the base. Their use in this circuit allows for precise control of the headlight activation based on battery voltage.