This automatic bicycle light significantly enhances nighttime cycling (though pedaling is still necessary, of course). The circuit is designed to consider the surrounding light level, activating the light only in darkness. It turns off the light if there has been no cycling activity for more than a minute or when it becomes light again. The key advantage of this circuit is its absence of manual controls. This ensures you never accidentally forget to turn the light on or off, making it especially convenient for children and those prone to forgetfulness.
To detect bike usage (when the wheels are in motion), the circuit utilizes a reed switch (S1) mounted on the frame near the wheel. A small magnet, similar to those used with most bicycle speedometers, is attached to the spokes. This magnet triggers the reed switch once per wheel revolution. As the wheel rotates, pulses are sent to the base of T1 through C1, charging a small electrolytic capacitor (C2).
In low light conditions, when the LDR exhibits high resistance, T2 becomes conductive, illuminating the lamp. With each wheel revolution, C2 is recharged. The charge stored in C2 ensures that T2 remains conductive for approximately a minute after the wheel stops moving. Virtually any light type can be connected to the circuit’s output. At a 3V supply voltage, the idle current is just 0.14 μA when the reed switch is open. When the magnet aligns to close S1, the current rises to 3 μA. In either case, using batteries to power the circuit poses no issues.
The supply voltage can range from 3 to 12 V, contingent on the connected lamp type. Considering the circuit will likely be housed inside a bicycle light, its compact dimensions are crucial. Consequently, the board has been designed to be exceptionally compact, utilizing surface-mount device (SMD) components. Most components are in an 0805 package, with C2 being a chip version. The board is single-sided, with the top serving both as the solder side and the circuit’s surface.
The printed outline for the LDR (R5) may not precisely match the one specified in the component list. This variation is because there are various LDR packages available in the market. Consequently, it’s possible to use a different type of LDR if the light threshold needs adjustment. The placement of the LDR on the board might also depend on how the board is mounted inside the light fixture.
Regarding the MOSFET, there are multiple alternatives, including the FDS6064N3 from Fairchild, the SI4864DY from Vishay Siliconix, the IRF7404 from IRF, or the NTMS4N01R2G from ONSEMI.
Reed switches also come in diverse shapes and sizes; some are even waterproof and come pre-attached with wires. For the supply and lamp connections, you can either use PCB pins or solder the wires directly onto the board. If using pins, it’s advisable to slightly shorten the soldered ends to prevent them from protruding beneath the board, minimizing the risk of shorts with any metal parts of the light.
When using a dynamo to power the circuit, it’s crucial to rectify the alternating voltage first. The same applies to hub dynamos, which often output alternating voltage. Careful consideration of these aspects is necessary for effective circuit operation.