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Automatic Rear Bicycle Light Schematic Circuit Diagram

This is a design for a rear bicycle light that automatically switches on and off according to ambient lighting conditions. The red LEDs flash with a 50% duty cycle to save energy; you can modify the circuit to light the LEDs continuously if local laws require it. The circuit can of course also be used as a safety light by pedestrians.

Automatic Rear Bicycle Light Schematic Circuit Diagram

Custom Bicycle Light Circuit Implementation

Unique Circuit Integration for Rear Bicycle Light

The author embarked on a project involving a commercially-available rear bicycle light, replacing the original printed circuit board with a custom design showcased in the circuit diagram. Due to spatial constraints, surface-mount devices were chosen for the prototype construction, though leaded devices can be employed interchangeably. The 10 µF SMD film capacitors are optional and can be substituted with electrolytic capacitors.

Incorporating High Brightness Red LEDs

The original bicycle light featured five high brightness red LEDs, retained on their dedicated circuit board with corresponding series resistors. This section remains intact, accounting for variations in series resistor values, adaptable according to desired brightness and specific LED characteristics. A green LED (D6) from the original unit is not used in this particular design.

Operation with Vibration Sensor and LDR

The circuit incorporates two sensors: a vibration switch (S1) and an LDR (R5). Movement of the bicycle triggers the vibration switch (S1), generating pulses that activate Darlington T1 via C1. This, in turn, charges C2 and drives the input of gate IC1.A (pin 1) to a Low state. Simultaneously, if the ambient light is below a certain threshold, voltage division between R4 and LDR R5 yields a voltage greater than 0.6 V, allowing transistor T2 to conduct and charge C3. A Low level is then applied to the second input (pin 2) of gate IC1.A.

Flashing LED Operation

If both inputs to the gate remain at logic zero, the NOR gate’s output goes high, enabling FET T3 to conduct. This supplies power to the astable multivibrator consisting of R9, R10, R11, C4, C5, T4, and T5, causing the LEDs to flash at 5 Hz. This flashing continues as long as the vibration sensor S1 generates pulses and the ambient light remains below the specified level.

Controlled LED Behavior

If the vibration sensor ceases to produce pulses (as the bicycle comes to a stop), C2 gradually discharges through parallel resistor R3, creating a 25-second delay before the gate’s output goes low. Consequently, T3 blocks, and the LEDs extinguish. When the bicycle is in motion but the LDR senses light (e.g., from passing vehicles or street lighting), the LEDs remain active for approximately 70 seconds due to C3 holding the gate’s input low.

Efficient Battery Operation

The circuit operates on 3 V (provided by two AAA cells) and boasts a quiescent current consumption of less than 2 µA. As such, the batteries are expected to last for over 300 hours of operation.

Sensitivity and Light Threshold Adjustment

In practice, the vibration sensor proved highly sensitive, generating pulses even when the cyclist halts at traffic lights, resulting in continuous LED flashing. The LEDs only extinguish when the bicycle is entirely stationary. The ambient light threshold can be tailored to suit LDR characteristics by adjusting R4.

Steady Light Modification

For those desiring a steady light instead of flashing, the modification involves removing T4, T5, C4, C5, R9, R10, and R11. Subsequently, connect the cathodes of LEDs D1 to D5 directly to the drain of FET T3.


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