Bicycle Back Safety Light Circuit Schematic

Flashing 13 LED unit, 3V supply, Also suitable for jogger/walkers

This circuit has been designed to provide a clearly visible light, formed by 13 high efficiency flashing LEDs arranged in a pseudo-rotating order. Due to low voltage, low drain battery operation and small size, the device is suitable for mounting on bicycles as a back light, or to put on by jogger/walkers. IC1 is a CMos version of the 555 IC wired as an astable multivibrator generating a 50% duty-cycle square wave at about 4Hz frequency.

At 3V supply, 555 output (pin 3) sinking current operation is far better than sourcing, then LEDs D1-D6 are connected to the positive supply rail. In order to obtain an alternate flashing operation, a second 555 IC is provided, acting as a trigger plus inverter and driving LEDs D7-D12. D13 is permanently on. The LEDs are arranged in a two series display as shown below, with a center LED permanently on. This arrangement and the alternate flashing of the two series of LEDs provide a pseudo-rotating appearance.

Circuit diagram:
Parts:

R1 = 10K
R2 = 100K
R3 = 10R
R4 = 10R
R5 = 10R
R6 = 10R
R7 = 10R
R8 = 10R
R9 = 100K
R10 = 100K
R11 = 10R
R16 = 10R
R17 = 150R
C1 = 1µF-63V
C2 = 10nF-63V
C3 = 100µF-25V
B1 = 3V Battery (2 AA 1.5V Cells in series)
SW1 = SPST Slider Switch
D1-D13 = Red LEDs 5mm. or bigger, high efficiency
IC1-IC2 = 7555 or TS555CN CMos Timer IC

LED arrangement:
Notes:

• Flashing frequency can be varied changing C1 value.
• High efficiency LEDs are essential.