Bicycle back Safety Light

Posted in LED, on 2015-06-21

Circuit diagram:

Bicycle back Safety Light


  • R1_____________10K 1/4W Resistor
  • R2,R9,R10_____100K 1/4W Resistors
  • R3-R8,R11-R16__10R 1/4W Resistors
  • R17___________150R 1/4W Resistor
  • C1______________1µF 63V Polyester Capacitor
  • C2_____________10nF 63V Polyester Capacitor
  • C3____________100µF 25V Electrolytic Capacitor
  • D1-D13_____Red LEDs 5mm. or bigger, high efficiency
  • IC1,IC2________7555 or TS555CN CMos Timer IC
  • SW1____________SPST Slider Switch
  • B1_______________3V Battery (2 AA 1.5V Cells in series)

Device purpose:

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.

Circuit operation:

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.

LED arrangement:

Bicycle back Safety Light


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

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