Bike Light

The idea was to build a high power, multi‐utility flash light powered by only 2xAA rechargeable cells. The flash light can be operated in two modes: flashing/non-flashing. This allows it to be used as a safety light on bicycles or as a standard portable flash light. It uses a total of 8 Cree 5mm LEDs which provide the highest luminous intensity for their class (at around 39000mcd max).

The whole design is based around a DC‐DC step up converter. Since most of the standard white LEDs operate at 3.3Volts, it was necessary to choose a converter that could convert voltage from two AA cells (2.4 – 3volts) to 3.3 volts with sufficient current to drive 8 such LEDs (30mA x 8). MAX756 meets these specifications perfectly and even has an inbuilt battery low indicator terminal.

The flashing control signals are generated by a low power CMOS version of the popular 555 timer IC configured in an astable mode with 66% duty cycle at 4.8 Hz. LMC555 can operate to voltage down to 1.5 volts, well below the available voltage. The output of this IC is connected to the Shut Down terminal of MAX756, thus enabling/disabling it at the set frequency. For non‐flashing mode LMC555 is shut off and the MAX756’s shut down terminal connected to Vcc via a DPDT switch.