1. Safety Lights
Download .CKT file
This is the circuit from Discovering Electronics with crocodile clips ©, developed as the basis for cyclist and pedestrian flashing safety lights.
The three LEDs flash at around 10 Hz. The flashing rate is controlled by the 220k and 470k resistors, together with the 220 nF capacitor. Usually, the output voltage of a 555 astable is HIGH for at least as long as it is LOW, so that the LEDs would be ON for at least half of the time. The addition of the diode makes it possible to reduce the ON time, helping to conserve the battery, without reducing visibility. The capacitor fills up only through the 220k, and empties only through the 470k.
The 22 µF capacitor is a decoupling capacitor which helps to prevent the transfer of signals along the power supply connections. If you use a 555, you need one!
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