This LED flasher is made up of just five components and is an ideal circuit for novices to experiment with. Operation of the circuit is quite easy to understand. With a battery connected to the circuit capacitor C1 is charged through the 1 MΩ resistor R1. The capacitor is connected to the emitter of the PNP transistor (BC557). This transistor’s base junction is connected to the positive 9 V supply via an LED. So the potential at the base junction will be equal to the supply voltage minus the forward voltage drop of the LED. A red LED will give a forward voltage drop of around 1.6 V so the voltage level on this transistor’s base junction will be 9 V – 1.6 V = 7.4 V.
When the rising voltage on the capacitor reaches a level to forward bias the base-emitter junction of the PNP transistor current will start to flow through its emitter-collector junction. The current flow causes the base-emitter junction of the NPN transistor to become forward biased, switching it on. Now it’s conducting its collector will be close to ground potential, pulling both the LED cathode and the PNP transistors base to ground, reinforcing the ON condition of the PNP transistor and causing a relatively high current to pass through the LED to emit a flash.
When the capacitor is discharged the transistors turn off again and the process is repeated. The values given in the circuit diagram (C1 = 1 µF, R1 = 1 MΩ) will make the LED flash briefly once every two seconds. The circuit will still run with a battery voltage as low as 2 V and uses so little current that a new 9 V battery will keep the circuit flashing for many months in continuous operation.
Even old 9 V batteries with too little charge left to power other applications could be used to power the circuit. The second circuit diagram shows that the basic circuit can be simply modified to make metronome or tone generator. A low-power 8 Ω loudspeaker is now connected in series with the LED. The sound produced by the loudspeaker will either be a repetitive click or a tone depending on the values of capacitor C1 and resistor R1.
Reducing the values of R1 and C1 will make the circuit oscillate more quickly. The second circuit uses values of 22 kΩ for R1 and 100 nF for C1.