CIRCUIT #25: The Timer

This circuit is also a multivibrator, but it is a special kind called a one-shot multivibrator. When you finish wiring you’ll see why it is called that. Press the KEY and release it immediately. The LED will light and stay on for a few seconds and then go off. It will stay on for the same amount of time every time you press the KEY, even if you hold the KEY down longer sometimes. The time the LED stays or is controlled by the 100 .F capacitor, so you could change the time by changing the capacitor — or the resistor that controls its discharge (the 100K ohm). The name “one-shot” comes from the fact that the LED only comes on once for each time the input is connected by pressing the KEY.