555 Timer Driving LED

For a monostable circuit, we join the threshold and discharge pins for the timing circuit, and apply the trigger pulse through pin 2. Therefore, this circuit will light the LED after a delay.

1. Section A is responsible for the proper triggering of the circuit. It consists of a 4.7 kΩ pull-up resistor, and a push to make switch. Pressing the switch generates a negative going spike into pin 2 forming the basis of a trigger.
2. In section B we see pins 6 and 7 joined, and there is a resistor and capacitor also joining to that common point forming the basis of the timing circuit. It is usually a monostable circuit when pins join in this way. The value of the capacitor and resistor determine the time delay.
3. Section C is the 555 timer IC.
4. Section D is responsible for the output. When the output is high, the voltage there is close to Vcc, which is 9 V. A high output would therefore light the LED. The 680 Ω resistor provides a voltage drop for the LED.

To calculate an approximate value of the time delay we use the following formula and calculation.

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