Looking for a simple time delay circuit? In the design of analog circuits, there are times when you would need to delay a pulse that came into a circuit before being used for the next process. This circuit uses a 555 timer to delay a pulse that comes in to a maximum time of 75 seconds. The timing of the delay can also be changed by changing the resistor value of VR1 and the capacitor value of E based on the time delay formula of t=0.69RC.
In order for the output to go high, the reset pin of 555 timer (pin 4) must be high and the TRIGGER pin (pin 2) voltage level must be below a third of the level of the power supply to the IC. When there is no pulse being applied to the input, transistor Q1 will turn ON and capacitor E is charged.
Once a pulse is applied to the input, transistor Q1 will turn OFF and pin 4 reset pin is held to high. This caused the capacitor E1 to be discharged through VR1 resistor. The time delay will depend on the discharged of capacitor E to a third of the supply before the output of 555 goes high. Experiment with different values of VR1 and E to get different time delay.
If the maximum value of potentiometer is set to 5M ohm, the time delay of the pulse will be 75 seconds.
This is a good project to embark on if you are a beginners to electronic design. There are only 5 components needed to construct this circuit. It will provide you a good hands-on experience of using the 555 integrated circuit, one of the most commonly available IC widely in use for the past decades.