Delay circuit using Transistor

Delay before turn on circuit based on Transistor.   

In this article, we will show how to build a delay before turn on circuit with..
(1) - Transistor.

(2) - Capacator.

(3) - Resistor.

(4) - Zenerdiode.

A delay before turn on circuit is a circuit that once you apply power to it doesn't turn on the output right away. There is a delay before the output turns on. For this circuit, it's a few seconds delay. Once these few seconds pass, then the output turns on.

A delay circuit can be useful for any circuit that needs a delay before output turn on.

How we achieve the delay effect is through the use of a capacitor. Basically the capacitor takes time to charge up. This time is what creates the delay.To adjust delay time adjust the value of R1,C1.

COMPONENTS NEEDED

• BC548 transistor (npn)
• BC558 transistor (pnp)
• 47μF capacitor
• Resistor (10k,3.3k,22k,32k)
• Zener Diode (6.2v)
• Small potentiometer(10k)

HOW THE CIRCUIT WORKS

This circuit works is through the RC network(3.3k+pot,47uF). The combination of the resistor and capacitor forms the RC network. This network determines the length of time it takes to charge the capacitor.

There is a delay of about 3-4 seconds with this circuit. once you turn the power on, the LED doesn't turn on until about after 3-4 seconds.

The RC network is what determines this. The bigger the resistor and capacitor value, the longer the delay. So if you want to increase the time of the delay, you would choose a bigger resistor and capacitor. If you want to shorten the time of the delay, you would choose a smaller resistor and capacitor value.

When the power is on the capacitor 47uF start to change through 3.3k+pot. when the capacitor voltage reach 6.6v or above the transistor BC558 is on by BC548.

When power is off, the capacitor start to discharge through (pot+1k).

One Drawback of this circuit is discharging of capacitor (47uF)little slow. This circuit doesn't work properly on rapid On-off situation.

For this reason I create a different circuit which I will post on my next blog. If you interested please visit my next blog.

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