Timer circuit Explanation. Auto ON / OFF.

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These two circuits demonstrate time delay control of an LED using BC547 transistors. The first circuit provides a time delay ON function — the LED lights up after a short delay when power is applied. The second circuit provides a time delay OFF function — the LED turns ON immediately but switches OFF automatically after a preset delay. In both cases, the delay duration can be adjusted by changing the resistor and capacitor values.


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1. First Circuit (Time Delay ON)
This is a time delay ON circuit. When you apply +5V to Vcc, the LED does not light up immediately. Instead, the capacitor (connected near the base of the transistor) charges slowly through the 97K resistor.
Once the capacitor voltage reaches the base-emitter threshold (~0.7V), the BC547 transistor turns ON and the LED lights up after a delay.

2 . Second Circuit (Time Delay OFF Circuit Description)

This circuit is designed to automatically turn OFF an LED after a fixed delay when the power supply is applied. The circuit utilizes two BC547 NPN transistors in a Darlington-like configuration to control the LED operation based on the charging of a capacitor.

Working Principle:

When the circuit is powered ON (Vcc = +12V), the capacitor is initially discharged.

In this state, both transistors are in the OFF condition, so current flows through the LED, causing it to glow immediately.

The capacitor starts charging gradually through the 1MΩ resistor. As the voltage across the capacitor increases, it eventually reaches the base-emitter threshold voltage (~0.7V) of the first transistor.

Once the first transistor turns ON, it drives the second transistor ON as well. Together, both transistors create a low-resistance path from the collector to ground.

As a result, the LED current is bypassed to ground and the LED turns OFF automatically.

The LED remains OFF as long as the capacitor stays charged and the transistors stay ON.


Applications:

LED indication with auto-off feature

Delay-off indicator circuits

Simple timing-based electronic projects


How to Adjust the Delay:

Increase capacitor value (C) → longer LED ON time before it turns OFF

Increase 1MΩ resistor → slower charging → longer delay



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