You can connect any AC/DC electric appliance with the relay to operate it with this circuit such as lamp, fan, LED lights, etc. The relay’s voltage should be the same as the input voltage. By increasing its value you can increase the preset time period, 1uf gives a time delay of approximately 30 seconds. The preset time period for the activation of LED or the relay depends on the value of the 1uF capacitor. LM741 op-amp offers many features which make its application nearly foolproof: overload protection on the input and output, no latch-up when the. The main function of this IC is to do mathematical operations in various circuits. A 1N4007 diode is used across the relay to save the circuit from the back EMF of the relay. LM741 IC is a DC coupled, high gain, low power voltage amplifier. Now the amplified output is sent to the relay.

Whereas in figure #2, the output of this IC is not enough to drive a relay directly therefore a 2N3904 transistor is used. The working of this circuit is simple on pressing the push button S1 it sends an input signal to the operational amplifier IC LM741, and it sends the signal to the LED. the difference between both the circuits is of the output, in figure 1 the LED will glow for a specific time period on pressing the push button S1 and in figure 2 the relay will be activated by when push button S1 is pressed. The operating voltage of both the circuits is 9 to 12 volts DC.

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