The figure below shows two very simple projects of 741 timer circuits. The heart of the circuit is a LM741 IC. LM741 is a very famous op amp IC built to easily use in many electronic circuits. In this circuit the IC is working as a comparator.
The circuit shown in the figure 1 will illuminate an LED for a preset time period after the push button S1 is pressed and the circuit shown in figure 2 will switch ON a relay for a preset time period when the push button S1 is pressed.
period when the push button S1 is pressed. The output of the IC is not enough to drive a relay directly therefore we have used a 2N3904 transistor. The 1N4007 diode across the relay is used to save the circuit from the back EMF of the relay.
Increasing the value of the 1uF capacitor can increase the preset time period. During the testing of the circuit the 1uF capacitor gave a time delay of around 30 seconds. By changing the value to 1000uF, 2200uF or 4700uF will increase the time period to several hours. The operating voltage of the circuit is 9V to 12V DC.
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Figure 1.
Figure 2.