Touch Switch And Delay Circuit Using 555 Timer IC

Touch Switch And Delay Circuit Description

In this simple touch switch circuit, the 555 timer is configured as a one shot multivibrator that is triggered by touching the touch terminal.

In this monostable mode, the timer generates a fixed pulse of about 4 seconds whenever the trigger voltage falls below Vcc/3. When the trigger pulse voltage applied to the #2 pin falls below Vcc/3 while the timer output is low, the timer's internal flip-flop turns the discharging Tr. off and causes the timer output to become high by charging the external capacitor C2 and setting the flip-flop output at the same time.

The voltage across the external capacitor C2, VC2 increases exponentially with the time constant t=R3*C2 and reaches 2Vcc/3 at td=1.1R3*C2. Hence, capacitor C2 is charged through resistor R3. The greater the time constant, the longer it takes for the VC2 to reach 2Vcc/3. In other words, the time constant R3*C2 controls the output pulse width. When the applied voltage to the capacitor C2 reaches 2Vcc/3, the comparator on the trigger terminal resets the flip-flop, turning the discharging Tr. on. At this time, C2 begins to discharge and the timer output converts to low. In this way, the timer operating in the monostable repeats the above process.

To increase the time of the delay from 4 seconds, the value of R3 and C2 need to be increased. Similarly, to decrease the output time delay, the value of R3 and C2 need to be reduced.

The output from the 555 timer can be used to drive a transistor which in turn can drive a delay to control device like lamp, and other equipments.

Parts List

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