When the plate A is touched (plate b is grounded), the LED will light up to 3.3 seconds then it turns off.
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| Touch indicator schematic |
R1 - 30 kilo-ohms 1/4W
R2 - 5 kilo-ohms 1/4W
R3 - 220 ohms 1/4W
R4 - 100 kilo-ohms potentiometer
C1 - 100uF electrolytic capacitor rated 10V or higher
C2 - 100nF Mylar, ceramic or any type of capacitor
Q1 - 2N3906, 2N4403, or similar PNP transistor
LED - red, yellow or any 3mm to 5mm LED
Touch plate -any conductor, a piece of wire or all plate
555 timer IC
DC - 6V to 9V battery
Touch Alarm
The circuit above can be converted into an alarm circuit by just replacing or adding buzzer and doing some modifications. See circuit below. At the moment plate A is touched (grounded plate b), the buzzer will sound for 3.3 sec.
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| alarm circuit |
R1 - 30 kilo-ohms 1/4W
R2 - 5 kilo-ohms 1/4W
R3 - 49 kilo-ohms 1/4W (buzzer sound control)
R4 - 100 kilo-ohms potentiometer
C1 - 100uF electrolytic capacitor rated 10V or higher
C2 - 100nF mylar, ceramic or any ype of capacitor
Q1 - 2N3906, 2N4403, or similar PNP transistor
Q2 - 2N2222, 9013, or similar NPN transistor
Buzzer - 6V to 12V buzzer with min 20mA current rating
Touch plate -any conductor, a piece of wire or all plate
555 timer IC
DC - 6V to 9V battery
Diagrams of part list: Click to enlarge.
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| 555 pins |
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| 2N222 and 9013 pins |
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| LED pins |
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| Buzzer |
The heart of the circuit is the 555 timer IC operated in monostable mode. When the voltage across pin 2 is less than 1/3 of supply voltage, the timer will output a pulse in pin 3 with the duration equal to 1.1*R1*C1. Meaning you can increase or decrease the 3.3 sec time duration by just replacing the values of R1 and C1. You can also use the 555 timer calculator to test and check the time constant or duration. The use of R4 is for adjusting the sensitivity of the circuit. Increasing the value increases the sensitivity.
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