It is an interesting application of IC 555. The trigger pin-2 of the IC is sensitive to human touch. This property is used in the circuit.
In this circuit when supply is connected, the output of the IC remains LOW and the relay remains deactivated. The timing capacitor C1 remains discharged into pin-7 of the IC, ready to receive the trigger pulse. Remember that, this condition of the circuit is also known as STABLE STATE of the circuit.
But when the touch plate TP is momentarily touched with your naked finger, (without any gloves, etc.), the internal trigger comparator triggers the internal flip-flop of the IC and now the output of the IC becomes HIGH. Due to this, the relay is ACTIVATED.
In this condition, the capacitor C1 starts charging through upper resistor R1 (it is timing resistor R1, 500k – not drawn due to mistake while drawing the circuit). The voltage across the capacitor starts building up. When it becomes slightly greater than 2/3 of supply voltage, the internal threshold comparator SETs the internal flip flop of the IC and now its output becomes LOW.
Now the relay is DEACTIVATED and the circuit remains silent till next time you touch the TP. The state of the circuit during which the capacitor charges and the relay activates is called as QUASI-STABLE STATE.
Important points to note
- DUS means DIODE UNIVERSAL SILICON. It means that you can use any type of silicon diode at the place shown as DUS.
- The length of the wire connected between pin-2 and the touch plate TP should be as small as possible. Practically I tested the circuit with wire length of just 3 inches.
- The coil resistance of the relay should be greater than or equal to 100Ω since the output current of the IC should not exceed 200mA, if you use upto 18V supply voltage for the circuit.
- You can use the relay to control a small load running on mains supply through its N/O contact.
- You can also adjust the timing cycle of the circuit by changing the values of R1 and C1 depending on your requirement.
- The capacitor C2 is used to absorb the noise that pin-5 may pickup during normal working of the circuit.
If you use a pull-up resistor of 10k between pin-2 and positive supply line of the circuit, then you can use the circuit with same application without having the problem of short length of the wire connected to TP.
In this modification, you will have to use a small push-to-on type trigger switch between pin-2 and ground line of the circuit. So whenever, it is pressed, same action will be done, as you touch the TP.
- You can use the circuit for controlling any load with the help of just a touch.
- It is also useful as fencing alarm in an architectural model of a bungalow or a building, such that if anyone touches the metal fencing wire of the model bungalow, the the alarm will sound.
- Useful in other applications of touch alarms, if an external alarm circuit is also used at the output of the IC.
- The circuit is very much suitable for demonstrating experiment on IC 555 working and showing its property of converting sine wave into its proportional square wave.