Touch Doorbell, Ready Kit, 100% Tested


Touch Doorbell circuit is presented here is based on the readily available touch sensor, comparator IC LM393 and relay. The touch  sensor is used as the touch detector while IC LM393 used as comparator. In the absence of touch, no touch detected on the touch sensor. As a result touch sensor stop conducting and buzzer stays off. However, when touch sense on the touch sensor, touch sensor starts conducting and buzzer sounds.

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Complete working video can be seen from

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Application of the circuit:

Most of the touch sensor based circuits available in market use one or another kind of touch sensor which increases the cost and also require complicated circuitry around it. But this touch sensor based circuit uses readily available touch sensor as touch detector. The functioning of this Touch Doorbell is to detect touch and automatically activate the buzzer in presence of touch. Buzzer sounds continuously indicating the presence of touch till touch senses on touch sensor.

Why this particular circuit:

Touch sensor used here as a touch detector. The variation in the sense of touch wanted can be easily done by touching touch sensor using finger. The circuit operates normally over a DC voltage of 5V which can be obtained by DC supply cable.

Explained working of circuit:

The basic idea behind the circuit operation is to turn ON buzzer in presence of touch. The touch sensor  is needed for this purpose. Touch sensor is device that converts touch sensed to electric energy. Here, Touch Doorbell works as touch detector while touch sensor sounds buzzer. IC LM393 is used as comparator. Touch sensor output is connected as input to comparator.  Pin no. 4 of LM393 is pulled towards ground. Relay is used as a switch. When relay is activated, normally closed contact becomes open and normally open contact becomes closed. In normal condition, touch not detected on  touch sensor and buzzer stays off. Now consider second scenario when touch detected, the buzzer sounds in the presence of touch.

How to build:

First of all read the given manual thoroughly and study the circuit given in the figure. Also have look at PCB and components supplied along with the kit. Each component has to be soldered in its position on PCB.

Identification of resistors is done by color coding. The color band on each resistor corresponds to its exact value.

There are different methods in which values are defined on capacitors. But usually values are specified numerically on them.

Can you make out the whole working of circuit and are you able to identify each component separately as to where each of them has to be placed?

If yes only then proceed further to actually mounting and soldering the parts.

Not ICs but their sockets are to be soldered on PCB. This is to make mounting and dismounting of ICs easy while troubleshooting.

Start from left most corner of PCB and solder the components one by one on their correct position on PCB.

Before soldering any component see that you have placed it at its right position and with correct polarity. Give due attention to diodes and electrolytic capacitors as they are polarity dependent.

Do the soldering of other components in the same way while keeping in mind that components with long and sensitive leads like capacitors and transistors are soldered last.

Testing the kit:

After assembling and soldering of components on PCB, connect external DC power source of 5V.

For testing purpose manually hold finger over touch sensor. Buzzer sounds in this case.

Now set the desired intensity at which you want detection of light. This can be done by changing the resistive value of R2 variable resistor.

Install the unit at ceiling of room.

ICs used here:

The only IC used here is LM393

It is dual differential comparator IC.

This 8 pin IC is popular and commonly used IC

Part List


U1               – LM393

Base            – 8 pin

Resistors(all ¼ watt +/-5% carbon unless stated otherwise)

R2               – 10k Ohm(variable)

R3               – 1k Ohm

R4               – 1k Ohm

R5               – 10k Ohm

R6               – 10k Ohm

Capacitor C1               – 1uF


D4:              – Touch sensor

D1:              – Red LED

D2:              – Green LED

5 V USB Cable