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Constuction of a Simple Touch Sensitive Switch Circuit Explained

written by: Swagatam • edited by: Lamar Stonecypher • updated: 11/10/2010

With a mere touch of your finger you can operate any electrical load you wish to. Learn how simple it is to construct a touch sensitive switch.

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    What Is a Touch Sensitive Switch?

    In simple words it is a kind of switch which can be operated or toggled just by a touch of the finger. Before moving into the construction details of a touch sensitive switch, let us try to understand how its superior than its mechanical counterpart through the following couple of points:

    • Since there are no moving parts inside a touch sensitive switch circuit, its permanent. Contrary to this the moving parts inside a mechanical switch are prone to degradation and thus are not long lasting.

    • In units like CVTs, mixer grinders, UPSs, the addition of a touch sensitive switch may give it a sleeker look than the contemporary protruding types of mechanical switches.

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    Construction Details

    typical touch sensitive switch The construction of this touch sensitive switch circuit can be easily carried out with the help of the circuit diagram through the following simple steps:

    • Begin by soldering the IC at the middle of a rectangle 3" * 3" general purpose board.

    • As per the circuit diagram given below, interconnect the associated parts to the IC and each other. This can be simply done by and joining the component leads itself.

    • After the circuit assembly is complete, it can be tested by connecting to an external regulated power supply (for example to an Universal Power Supply Unit).

    • Now if all the connections are OK, a repetitive touch of the finger on the touch pad should alternately make and break the relay.

    • This whole assembly is now ready and may be added to any of your existing home appliance after certain external modifications as per your choice.
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    Circuit Description

    Simple Touch Sensitive Switch, Circuit Diagram, Image A single quad 2-input NAND gate IC 4093 along with a few handful of other components is all that is needed to construct the present circuit. Here the CMOS IC 4011 solely performs the entire function of an outstanding touch sensitive switch.

    With the help of the circuit schematic let us try to understand the functioning of the circuit through the following easy to understand steps:

    • The transistor at begining of the circuit is used to amplify the touch signals at its base. A darlington paired transistor is chosen to increase the current gain and to make the circuit respond accurately to the repetitive finger touches.

    • The amplified signal is rectified by diode D5 and a logic hi is created at the input of the first gate N1. Its output immediately changes state, so does N2 and the circuit latches via resistor R5. This part of the circuit forms the heart of the touch sensitive switch circuit.

    • Capacitor C5 has been included, so that the circuit does not behave erratically during prolonged touches.

    • In the latched position a logic hi is obtained at the output of N2. This is used to switch ON a relay through the transistor T1.

    • To switch OFF the relay it becomes necessary to break the latch of N1-N2, so that a negative pulse can appear at the base of T1.

    • This is done by a third gate N3. The latched N1-N2 slowly charges a capacitor C4. When fully charged one input of N3 becomes logic hi. Its other input is held at logic low by the resistor R2. Thus the output of N3 at this point of time is logic hi but does not effect N1-N2 because of D2.

    • The moment a touch is made at the base of BC 517, N3 gets a logic hi at the input which is connected to R7 and at this instant both of its inputs are at logic hi. Its output immediately goes low and breaks the latch of N1-N2.

    • The relay is switched OFF and the touch sensitive switch reaches back to its initial position. It sustains this position till the next touch is made at the input.

    image credit: http://www.globalspec.com/FeaturedProducts/Detail/Barantec/Standard_point_illuminated_Touch_sensitive_switch/20205/0?deframe=1