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Touchless ON/OFF Human Interface Capacitive Switch

This sensitive ON/OFF human interface, non-touch, proximity switch is a fun experiment and may just have some useful applications. A 74HC02 NOR gate acts as a high impedance capacitive sensor. It sets /resets a latch circuit and drives an LED & relay for high power control. Simply placing a finger near a ‘touch pad’ is sufficient to toggle the latch –no human contact is required so the pad may be insulated. A range of 15mm is easily obtained and sensitivity is adjustable.
Power is supplied via a 12VDC source.

Schematic of the Capacitive Switch Circuit

This is an enhancement of an earlier circuit that required two contact pads: Non-Contact Human Interface Capacitive Switch

Touchless ON OFF Human Interface Capacitive Switch Update

How it works

The gate input capacitance is typically only 3pF. To make it highly sensitive to body capacitance a 100M bias resistor is recommended. The 100M resistor is available from DigiKey at a cost is only $0.52, Or you can do what I did –fabricate a resistor via series 10 or 22M resistors –that is what I did to obtain 62M. The ‘low side’ of the resistor is tied to a bias potentiometer so that the static gate input voltage may be set very close to the digital threshold (about Vcc /2). Sensitivity is also increased by reducing Vcc to 3V (minimum specified operating voltage is 2.0V). I was able to obtain a range of about 15mm using a 62M resistor. The pad in this case is a penny soldered to a 1M resistor. A range of 25mm may be demonstrated by using a hand instead of a finger.

After the latch is set or reset, capacitor C2 charges or discharges via R7. When it reaches the threshold (Vcc /2), gates U2A & B steer the input signal to the opposite latch input.

A 2N7000 N-Channel MOSFET (Q1) drives a 20mA ultrabright white LED and a 12V relay.

The 1M input resistor protects the IC against static discharge and actually allows touching of the pad without danger of ESD damage.

Circuit evolution

This circuit evolved somewhat as I got it working –the biggest problem was locating the gate U2C inside U1. What happened was that U2C oscillated wildly as it crossed the threshold thus creating such an electrical disturbance that the circuit malfunctioned. Another issue was that one of the gate inputs was damaged (probably ESD) so it had excessive leakage that prevented high impedance operation. Still another issue was that when the gate voltage is biased close to the gate transition voltage, the IC goes linear and starts conducting appreciable power supply current –this current precluded the high impedance power supply arrangement that I employed in the two contact version of this circuit. As a result, I put in an LM317LZ to provide a low impedance 3V power source. Then I increase the input voltage to 12V so I could add a 12V relay.

Numerous applications

The pads may also be insulated or placed on the back side of a thin board such as a canvas painting etc.

Limitations

The closer the sensitivity pot is set to the threshold, the more probable that an electrostatic or electromagnetic disturbance will accidentally cause it to change states. As a result, do not set for highest performance. Certainly, do not apply this circuit in critical applications.

Capacitive Switch Photos

Video presentation

For the future

Additional logic to prevent the latch from free-running if the finger is not immediately moved away after setting /resetting the latch.

4 Comments

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  • fgyimah11gmail-com

    Hey Jim I want to do this for my engineering project.. What is the circuit on the top

  • Jim Keith

    Hi Frank, you are correct –there is an error in connecting the LM317LZ –R4 is incorrectly placed –will update the schematic shortly.

    Note that it takes a good 74HC02 –in other words, zero leakage on the input pin –I had one that would not work well. The 100M (or whatever) resistor is to provide a DC bias to the input without adding much load. The body capacitance couples an AC power frequency signal to the input pin via the 1M resistor –this is the same signal that you see when you touch a scope probe. Good luck –it worked well for me.

    • Frank

      Thank you for the quick reply!

      I’m actually doing a project at school and I chose to try your circuit because it was impressive. That said, I do mot completly understand the concept of capacitance. I won’t ask you to explain everything but I’d really like to understand exactly what happens when your finger is near the penny. It generates a ac power frequency which is strong enough to pass through the 1M ohm resistor? And then, what does it do to the input?

      Thanks a ton!

  • Frank

    Hello,

    I would really appreciate little more explanation on how this circuit should function because it is not working for me. The capacitive effect with our body is used to ground the IC pin? What’s the use of a 100M resistor bias? Isn’t a resistor that big considered open circuit? Even the 1M seems too big for a 3V circuit (and the sensor).

    Thank you

    PS: I think the LM317 is badly connected on the schematic. The upper end of the 220 resistor should be connected to OUT (pin2) directly to adjust the voltage at 3V. Pin1 should be between the 2 resistors.

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