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Non-Contact Human Interface Capacitive Proximity Switch

Non-Contact Human Interface Capacitive Switch

Simple, highly sensitive capacitive ON/OFF switch pads change the state of a latch and turn on an LED without requiring actual physical human contact. The pad may be insulated. A range of 12mm is easily obtained and sensitivity is adjustable.
Power is supplied via a common 9V battery.

Non-Contact Proximity Switch Circuit Schematic

Non-Contact Human Interface Capacitive Proximity Switch

Proximity Switch Bill of Materials

Non-Contact Switch BOM

Bill of Materials File

Proximity Non-Contact Switch – How it works?

Central is the 74HC02 CMOS NOR Gate IC. The gate input capacitance is typically only 3pF. To enable a sufficient voltage to change states reliably a 100M bias resistor is recommended. I had to check to see if such a resistor is easily available — yes, it is available from DigiKey at a cost is only $0.52. Or you can fabricate a resistor via series 10 or 22M resistors — that is what I did to obtain 50M. The ‘low sides’ of these resistors are tied to a bias potentiometer so that the static gate input voltage may be set very close to the threshold. Sensitivity is also increased by reducing Vcc to 2.7V (minimum specified operating voltage is 2.0V). I was able to obtain a range of about 16mm using a 50M resistor and paperclip ‘non-contact pad.’

I also experimented with the CD4011 NOR gate—it worked reasonably well, but it has higher gate input capacitance (5pF) and minimum operating voltage is also higher at 3.0V.

A 2N7000 N-Channel MOSFET (Q1) drives a 20mA ultrabright white LED. Note that the gate output voltage may be insufficient to fully turn on the transistor because the maximum Vgs threshold voltage is 3V. Fortunately, most run close to the typical 2.1V. In case one does not work, try another device.

Quiescent battery drain is only 56uA.

The 1M input resistors protect the IC against static discharge and actually allow touching of the pad without danger.

Numerous applications

The pads may also be insulated or placed on the back side of a thin board such as a canvas painting etc—this is the initial application. Another possibility would be an anti-theft deterrent/alarm that would be easily turned on, but not easily turned off. A sensitive coil 6V relay with back diode may also be driven by Q1 — DigiKey PB1095-ND would be a good choice.

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.

Proximity Switch Photos

non contact proximity switch photo

Note the paper-clip pads on the top of the photo—the pads are non-critical, but the larger, the more sensitivity.

For the future

Single pushbutton non-contact switch circuit (‘push’ On /Off)

6 Comments

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  • Abu yousaf

    thanks for reply
    As u told me about the 100v voltage and 3pf capacitor but still my Multisim circuit not working.
    Is it possible that i can send you my sreenshot or Multisim circuit and you have a look please

    • Jim Keith

      I do not have Mulitsim, but have used it in the past. I have found by experience that it is easier to get actual circuits working than to get Multisim working –not altogether friendly because it has numerous hangups –just keep struggling, making no assumptions.

      One suggestion is that you attempt to get it working with DC input voltages –then progress to capacitively coupled high voltage AC.

  • Abu yousaf

    hello
    I want to built a water detector by using same plates in water but before building, i have simulate this circuit on Mutisim software but this does not work. Can u help me please
    thanks

    • Jim Keith

      I doubt that the sensitivity adjustment will work on Multisim because the device is being operated outside of its specifications.

      However, try this: Connect high voltage generators (e.g. 100VAC relative to common) to the inputs via very low values of capacitance (e.g. 3pf) to see if it will change states when the voltage is applied. This is a means of simulating body capacitance to stray electric fields.

  • J. Muffalo

    I think this may be my beginner’s project. Seems relatively uncomplicated yet there is lots of potential for experimentation and expanding learning opportunities.

    • Jim Keith

      To prevent static discharge from destroying the input resistors and IC, it may be necessary to cover the pads with a high dielectric strength plastic film. Static discharge is more of an issue in arid climates and in the winter when heating is required. Small MOV transient suppressors may also help, but tend to add sensitivity deadening input capacitance.

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