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.
Proximity Switch Bill of Materials
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.
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.
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
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)