touch on touch off switch 01 PUBLIC

A basic touch on touch off switch with a single electrode capacitive sensor input.

See thread:

https://www.circuitlab.com/forums/power-electronics/topic/z3c43kph/single-touch-sensor-to-turn-on-and-off-12v-out/

A basic touch on touch off switch with a single electrode capacitive sensor input.

See thread:

https://www.circuitlab.com/forums/power-electronics/topic/z3c43kph/single-touch-sensor-to-turn-on-and-off-12v-out/

There input signal is a crude representation of mains hum induced in the human body and that signal being connected to and disconnected from the capacitive sensor input via a finger.

The first stage is an AC coupling capacitor forming an approximately 10Hz high pass filter with R1//R6 (where // means in parallel with) and two diode to protect the following opamp input from signals beyond the rails.

This signal is amplified by a non-inverting high gain low bandwidth stage which has an approximately 10Hz high pass filter and a low pass filter defined by the gain bandwidth of the opamp. The midband gain of the stage is approximately 10k (R3/R2+1). The minimum gain at very low frequency is +1.

The opamp is biased to a zero signal point of mid supply.

The opamp output is fed into a peak detector but with a small amount of low pass filtering to improve immunity to noise spikes and so reduce false triggering of the switch. The peak detector output is also biased to a zero signal point of mid supply.

The filtered peak detector output is fed into a non-inverting Schmitt Trigger input to speed up the edges and to reduce false triggering.

Note that the threshold levels are offset from mid rail by R8 and R9 so the hysteresis levels are also offset compared to a simple split supply circuit with a single resistor to ground.

See:

http://en.wikipedia.org/wiki/Schmitt_trigger

for more information.

The Schmitt Trigger output is then used to clock a D-type flip-flop so that successive rising edges alternate the output state of the D-type.

The D-type output is then level shifted by a low power NMOSFET and a higher power PMOSFET to switch the 12V supply into a load notionally representing the LED power supply.

No attempt has been made to model this LED supply.

Note that power supply decoupling is omitted from the circuit.

This is a fully simulated circuit:

Simulate > Time Domain > Run Time-Domain Simulation

The notes in the schematic are essential reading.