Circuit To Simulate Capacitive Touch Button Press - Raspberry Pi Zero

Question

I realise similar questions have been asked in the past about automating capacitive touches, but I'm aiming to put this into practise with a simple circuit.

I want to be able to simulate / automate the press of a capacitive touch button on a tumble dryer. I want to do this without opening up or modifying any of the existing circuitry.

I've tested placing some aluminium foil on the button and either touching it or running a wire from it to ground (metal sink) and both work in activating it. I want to take this to the next step now and build a circuit to automate this.

I have designed what I think is the circuit to achieve this, however my electronics experience is extremely rusty going back 30 years to high school physics and some recent Googling on transistors and capacitive touch.

Before I go out and buy the components, I'm looking for some input as to whether I'm overlooking anything.

Proposed circuit diagram:

In the diagram, L1 is just an indicator to show me the "press", R1 is to bring the current down to about 20ma for the LED (with ~1.7V across the LED and ~0.7V across the transistor), enough to active the transistor but to keep it below the collector voltage (potential).

I've read that the potential on a capacitive touch button is between 1.8V to 5V (so I'm estimating somewhere in the middle) and R2 is just to place some load between the button and ground so there's no short to ground.

My belief is that when the GP26 is activated at 3.3V, the LED will light up and the 0.7V on the transistor will then allow the flow from the collector to emitter (i.e. ground) which would result in the button sensing a change in capacitance and activating a "press".

Am I missing anything?

Answer 1

You can turn on and off a capacitive sensor load using a JFET as switch. The advantage compared to a MOSFET is, that there is no body diode remaining connected to the capacitor during the off state and the output capacitance is pretty low.

If BTN_PRESS is high (close to 3.3 V, blue track) J1 is conducting and so your touch foil C1 has an AC GND connection. The amplitude at FET_SW is very low in this phase.

The amplitude the sensor sees, is reduced and the phase is shifted, so a touch will be recognized.

If BTN_PRESS is low, J1 is turned off and this disconnects the foil (C1) from GND. FET_SW shows the coupled sensor signal amplitude, because this point has a high impedance now.

This, however, does not work properly if GND of the MCU circuit has no connection to the EUT GND.

The cutoff voltage of the JFET (e.g. MMBF5484) must be below the MCU supply voltage to turn it fully off.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

A NPN will work better than a mosfet for this because most FETs you can buy will have too much D-S and D-G capacitance even when off.

The NPN needs a pull-up (when off). Change your R2 to 1 MΩ; tie the top to V3v3_out and the junction of the R and collector to the capacitive button.

Now when the npn turns on (and saturates) it will appear to have a large capacitance at its collector.