How to physically stimulate a touch screen with an external device

Question

I need to develop a small autonomous device that could stimulate a touch screen. For the record I need such a device to be able to run some automatic tests on some specific smartphone apps, where touch related functionalities cannot be tested by injecting touch events from USB.

Here is my first design idea: a first prototype would consist of a grid of copper pad on a PCB, like on the following picture.

Each copper pad could be connected with a transistor to a metallic mass that would simulate the fingertip capacitance. Obviously each transistor would be commanded by the MCU comprised in the device.

^{simulate this circuit – Schematic created using CircuitLab}

However I don't have any idea if this design would work. What I can't figure out is how I should connect my battery to the metal part and/or the copper pad, and how big should my metal body be to yield enough capacitance to the device to be detected by the touch screen.

Thank you for any insight you stackexhangers could provide!

Answer 1

I'd suggest that you will have too much capacitance in the FET to give reliable results.

When the FET (and I'd use P-Chan in your circuit) is off there is typically hundreds of pF of output capacitance.

I'd use a Reed Relay where the open circuit capacitance is extremely low, just a couple of a pF maximum. This gives you the maximum possible change in capacitance to be recognized by your touch matrix. I'd also use round capacitor targets rather than square.

^{simulate this circuit – Schematic created using CircuitLab}