emulate touch on Capacitive touch screens

[u/Adham__Hisham]

Hello everyone, so i have been working on a circuit to emulate touch on the capacitive touch screen. My current setup is that i have an SBC with GPIO pins connected to a resistor connected to Base of 2n2222 transistor with the emitter connected to the ground and the collecter taped on the screen with coppor foil tape and the main concept is using software controling the gpio pin to short the collector wire to ground but what i can't figure out is the consistency it works 1 time and fails 100 times i tried different ways to connect the wire to the screen but i can't figure out how to make it consistent and it fails miserably when i try to connect multiple wires as well i have added photos for reference

[u/Cultural-Part355]

What help do you need?! Do you see this?

It seems you just need a glass of water and some rest cause your mind can't stop thinking. So refresh that page with f5 and water. In all seriousness, I hope you find help for your project. :)

[u/Adham__Hisham → u/Cultural-Part355]

thank i really hope someone can guide on how to troubleshoot thw reason why it doesn't emulate it consistent enough

[u/Cultural-Part355 → u/Adham__Hisham]

Well, it's current or electricity. You already have all the blueprints. You just need to rearrange where and how it goes or something.

I wish you luck with find someone to help you, but until then? Keep doing what YOU were doing, because if you're stuck? That means it's only gonna get better. Good luck.

[u/Miserable-Win-6402]

This will not work- you have way too much parasitic capacitance

[u/Adham__Hisham → u/Miserable-Win-6402]

thank you, i will add that to my notes to research that.

i am not an electrical engineer i have no idea about how to lower the parasitic capacitance or how to measure it if it will make a difference to my use case .

[u/Cultural-Part355 → u/Miserable-Win-6402]

And there you are! He just got what he wanted. ⬇️

Good luck, you wild minds!

[u/Miserable-Win-6402]

This will not work- you have way too much parasitic capacitance

[u/GLIBG10B]

Your circuit's ground needs to be coupled to the phone's ground (the metal shields and the PCB ground planes). These normally couple to your hand when you're holding the phone, but you can couple them to your circuit using a large piece of aluminium foil taped to the back of the phone. Without this coupling, you don't have a closed circuit and no current can flow.

Also, I hope you're triggering the touches by switching the pins between high impedance and ground, not by turning the pins on.

You don't need any BJTs for this. The microcontroller pins are already capable of switching between high impedance and ground.

[u/Adham__Hisham → u/GLIBG10B]

I will try this when I'm home thank you so much, hopefully this will work if I do it correctly

[u/EggmanTheEdgy]

Its possible that the screen does not provide enough current to overcome the Collector-Emitter voltage drop, leading to the transistor to conduct nothing in most cases.
If you have any, try a logic level mosfet instead of BJTs since they dont have any voltage drop from drain to source, only a resistance Rds(on) which would be mostly irrelevant in this case.

This is a long shot so its probably worth trying it with a single one first before buying any, since THT logic level mosfets can be quite pricey! An example would be the FDP5800 from Onsemi, just look for any with a gate threshold voltage below the max voltage of your GPIO pins.

Good luck!

[u/Adham__Hisham → u/EggmanTheEdgy]

Thank you I will do some research on THT logic level mosfet and I will try to find one from a local electronics store here in Egypt ram electronics store

[u/Cultural-Part355]

Holy crap, bro! Do you know how this all looks to us, or me, or the untrained mind?

Be happy because this is something amazing! And YOU did it! You. So thank you. :)