I have a little keyfob style remote that I want to mod. It runs on a 3 volt button cell. I want to control the three switches with an ESP8266. The switches are connected to ground, and directly to a package without any identification on it. I figure it has a pull up resistor internally. The voltage on the input pin is 3.00 volt. The output of the microcontroller is 3.3 volt.
I'm not keen on soldering 3 extra transistors to the contraption, if I can help it.
If I were to connect a gpio pin directly to the input pin of the mystery package, will the backwards current break it?
If I were to power the entire board with 3.3 volt, would that allow me to forego the transistors? It's a shame I cannot identify the package, but would those extra .3 volts usually pose a problem to IC's? I reckon it makes a difference that this board has an rf circuit on it.
Best Answer
There's no reason to drive the GPIO pins high at all. Just configure them as open-drain — drive the pin low when you want to activate the corresponding "button", and tristate it otherwise.
As long as the fob voltage is not higher than the MCU voltage, it will be fine.
And running the fob at 3.3 V should be fine, too.