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I have a video game controller that I want to control with an Arduino. The PCB has two pads for each button, and connecting those two pads "presses" the button. I have attached a picture of what the pads look like.enter image description here

Right now I am controlling the flow between them using a relay. But before using a relay, I connected a BJT (a 2N 2222A -1726) having the collector on one pad and the emitter on another pad. However, this did not work and I can not figure out why. I am relatively new to electronics so this might be a simple misunderstanding. The battery that is powering the PCB in the controller is 3.7V 1,300mAh 5Wh. So my questions are:

  1. Why doesn't the transistor work?
  2. Should I use a relay or a transistor for this purpose?

EDIT: I looked online for a schematic but I could not find any unfortunately, although maybe I am just not looking in the right spot. The controller is a Nintendo Switch Pro Controller.

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    \$\begingroup\$ There's a variety of reasons why a transistor may not work. A transistor is a switch, in a sense, but it's not an all-around replace-any-switch sort of switch. Knowing whether and how one could fit a transistor in there to actuate the switch depends on the design of the controller. If you can get your hands on a schematic of the controller and post it, we can help more. As to whether you should use a relay or transistor -- that's a judgement call. Relays will certainly do the job, and require less engineering work to do it. But there's certainly a chance that transistors will work. \$\endgroup\$
    – TimWescott
    May 8, 2021 at 22:14
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    \$\begingroup\$ You need to attempt to draw the circuit. You didn't say what you did with the transistor base. It needs a reference to the emitter. \$\endgroup\$
    – Mattman944
    May 8, 2021 at 22:48
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    \$\begingroup\$ You soldered the pins? Next time cut the pins off and solder to the bare wire. Its easier and less likely you'll torque it off. \$\endgroup\$
    – Passerby
    May 8, 2021 at 23:09
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    \$\begingroup\$ Salty, the very first thing I do when considering modifications like this (besides learning to solder just slightly better) is to figure out what each of those two conductors are tied towards. If you are lucky, one of them will be grounded... or else tied directly to a (+) power rail. If not, though, you may find that neither of them are tied to a voltage rail directly but perhaps each of them through some resistance or some other component. It's important that you do some trace-out, first, so that you know what you are working with. Then you can consider modifications. \$\endgroup\$
    – jonk
    May 9, 2021 at 0:09
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    \$\begingroup\$ When people asked for a schematic they meant a schematic showing what you did. What did you connect to the base of the transistor? What did you connect to the collector and what did you connect to the emitter? \$\endgroup\$
    – mkeith
    May 9, 2021 at 2:14

2 Answers 2

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It's hard to say without knowing more about the circuit, but here are some things you can try:

  • Check the voltage on each pad with a multimeter. The collector of the transistor should be connected to the pad with the higher voltage.
  • Check the voltage across the transistor when it's turned on. BJTs always have non-negligible voltage drop. If the open-pad voltage is low enough, the transistor might not be able to pull it down far enough.
  • Measure the current when the pads are shorted together with the relay vs. the transistor.
  • If you have an oscilloscope, plot the pad voltage during turn-on of the transistor. Make sure the transistor turns on reasonably fast.

A transistor should be fine for this purpose, but you might need a different one. A MOSFET would probably do better, if you can find a logic-level one.

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Transistor need a certain voltage and polarity that can be switched. In addition the controller signal must have right polarity and ground, the ground should be one of your switchboard pads. It's well possible that right polarity common voltages and ground do not exist. Without a schematic and without having your toys in my hands it's impossible to say does a way to place a transistor switch exist and what it is (see NOTE1). If there's no schematic some oscilloscope measurements may bring the light if the circuit functions in simple enough way.

Relay allows a large range of voltages and currents and it has totally separate in and out. Use it if it works otherwise well enough. Optocoupler have the same separation property, but the polarity of the voltage is critical if the optocoupler has transistor output. LDR output optocoupler very likely works as well as a relay, but with no moving parts.

NOTE1: Matrix type multi-button interface has neither of the plates = GND or operating voltage. Then there's a button reading principle which uses AC signals. It's based on capcitance change. The ultra thin PCB stripes hint (but not prove) that the button reading principle isn't the simplest possible.

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