digital-logicmosfetrelayswitchestransistors
If I have a switch in some consumer electronics that I take apart, and want to replace a mechanical switch with a digitally controlled one, which component should I use? The only problem I see with MOSFET/transistors is don't I need to know if it is going to be a high side switch or a low side switch? What if I don't have access to the whole circuit when choosing?
Just wondering if there is a rule of thumb for choosing which method to replace a switch I'm missing.
Basic P-MOS switching circuit:
The problem is that the body diode of the FET is going to conduct if an external power source is present.
You can connect two FETs in series with their body diodes opposite each other so they don't conduct when off, and get low drop-out when both are on. Here's a crappy illustration of the concept from a product:
As Matt says, the first thing that jumps out is more capacitance needed.
Breadboards are terrible for stray inductance/capacitance so you really need to try and keep wires as short as possible, keep sensitive signals away from higher current/voltage/fast changing signals, and add plenty of bypass and bulk capacitance around the circuit.
I would start with a couple of >100uF electrolytics on each power rail and near the FET, with a few 100nF or higher ceramic caps on the power pins of the ICs.
Another thing to look at is using a separate rail for the relay, and routing the ground return separately from the uC's ground. Of course they need to be tied together, but you can ensure the high current return does not flow through the uC board's ground this way.
Best Answer
Without knowing the exact details regarding the circuit connections that the switch was connected to it is not possible to give a generic answer as to what semiconductor components could replace the switch.
What you can do though is to select a relay that has contacts with ratings similar to the switch. Ratings being the current carrying capacity of the contacts and the open circuit voltage rating of the contacts. And whether specified for AC or DC service.
With the relay in place you can now control the relay coil as suitable to your remote control system and be totally isolated from the device which is being switched.