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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.

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    \$\begingroup\$ "I want to travel, which means of transport should I use?" We can't answer your question in general, we need all the details. Current, voltage, switching frequency, maybe an inductive/capacitive load, galvanic isolation or not? If you don't have those answers, the only solution is a relay rated for at least the ratings of the switch you are replacing. \$\endgroup\$
    – Wouter van Ooijen
    Commented Oct 4, 2014 at 8:51

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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.

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  • \$\begingroup\$ I know some details but not many, i have the switch that connects DC ~10v, but this is a control module. So it comes into the box, through a switch, and out the box again... \$\endgroup\$
    – Smanger
    Commented Oct 4, 2014 at 9:00
  • \$\begingroup\$ @Smanger - That's why you use a mechanical relay. The relay contacts are a switch that happens to be "controllable" via a separate circuit. \$\endgroup\$
    – Michael Karas
    Commented Oct 4, 2014 at 9:04
  • \$\begingroup\$ Gotcha, so a relay is the only option when you don't know the circuit? \$\endgroup\$
    – Smanger
    Commented Oct 4, 2014 at 9:26
  • \$\begingroup\$ I wouldn't say only option. But a relay is a good option and in most cases may be the best option. To elaborate let me propose another solution so you can see what I mean. You could set things up so that an R/C servo actuator would turn the actual switch on and off. A wireless RF signal remotely activates the R/C servo which uses a small motor and a gearing mechanism to rotate a cam or lever. The cam or lever would be mechanically interfaced to either the existing switch on the electronics unit or a replacement switch that you put there because it is easier to activate with the R/C servo. \$\endgroup\$
    – Michael Karas
    Commented Oct 4, 2014 at 9:35
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If you can't determine what the circuit needs then use a relay. Of course you must match the contact ratings to the load.

If you are replacing a tactile switch then it should only be switching a few mA, so a PhotoMOS relay (eg. AQW212) will work well. You could also try a standard opto-coupler (eg. 4N35), but bear in mind that its output is polarity conscious, so you may to reverse the connections to get it to work.

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