Using a tip 120 to turn on a computer

Hi im new to circuits and im wondering if its possible to use a tip 120 like a relay to close the circut of 2 pins on a motherboard. I know it might not be the ideal solution but its what I got.

Maybe but I don't think it is a good choice.


  1. what would you have controlling the TIP120?
  2. do the two pins need to be on for a moment then open? That's how my PC works.
  3. the hardest to know (unless you have a voltmeter) is at what voltage are the two pins?

for instance is one pin +5V and the other a signal to be connected to +5V?
or is one pin ground and the other a signal to be connected to ground.

I plan to use a d1 mini with blynk to use as a remote power button anywhere. As far as i know the two pins just need to be connected for a second.

That transistor is a bad choice for that purpose. Better use a MOSFET, preferably as a high side switch.

Opto-Coupler with integrated MOSFET.


Tom... :+1: :smiley: :+1: :coffee: :australia:

You could do it with an isolated mosfet, I would go the old classical route reed relay

  • Complete isolation
  • Not sensitive to polarity
  • I trust LittleFuse / Hamlin

The question was: "can this be done with TIP120?" not "what are better ways?"

I think it can be done. It is a Darlington but with enough Base drive the voltage drop should be low enough to be recognized as pressed button by the computer.

How will you power the D1?

The TIP120 may possibly work, we are suggesting some sort of isolation between controller and PC power supply.

Give it a try, just remember that the gnd of the D1 and the gnd of the PC power supply, in your case, will have to be connected together.

Tom... :smiley: :+1: :coffee: :australia:

It will be better to connect negative side of the button to D1 GND (provided the power supplies are isolated). If D1 is powered from the computer PS you need to determine if a pull-up or pull-down is used for the button.

Whereas that is strictly speaking true, often the risks, benefits and pitfalls of a solution proposed by the OP can best be explained relative to other, more conventional, ways of achieving the same thing.

So here goes:
A relay would be ideal in the sense that you don't have to understand anything about the circuit you are connecting to and offers good isolation.
An opto coupler would be good because you don't need to think about a common ground with the device you are switching.
Your transistor method is easy enough to test without an Arduino just using say a 5 volt power supply and a say 1k base resistor.

I would use an Opto-Coupler. The current draw would be insignificant, pretty much any opto-coupler should do the trick, just the direction will have to be discovered by trial and error.
As for Mosfet's or Transistor, the Common GND is what would make me discard that option, unless you are powering the d1 mini form the same source as the computer, in that case it would be easy enough but you will have to figure out whether the one side of the switch is 'Grounding' the other or 'pulling-up' the other, in which case you may actually need a PNP not an NPN transistor
A Relay will always work but sounds like serious overkill. So does a TIP120 btw, a BC547B should do the trick just fine, but i would go for a PC817 Opto-coupler to do the job. (i have plenty of those)


It won't be if the PC is turned off, I don't think the standby 5V supply would power much if you could get inside and tap it!!!!

Tom... :smiley: :+1: :coffee: :australia:

Really? ?

I belive I can power it while the computer is off and that would mean the grounds are connected. To wire it would I do it like the photodownload
If not how would I.

How do you define "should"?

I don't know what the OP's computer requires but the TIP120 by spec will have trouble getting below 1 volt when ON.

See the first post

I know it might not be the ideal solution but its what I got.

Perhaps my goal expected too much for a someone with little / no electrical experience. It would be unfortunate if s/he did everything right but the TIP120 just could not deliver what was needed.

OP has TIP120. There is a good chance it will work. Why not try it first and looking for a better solution only if it fails?

@noobie_69000 I think you have confused the forum members!

First you said:

Later you said

which seems to show a solenoid, presumably to physically push the PC's power button.

Please clear up this confusion!

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