# What does short circuiting a RST front IO header do?

My motherboard has a connector to enable the use of a front panel RESET button:

I would like to simulate the press of the button (equivalent to short circuiting the wires) by a Raspberry Pi, by connecting one (or two) of the GPIO pins to this connector.

What does a short-circuiting do, from a signal perspective? The pins on the motherboard are noted RES+ and RES-, suggesting (wild guessing, which is never good in electronics) that the result is 0? Since GPIO voltage levels are 3.3 V and are not 5 V tolerant I would like to make sure that I am not going into a dangerous land (frying the RPi would be sad, the motherboard - disastrous).

As you certainly guessed, my electronics background is close to nil so thank you in advance for adapting the answer to a beginner entry level.

The safest option is to use some means of isolating the two circuits from each other. The standard methods are relay and opto-isolator.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Relay isolation. Figure 2. Opto-isolation.

Relay

• When GPIO goes high Q1 is turned on, RLY1 turns on and the reset contact closes.

Opto

• When GPIO is pulled low D1 turns on. The light hitting the base of the transistor turns Q2 on shorting out its collector and emitter.

With either of these arrangements the two systems are electrically isolated from each other and a fault on one won't affect the other (other than, perhaps, the Pi shutting down the MB).

• Thank you. I was hoping that a direct wiring would be fine (to reduce complexity, soldering / breadbording) but I will go for your safer solution.
– WoJ
Apr 19 '16 at 14:16
• The problem is that we don't know whether the reset line is being pulled high or pulled low by the reset button. This solution doesn't care. I hope it works. Thanks for accepting my answer. Apr 19 '16 at 17:23
• "this solution does not care". You are right, what you suggested does exactly this : simulate the button press / short-circuit. I will now be looking for a small breadboard :). Thanks again.
– WoJ
Apr 19 '16 at 17:33