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It's a common problem, when using desktop hardware to run a server: after a power failure, the machine needs human action to restart. High-end desktops and server hardware often have a BIOS setting to automatically start when power comes back. Using a low-end, older PC hardware to run some low demanding services is very tempting, but what to do when the UPS runs out of battery, or when there are no UPS at all (for security reason, like a system that controls a door, it may be prefered to avoid using a UPS).

Until recently I used to modify the power supply unit so the PC would turn on as soon as the power was ailable, but I did not like that solution very much: it means opening the power supply unit, which voids the warranty and even if the solution was working, it was all done upon guessing how the power supply is built, no real guarantee that it would not do more bad than good.The relay

I was looking for a device that would plug on the mother board, when the power button is usually pluged, and do just what is needed: close the contact on these two pins for a short delay (the dealy one presses on the power button to turn on the PC). A permanent contact would be equivalent to pressing for five or six second and stop the system.

Relay cablingThe device happens to be a simple 12 volts DC relay with normally-closed (NC) contacts. When there is no main power, the contacts are closed, teh two pins on the mother board are connected. As soon as there is power (12V) provided by the power supply unit of the PC, the relay is activated, the NC contacts open, the pins of the mother board are diconnected. This works exactly the same as if you keep your finger pressing the power button on the PC, and release it once the system has started. The response time of the relay will make it for the delay needed while one keeps his finger on the power button while waiting for the sound of the PC that is starting.

The coil of the relay is connected to the 12 V of the power supplpy, for example using a connector like the ones found on hard disks. The relay contacts are connected to the mother board by a connectopr similar to the one used by the power button.

Et voilà.

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