Since one of the servers owned by the author would not start up by itself after a power failure this little circuit was designed to perform that task.
The older PC that concerned did have a standby state, but no matching BIOS set-ting that allows it to start up unattended. Although a +5 V standby supply voltage is available, you always have to push a but-ton for a short time to start the computer up again. Modern PCs often do have the option in the BIOS which makes an automatic start after a power outage possible. After building in the accompanying circuit, the PC starts after about a second. Incidentally, the push-button still functions as before.
The circuit is built around two golden oldies: a NE555 as single-shot pulse generator and a TL7705 reset generator. The reset generator will generate a pulse of about 1 second after the supply voltage appears. The RC circuit between the TL7705 and the NE555 provides a small trigger pulse during the falling edge of the 1 second pulse. The NE555 reacts to this by generating a nice pulse of 1.1RC. During that time the output transistor bridges the above mentioned pushbutton switch of the PC, so it will start obediently.
Other applications that require a short duration contact after the power supply returns are of course also possible.
Author : Egbert Jan van den Bussche – Copyright : Elektor