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
