NE555: Power Switch emnulator

Ramin Tavassoli

Part Number: NE555
Other Parts Discussed in Thread: SN74LVC1G123

Tool/software:

I want to use logic circuit to act as power button on a PC.

The PC is not accessible when DC power is applied to the motherboard and can not access the button to power up the system.

I tied using the attached file.

But, the 555 timer stays on since PIN 2 (Trigger) is held low and timer does not expire after 5 seconds or so.

The Power GOOD from 5VDC supply suppose to trigger the 555 time and the PIN 3 output is inverted and sends a low signal for 5 seconds to power up the motherboard.

I need a better solution.

Thank you

3 days ago

0 Ron Michallick 2 days ago

TI__Guru**** 152966 points

Hello Ramin,

I suggest SN74LVC1G123 because it can trigger directly from PG rising edge and it is fine that PG stays high. The output pulse will be high same as 555.

For your power switch output, would an open collector/drain be more appropriate? For the final pulse high to low inversion, some logic devices have open drain, or you could use a 2N7002 NMOS to invert.