Hi guys, this is my first post so first off an introduction. I live in Vancouver, BC, Canada and I work as a network engineer and computer teacher, but in my free time I have been working to set up a computer museum for many years now. Now, here's why I am posting.

I am building a tower case for my Atari STe, and the case I have chosen to use is an ATX case, and as such has a momentary pushbutton as a power switch. I am going to modify an ATX power supply to drive the STe, but an ATX power supply requires a pin on the ATX connector to be latched to ground to turn the power supply on, and opened to turn it off. So I am looking to build a simple circuit to connect to the power button, the on/off line, ground, and the +5v trickle line from the atx supply so that when I push the power button the power on line is latched to ground to power up the system and when I push it again the connection is broken and the supply shuts down. I know this can be done simply, but I'm a little rusty with my low level logic. Can someone help me out with this? Thanks guys!

 

Simple enough.

555 Bistable Multivibrator

Welcome to AAC!

I have quite a few tutorials you may be interested in. Most of them are going into the text book here.

Bill's Index

 

Thanks for the welcome Bill! This circuit looks to be like what I am looking for but I think it can't be used as is without modification. I think that in this circuit the 555 drives the LEDs directly whereas with what I am doing I want the pushbutton to connect a separate line to ground and disconnect it with another push, not just feed +5 to a particular device, so I think (unless I am totally off) that I will have to modify it. Am I totally off here? If not how should I modify it to my needs?

 

A simple transistor, either a logic level MOSFET or a BJT (regular transistor) will make a good on/off switch.

Modification by any other name is designing.

 

Ok, so I should just replace one of the LEDs with a transistor, with base connected to where the LED would be, and the emitter and collector connected to the power on line and ground?

 

...So I am looking to build a simple circuit to connect to the power button... so that when I push the power button the power on line is latched to ground... and when I push it again the connection is broken...

You will not get any simpler then using a SPST Push On-Push Off Switch

 

You will not get any simpler then using a SPST Push On-Push Off Switch

I understand that, the only problem is I want to keep the original power button, not hack a toggle button into the case.

 

You can also use a flip-flop, such as shown here.

 

I think I can use this circuit without any modification. I think I would just connect the Power On line in the place of the +5V going to JP1. Please someone correct me if I'm wrong.

Circuit sourced from here.

 

Sorry I left something out of the schematic before, here is the corrected one.

 

The first schematic is correct. You must have pin 4 connected to Pin 8.

Basically the same circuit I showed you.

 

Hi Bill. Yes it is pretty much the same just slightly changed. The reason I connected pin 4 to a cap and then ground is because someone told me that if you set it up that way the device will receive a reset pulse on power up and always come up in the off setting. Is this not correct?

 

You can believe me or not, but I'm telling you pin 4 must be connected to pin 8. This is not optional. It is fundamental to how 555's work.

 

You can use a cap to ground from pin 4, but I suggest also using a 10k to 220k resistor to Vcc. That way, pin 4 will eventually wind up at near Vcc.

 

Oh believe me Bill, if you say so I believe you 100%, I was just confirming. I will build it according to the first schematic and let you know how it turns out. Wookie, are you saying leave pin 4 attached to pin 8 but attach the cap and the resistor as well?