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Power Switch for AT MB in ATX Case with ATX Power Supply

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First post, by Mandelbrot

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Because AT cases are hard to find and expensive nowadays, I want to build a machine with an AT mobo inside a more modern ATX case with an ATX power supply.

I have an ATX-to-AT PSU adapter which works well, but the green wire (PS-On) needs to remain grounded for the supply to be on, while the power switch on the ATX case is a momentary switch.

What is the easiest way to have the case power switch control the PSU? My initial thought is to program a PIC microcontroller so that a single press of the power button turns the system on, and holding the button for two seconds turns it off. I could probably also get something working with a latch circuit or a 555 timer.

But shouldn't there be an easier, off-the-shelf solution for this? Surely many others have wanted to do the same, so what simple solution am I overlooking?

Reply 1 of 5, by maxtherabbit

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the easy solution is to replace the momentary switch that came with the case with a latching one in the same form factor

post pic of the power switch and I'm sure we can help you find an appropriate one

Reply 2 of 5, by jakethompson1

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Some "smart" AT-ATX converters may already incorporate the logic you suggest.

I have done a few latching conversions with E-Switch TL2285EE. It does work, but it doesn't spring back when it's in the "on" position which looks a little awkward. And you need a soldering iron and heat shrink--cut the wires to the existing power switch, solder them on to the appropriate pins, then heat shrink them to cover them.

You can also, of course, just short the power wires together, use the rear power switch, and leave the case power switch for decoration.

Reply 3 of 5, by OMORES

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Well, I’m using a 386 AT motherboard mounted inside a Cooler Master horizontal ATX case. (you can see it in action in this video) For now, I’ve left the PSU on, and until I find a more elegant solution, I’ve placed a power strip with individual switches on my desk. I’ve labeled the switch for the AT PSU, so whenever I press it, the 386 springs to life and I just go back in time...

My latest video: NT 4.0 running from M.2 PCI-E AHCI SSD.

Reply 4 of 5, by Mandelbrot

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I finally got around to creating (IMHO) an elegant solution using a small 8-pin PIC12F675. Just connect your ATX power switch and a couple of PSU lines to the PIC, and now your ATX power switch works to power on/off your AT motherboard just like you expect. Pressing the power button powers on the PC instantly, but to power it off, you must press and hold the button for 1/2 second.

Here's the (nearly trivial) code if anyone is interested: https://github.com/timali/ATXPowerSwitch.

If anyone is interested in actually using this, you can buy a PIC for a couple bucks, or I have some spare PICs which are pre-programmed that I can send for the cost of shipping.

Reply 5 of 5, by Mandelbrot

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OMORES wrote on 2025-02-24, 22:41:

Well, I’m using a 386 AT motherboard mounted inside a Cooler Master horizontal ATX case. (you can see it in action in this video) For now, I’ve left the PSU on, and until I find a more elegant solution, I’ve placed a power strip with individual switches on my desk. I’ve labeled the switch for the AT PSU, so whenever I press it, the 386 springs to life and I just go back in time...

If you're still using the power strip for your 386, and if you would like to use an ATX power supply with the ATX power switch on the case instead, you might be interested in my solution (using a small PIC to adapt the ATX power supply and switch to an AT motherboard).

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