ATX power supply remote start with single push button.
This project explains the workings of a recent design to enable starting up and shutting down of a vintage PC's ATX power supply using a single push button instead of a toggle switch. This circuit features a micro-controller at its heart and also a simple network featuring a few resistors, a diode (D1) and a transistor (Q1) which assist in deciding the on/off state of the PSU.
I have designed this circuit so I can send AT type strings into the MCU's UART port for status updates e.g. if current/temp/voltage sensing and a bluetooth module is attached.
This is a snippet from a circuit I have running to power a HP desktop PSU (HP part no. 702307-001) and the MCU (which is a PIC18F26K80 , shown here is the PIC18F1220 ) also manages the HP PSU's internal fan speed by monitoring its temperature fed back to it by a remote NTC resistor attached just above its heatsink.
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How it works
To power up a PC's power supply the green wire labelled *PS_ON has to be held at the ground or zero volt state to keep the PSU's output active.
Initially *PS_ON is held in a high state by the PSU's internal pull up resistors and D1 is held in a revers biased state and no ground current will unless D1 or Q! is forward biased.
To start the PSU, SW_PB is pressed for about half a second and D1 forward biases while simultaneously grounding *PS_ON and starting the PSU.
Once started output PS_OK tells the MCU to enable output PS_HOLD to forward bias Q1 in the instant before SW_PB is released and input btn_PSU has changed from a low to a high state.
The MCU has now changed its state internally from OFF to ON. Now that its in the ON state it is waiting for SW_PB to be pressed for about 1.5 seconds to change back to OFF state. Pressing SW_PB pulls btn_PSU low and the MCU disables PS_HOLD after btn_PSU is released.
A similar commercially available power adaptor for a generic ATX-PSU is available via the link below but it's without a push button push and hold power down feature.