Thing-O-Matic: Reset on Power (Not So) Good

Having added a dummy load to pull a minimum current from both the +5 V and +12 V supplies, it seemed reasonable to connect the ATX +Power Good signal to the Arduino’s -Reset input. That ensures a power glitch will force a hard reset, rather than produce random crashes / instability / weirdness, and make the problem obvious.

Of course, that presumes the power supply notices the glitch and drops the +Power Good line. That’s probably an incorrect assumption, but the only way to test it requires hitting the power supply with a crowbar and I’m just not going to go there.


This also ensures the Arduino gets a hard reset when I turn the power off by triggering the manual shutdown button on the thermal lockout control box. The Arduino draws power through the USB cable from the PC (at least on the Foxconn box I’m using) and sometimes starts up crazy; that will no longer happen.

The +Power Good signal arrives through the Gray wire on ATX pin 8 (it also drives an LED on the lockout box) and the -Reset signal is the outside pin on the Motherboard’s manual reset pushbutton. The new connection looks like this:

Power Good to Reset - MB Schematic
Power Good to Reset - MB Schematic

Being no fool somewhat cautious, however, I added a switch that can disconnect the two lines; if it turns out the +Power Good signal has any glitches, I can use the original mode while scratching my head.

A giant blob of hot melt glue holds the switch in position:

Power Good Enable Switch to MB Reset Button
Power Good Enable Switch to MB Reset Button

A wire burrows through one of the unused RS-485 connector mounting holes under the switch on its way to the ATX connector. It’s the Blue wire below the board in the previous picture, enters from the top right here, and terminates on the third pad over with the Black wire that joins the cable on the way to the Lockout box

ATX Power Good connection
ATX Power Good connection

Now I’ll see whether the Thing-O-Matic begins resetting at random moments. After doing all the various mods & improvements you’ve seen over the past few months the printer has been quite reliable, so I have some hope that this change will produce … no change.

As a quick test, I let the printer sit all day with the Thermal Core and HBP temperatures cycling around 50 °C and all four steppers enabled. Any reset would disable the steppers and make itself obvious, but after two days it’s all good.

One good sign: the LED ring around the extruder head just barely changes brightness as the heaters cycle.

Oh, and the steppers don’t overheat, either. This thing is starting to behave like a real 3D printer should!

If it does start resetting, however, I’ll add a latch inside the Thermal Lockout box that captures short +Power Good glitches and lights Yet Another LED.