Thing-O-Matic: Thermal Lockout Circuit

Thermal lockout control box
Thermal lockout control box

This is a proof-of-concept lashup of a circuit to shut off the Thing-O-Matic’s power should the Thermal Core overheat. It vaguely resembles those doodles, but with the thermal switch cases grounded and an indicator for the main thermal switch.

[Update: You should read the rant at the bottom of that post to understand why this isn’t a firmware mod and doesn’t contain a microcontroller.]

Operation is straightforward:

  • The black NO (Normally Open) momentary switch energizes the DPDT relay, one NO pole of which then holds the relay power on.
  • The red NC (Normally Closed) momentary switch interrupts that circuit and releases the relay.
  • An NC thermal switch detects an overheated Thermal Core, opens that circuit, and releases the relay.

The other NO relay pole connects / disconnects the ATX power supply’s -Power On line from the Thing-O-Matic Motherboard. That connection requires a circuit-board cut to splice the relay into the Motherboard.

The LEDs:

  • Lower Green = ATX AC power on (from +5VSB power)
  • Upper Green = +Power On signal active
  • Red = Test / Fault (on = relay inactive)
  • Yellow = low over-temperature alarm
  • Orange atop box = high over-temp switch active

I included a second NO thermal switch that activates at a lower temperature, mostly because I had one, but that’s certainly not required. The multitude of LEDs makes for a happy-looking box; labels would be a nice touch, I agree.

When you turn on the ATX power supply, the Lower Green and Red LEDs turn on: the “Test” part of the “Test / Fault” indicator. Push the black button, the Red LED goes off, the Upper Green LED goes on, and the Thing-O-Matic is up & running. Push the red button, the TOM shuts down, and you’re back to the starting condition.

The Yellow LED goes on when the lower temperature switch goes on.

Shortly thereafter, presumably, the higher temperature switch opens, the Orange LED goes on, the TOM shuts down, and you’re left with the Lower Green, Yellow, and Orange LEDs: zowie! When the  high temp switch cools off a bit, the Orange LED goes off and the Red LED goes on. After a while, the Yellow LED will go off, and you’re back to Square One again.

What’s not yet done: mounting the thermal switches to the Thermal Core in a way that’s mechanically solid, electrically isolated, and thermally dependable. I just got a bag of 100 °C NC switches, which make more sense than the 65 °C NC switches I’d been fooling with.

The wiring uses 4P4C and 6P6C modular phone connectors and cables, which are cheap & readily available, if not exactly proof against high temperatures. In normal use, failures tend to be open-circuit that will shut off the heater power. Take care not to position the cables so they melt first; they’re not intended as thermal switches.

Achtung: modular cable color codes are not standardized, particularly on the jack side, so pay more attention to the pin numbers than the colors. If I ever meet the guys who rearranged the jack colors, There. Will. Be. Gibbage.

A back view of the box shows a nice rectangular hole that’s obviously a manual CNC job on the Sherline, with no corner filing whatsoever. Hot melt glue holds the connectors in place, so I’m not showing off the inside:

Thermal lockout box - rear
Thermal lockout box - rear

The -Power On connection to the Motherboard requires the single cut shown in yellow:

Motherboard PCB Modification
Motherboard PCB Modification

It looks like this in real life, with the wire soldered to the Arduino header pin. Another dab of my Shop Assistant’s orange nail polish seals the PCB wound:

Motherboard -Power On modification
Motherboard -Power On modification

The remaining wires attach to the ATX power connector pins on the bottom of the board. The yellow wire passes through an unused mounting hole on its way to the top side, as above. Use a cable tie to tie the cable to the board, through a pair of otherwise unused RS-485 connector mounting holes.

Motherboard Connections - Bottom
Motherboard Connections - Bottom

While you’re chopping away at the Motherboard, add that isolating diode to keep +5 V USB power from turning the ATX fan with the power off.

The overall schematic (clicky for more dots):

Thermal Lockout Schematic
Thermal Lockout Schematic

There is no corresponding PCB layout, because the circuitry forms a point-to-point hairball inside the box. If you were doing this for real, you’d want a PCB with a bazillion connections, but …

For example, here’s the FET driver for the Orange (it just looks Red) high temperature LED before a liberal application of heat stink shrink tubing:

Overtemperature LED driver hairball
Overtemperature LED driver hairball

You can test the thermal switches using a butane lighter.