Dual Thermocouple Meter Backlight Override

A cheap dual thermocouple meter, utterly devoid of branding, arrived:

It seems suitable for a semi-permanent laser cooling water monitor, particularly because it can perform arithmetic to show the difference between the inlet and outlet temperatures. The minuscule clock face at the center top of the display shows it’s in auto-power-off mode, which can be defeated by a Vulcan Nerve Pinch while turning it on.

Having a large backlit display was a selling (well, buying) point and the instructions have this to say about its operation:

Dual Thermocouple meter - backlight instructions
Dual Thermocouple meter – backlight instructions

The instructions say nothing about defeating the backlight timeout. The description is technically correct, because the two seconds before it goes dark is “within 30 seconds”, but I’d rather have a nicely lit display that’s on all the time.

Five screws hold the back cover in place, with no nasty prying required to pull it apart, and the build quality is about what you’d expect for a cheap meter. The circuitry fits on a single PCB and perhaps the thermistor over on the right serves as the cold junction compensation:

Doodling the backlight circuit layout suggests it’s pretty simple, even without filling in the component values:

I replaced the transistor base resistor with a somewhat larger 4.7 kΩ SMD part and added a flying wire to jam the transistor on all the time:

The IC is a serial EEPROM with its VCC and ground pins in the usual places, so, when the power to the EEPROM goes on, the backlight turns on and stays on.

The meter draws a bit over 8 mA with the backlight running, which means the trio of AAA cells won’t last all that long. When things settle down, I’ll conjure a simpleminded power supply running from a convenient voltage inside the laser cabinet.

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