Temperature monitor & alarm

This was one of those projects that should have taken an afternoon and ended up as a reasonably large build, however it’s now complete and working rather nicely. It’s also a perfect of example how I sometimes create a piece of test equipment for use with another project I’m working on.

In this case I was experimenting with buck/boost regulators and, well, had a little thermal accident that involved burnt fingers, smoke and a wasted afternoon.

This gizmo will monitor the temperature from up to 3 sensors and if any of them exceed specified maximums it will sound an alarm and open a relay contact, which can be used to cut the power to a project that start’s overheating.

I already had a box from a defunct project that had the 4×20 LCD display installed and power switch, mains transformer and a mains IEC socket on the rear so I decided to re-task it for this.

Inside is a PIC18F25K22 running this show, and the device can monitor up to three channels. Each channel has a 5-pin din connector and cable that connects to a Maxim DS18B20 temperature sensor.
You can set the alarm temperature independently on each channel, and the project starts beeping and flashing the LCD backlight if a maximum is exceeded. I also included a relay that can be used as a power interrupter for the project under test. This way I can leave a project on soak but if it starts to over heat, the power can be cut automatically.

The reason it took longer than anticipated was for some reason the PCB never etched correctly and I had some messing around to do. The software only took around an hour to write and debug. I love Proton BASIC.

Oh, and the reason it’s three channels and not more, well I happened to have three 5-pin din sockets in my junk box. If I’d had more… who knows.

If anybody is interested I’ll make available the circuit diagram, PCB foil and PIC firmware but this really is an easy project to design and build.

So, now that’s up and running, I can get back to the original task of designing a decent voltage boost converter.

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