Modding a cheap PID controller - Projects - MakeHackVoid Discourse

I’ll start this category off then since I suggested it…

I bought one of these cheap PID temp controllers off ebay which I’m sure lots are familiar with. Plan was to make a sous vide with it.
http://www.ebay.com.au/itm/310881980306

After a bit of investigation I was pretty unimpressed with a number of things about it:

• No substantial documentation in English. Sure you can glean enough by googling to do a basic setup, but it’s that old chestnut of different versions behaving a little differently with different bugs and a number of user manuals on the net showing different things.
• The thermocouple input appeared to have no cold junction compensation. My guess is they’ve just picked a standard room temp and a using that for the cold end (e.g. 25 degrees). I never investigated this enough to be sure, but there’s definitely no temp sensor of any kind near the back of the case where the connector is. There was a pair of terminals labelled Rt, which possibly was for an external thermistor to be connected, but again it’s not clear as one manual I found shows those terminals being used for a PT100 thermistor probe instead of the thermocouple, not in addition to.
• The K-type thermocouple was never really going to be that great for the temp range (in the 50-80 degrees.) They’re just not that accurate in that sort of range owing to the extremely small thermocouple signal levels. Buying a PT100 (thermistor) version of the same controller was easily twice the price.
• No easy way of moding the firmware. I wanted to be able to do things like stepped temp changes and logging of the PID performance, of which this controller has no capability for. The uC on it is a Chinese 8051 derivative of some kind, but I couldn’t find any good data on it. Getting a tool chain together for it was always going to be an uphill battle.

Solution:
Because the design of these controllers is modular, with a power supply board, display board and separate microcontroller board, I’ve designed a new microcontroller board that has the same physical form factor of the old board. I’m using a PSOC 4200 micro because it has some nice analog stuff for this applications, and I’ve got a bunch sitting around from when they were on a promo sale for $1ea. The board is designed to slip straight in with a small amount of soldering and make it somewhat of a universal controller. See attached kicad screen grab.

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