12F675 Temp controller project - with open C source

[Mr RB]

Hi, I made this a few days back but only got around to putting it up today.

It's a complete temperature controller for a small paint curing oven, using LM335 sensor and a relay to drive the "heater" which is some light bulbs.

A rotary encoder knob sets the temperature, and a button resets the 999 hour count-up timer 000:00:00. Display and control resolution is 0.5'C steps and range is the full range of the LM335; -40'C to +100'C (although this oven will only see about 30'C to 70'C).

WARNING!
This project uses some mains voltage wiring. This involves wiring up a mains
voltage transformer, and a mains-rated control relay. Please do not attempt
mains powered projects unless you are sure you can do this safely!

**broken link removed**

It's nothing too fancy to look at as it was put together fairly quickly, but it is a good performance temperature controller for the simple job of a drying oven.

The full project is here;
SH1_Temp2 Temperature Controller

The PIC 12F675 based controller is built on the cheap little Shift1-LCD board I designed recently;
Shift1-LCD Projects

 

[Mr RB]

Hi, it has been brought to my attention that the project is unsafe for beginners due to the fact it uses some mains wiring. I apologise for that, and have now added warnings on this thread and on the project page itself.

The mains wiring in the project consists of a 240VAC mains transformer and a relay with 12v coil and mains-rated contacts which turns the heater light bulbs on and off. This should be as safe as any typical mains powered project built by hobbyists (like a battery charger or DC power supply) but this is still NOT a suitable project for beginners.

Additional covers and insulation was added inside the case over the mains wiring, I did mention this in the text but it was not shown in the pictures as the purpose of the pictures was to show the wiring!

Also the low temperature drying oven has some other safety issues;

The heater "element" should be light bulbs in proper sockets, that are suitable to be continuously ON without causing any safety concerns. Their power rating should be chosen so that even if they are left continuously on by some fault the oven will still remain well within its safe temperature range. If any high-temperature elements are used there should be some type of safety thermo-fuse etc near the element to protect the oven from meltdown by cutting power to the element.

The project uses a 12v relay that MUST HAVE mains-rated contacts. This provides full isolation between the safe low voltage side (ie the PIC, LCD, sensor) and the dangerous mains voltage side. Take all the care with the mains wiring of the relay as you would with the other mains wiring like the cord, the fuse and transformer primary (the same care you would take with any mains wiring in a project).

I would also like to state that this was not meant to be an example of the best way to make mains equipment etc, it was an example of making a quick easy low-voltage temperature controller then adding a simple mains relay so it can control some mains powered light bulbs which act as a low-temperature heater element.

The brain part of this controller will be suitable to control a heater or cooler in the range -40'C to +100'C. If using such high or low temperatures or high powered heating elements you must use the appropriate type of relay and wiring that may need to be heat proof or condensation proof as required.

PLEASE do not attempt any mains powered project unless you know what you are doing!