Homemade digital temperature controller for New Original Bradley Smoker

I've been having fun using my Bradley smoker but the control on the model requires a lot of fiddling with to achieve and maintain the desired temperature.  To make things easier I decided to add a digital temperature controller to my setup, but to make things harder I decided to make one myself instead of purchasing a pre-made unit.  Pictured to the right is what I came up with.  It consists of a Mypin TD4 PID controller with 25A SSR and heatsink, a K-type thermocouple probe, a IEC320 C14 main power switch with 10 amp fuse all housed in a styrene plastic project box .

Power is output through a standard 15 amp duplex outlet.  These come standard with both plug sockets connected so you only need to wire up one set of terminal connectors to power both outlets.  For this application I wanted one outlet to be regular uninterrupted AC power while the other would be run through the solid state relay (SSR) to control the heating element.  To make each outlet independent you just need to use a set of clipper to cut out the copper tab that connects the two terminals on both sides.

Once I had all the parts it was time to get started.  The first obstacle was to cut openings in the styrene project box for the PID controller, power connector/switch and outlets.  My first attempt didn't work out too well, I tried to use a rotary tool with a cut-off disk attached to cut out an opening.  This resulted in big globs of very hot plastic landing on my hand and the cut-off wheel shattering.  I then used a zip saw attachment for the rotary tool, but wasn't really able to control it very well and ended up with a hole that was too large for any of the components (if anyone asks I'll claim that the odd hole on the bottom is there for ventilation).  What finally ended up working for me is using the cut-off wheel to cut a slit on each side of the cutout and then using a jab saw with a metal cutting blade to finish the cuts to make the openings.  The outlets and PID controller were mounted right on the open edge of the box to save a few cuts. Holes were drilled for the mounting screws for the SSR heat sink, switch and outlets and a hole was drilled towards the top large enough to thread the thermocouple leads through. Now came the wiring.  Three wires come from the power switch, black (hot), white (neutral) and green (ground).  The ground wire was connected to the green terminal on the outlet.  Wire nuts were used to connect three leads to both the black and white wires coming from the switch.  One set (black and white) was connected to the PID controller to power the instrument.  The other two neutral (white) wires were connected to the silver terminals on the outlet.  One of the two remaining hot (black) leads was connected directly to the top gold terminal of the outlet, the final black lead was connected to the bottom gold terminal after passing through the SSR.  Finally, the control signal wires (in this case I used thinner black and white wires because that's what I had) from the PID control were connected to the DC side of the SSR.  The polarity of these connections matters so you need to be sure to connect (-) to (-) and (+) to (+).  The thermocouple was threaded through the drilled hole connected to the PID controller (again, polarity matters) and everything was mounted to the styrene case with nuts and bolts.  Finally, the top of the case was screwed in to place.

This project took a lot longer to complete since it was the first time I had attempted something of this nature.  I have not been able to try out the control box on the smoker, but I did plug a lamp in to the SSR outlet and verified that it does what it's supposed to do, when the thermocouple probe was immersed in hot water the PID started turning the lamp on and off and finally keeping it off once the measured temperature exceeded the set-point.

I spent a lot of time trying to plan out this project to end up with something that would not only work for the original purpose, but also be able to be used for other things.  Besides being used on the smoker I could see using it for home brewing to control the temperature of the mash.  I could also switch the PID controller to cooling mode, plug in a small refrigerator and use it for lagering beer.  Plug a submersible heater in the bottom outlet and a circulating pump in the top outlet and you'd have a decent sous vide setup.  The other benefit of building a stand-alone controller is that I didn't need to make any modifications at all to the smoker itself, which will be handy if I ever decide to sell it for an upgrade.