Taking care of the cold resin problem

 It's been a while since last I updated my blog. Or even worked on the boat... Time to resume activities. But. Winter is coming. And it's getting cold in the shop.

Mindful of how much I hated working with cold syrupy resin last winter I decided I need to make a heat box to keep the glue toasty and thin. In the spirit of DIY and due to the fact I could not find one that would let me control both the temperature of the box AND the heating element (maybe they are there, but i'm sure they are expensive) I decided to make one. The heating element temperature needs to be limited for the obvious reason of not melting a plastic jar of resin, that would just be counterproductive. And I want to be able to use whatever heater I have at hand: light bulbs, whatever. I do happen to have a 200W printer heat bed, but that also likes to get pretty hot.

Requirements:

- Fun. Check. That has been had. The geeky kind.

- Cheap. Not sure if in the end it actually was cheap(-er) than commercial options, but hey, see point 1.

- Silent. None of that clickety-clack of mechanical relays. Hence a cheap chinese SSR comes into play. It is rated to 25A, I would not trust it beyond 10, in reality it will not control more than 1 or 2. Should be aok.

- Safe. As in: not melting down anything. Not the resin, not itself. So the temperature of the heater must be under control, and the temperature of self (as in: the solid state relay) must be kept in check. And since the latter is of the chinesium persuasion and likes to go up in flames - an additional safety (mechanical) relay comes to the rescue to disconnect the whole thing if overheated.

- Modular enough to be easily serviceable. That includes the micro controller - I picked an Arduino pro mini - simple, well supported and cheap.

- Simple interface: just one rotating knob to set everything (temperatures, hysteresis setting, saving defaults and the like).

The prototype (using resistors as heaters pressed against dallas temperature sensors) looks like this:

Instead of soldering everything on a prototyping board (which I considered) I designed a proper(?) PCB and had a well known chinese PCB manufacturer stamp out 5 of them (that is the minimal amount I could order). The boards will be arriving in a couple of days and if I did not make any oopsies in the layout I will have 4 perfectly good spares if anyone will be interested:

The code to go with the setup is published on github. I'm sure there is plenty of thermostat projects out there, but this one is mine and mine alone:)

Now on to trying to finish Piglet's cockpit. I'll post when done (in a few days).

doei.