Simple/ and or cheap variable thermostat?
Well I have a friend who is looking into the purchase of a snake, and meanwhile picking out the snake and the heating element isn't the hard part for him (he's looking at a 23 watt 120vac heating pad for it). He was also looking at a thermostat designed for snake tanks, however they want close to $150 for it, and it doesn't even have many features, it's basically a black box with an 3 digit lcd and a programable timer.
I know it wouldn't be hard to put something similar together from a pic and a few different parts, however it's been a long few years since I've designed a full schematic of something, let alone look at one. So I was wondering if someone so kind might aid me in making of a schematic so my friend can go order the parts he needs and I can build it for him (don't have a problem assembying circuits from raw schematics, I find it fun).
However, there is a couple things he wants out of it. Number one most importantly, he wants it to be able to adjust the output voltage so he doesn't fry his snake (most simple thermostats simply turn on until temperature is reached, then off), no he wants something more dynamic than that.
Second, he needs to have an external temperature probe so that he can keep the thermostat outside of the tank whilst keeping a handy eye on the temp and also obviously so that the thermostat knows when to come on (is there a limit to the length of wire you can put on a thermocouple?).
Third and last, we need a simple timer circuit that keeps track of the time (what's that involve? a 13mhz crystal and a few other parts?) so that at night the thermostat can change it's "heating schedual", I assume so that it tries to maintain a temperature say 3-4C above the programmed temp. (night mode anyone?) However the thermostat he was looking at buying wanted you to buy a second cord, which connected to an external appliance timer (yes you heard me, a $9 wall timer) so that it knows what time it was.
My friend really needs this as he's looking into getting this snake within the month. I appreciate anything anyone has to offer. Thanks guys!