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Electronic thermostat control for 12V cool boxes

Status: Proposal
August 16, 2015
My idea started when I whould like to use my cool box at home, but the built-in thermostat had a fixed switching-point. I had a thermostat module laying around; it has two switching points and two outputs, one for the lower level, one for the higher, both individually adjustable. So, hysteresis is adjustable too. I took a bistable relay with two coils (laying around of course, like the most parts of this project), and used a CMOS 4538 monostable multivibrator to generate the switching pulses for that relay. Working fine when it´s running... but when power is switched off, we have to consider the following points: 1. The battery of the module will discharge through the inputs of the 4538. 2. When powering on again, the state of the bistable relay is unknown, the switching pulse has to be repeated. C2/R2 and C5/R5 prevent the 4538 to generate an impulse when powering on, next, after a delay generated by C3/R7, T1 switches on the reed relay RE1 and the 4538 may generate an impulse to switch the power relay RE2 on or off. Now it´s ready and working (except for three buttons for the module), and I think, why I didn´t use a microcontroller? Hmm, I whould program more functions than necessary. Now it´s programmed in true hardware language!
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