Tnx John.
Looking around, it seems that an LM26 might be a possibility. You can buy them at DigiKey factory programmed to -25C or 0C.
Here is what I am trying to do. I am building a 2m CW beacon for high-altitude balloons. It is a fail safe beacon to be used as a last ditch method of locating the payloads (after the balloon bursts and parachutes back to the surface) in the event that other tracking methods fail. We recently lost a balloon with several expensive payloads attached. It would be ok for the beacon not to transmit during the flight, but it must come on after the payloads parachute back to ground level.
The beacon transmitter has to be dirt simple, reliable, light, run 5 days on a small battery, sort of like the radio collars on animals. To satisfy FCC regs, it must transmit a Morse Code ID every 10 min. For battery saving, the 50mW transmitter duty-cycle needs to be <10%.
I am using the Atmel chip (Arduino Nano) to control the timing of the transmissions and generate morse code ID. To minimize power drain, the Nano spends 99.95% of its time in deep sleep mode, meaning it is only drawing 6uA, so self heating of the processor is non-existant. I am using the sleep mode where all that is running in the chip is a low-freq RC oscillator which clocks the Watchdog timer, which is supposed to re-awake the processor.
The balloon gets to an apogee of 110K feet, where the ambient temperature is -50C. I fear that during such cold temperatures the Nano may not wake up reliably, so I want to use the thermostat to apply power and reset the processor after the payloads descend below about ~20K feet.
The purpose of the thermostat is to shut the entire beacon off while the temperature is < -10C which will occur after launch as the balloon climbs, the beacon can be silent during the high altitude portion of the flight, and then turn on after the payload descends back into warm air...
The thermostat can simply shut off power to the Nano (and the transmitter) while the balloon is aloft, and then just reapply power when things warm back up. There is an advantage to disabling the beacon transmitter during flight because some of the other payloads contain 2m uplink receivers that could be de-sensed by the beacon transmitter.