Junk Box Retro
Electronic Attic Fan Thermostat
OK, I could easily have coughed up $25 or so for a line-voltage SPDT mechanical thermostat, but decided instead to build an all electronic one with stuff from my junk pile and a few never-used old-stock components. Some scrounging, drilling and proto board fabricating produced this retro (~1970s-80s) electronic thermostat. The incremental dollar cost came in at a big fat zero, but more importantly, the project was fun and educational. You may find it interesting even if you have no need for an attic fan thermostat.
Since the unit would be a set-and-forget thing, hidden away in the attic and likely never seen again, I wasn't too interested in making the project look pretty. It fulfilled that non-goal, as you can see.
Mounted in the Attic
The circuit is based on the LM34 Fahrenheit temperature sensor from TI. The sensor outputs (on the Vout pin) 10 millivolts per degree F above 0°F. So, for example, 80°F produces 800 mV or .8 volts. The first opamp (U2a) amplifies the sensor voltage by a factor of 3. Thus 80 degrees (800 mV) produces 2.4 volts at the output (pin 1), 90 degrees produces 2.7 volts, 100 degrees produces 3.0 volts and so on. This voltage is compared with the voltage from the wiper of the 10k potentiometer (which can be set anywhere from 2.4 volts to 3 volts) by U2b. If the output of U2a is greater than the voltage from the potentiometer, the output of U2b goes high, turning on the 2N2222, energizing the solid-state relay and turning on the fan. Same thing in reverse on the trip down: when the output voltage from U2a is lower than the wiper voltage, the output of U2b goes low, shutting off the fan.
Some Fine Print
Schematics produced with DCCAD.