email@example.com (Bob Valentine) wrote: >I'm looking for a simple circuit to trigger a 12-volt relay at a >certain temperature, and then turn it off at another pre-set >temperature, or at at x degrees less than turn-on. This is not a very difficult task. The first answer is to use an LM75 temperature sensor, which has a programmable over-temperature set point and a hysteresis set point. When the device temperature exceeds the over-temperature set point, the overtemp shutdown (O.S.) output goes low. When the temperature comes back below the hysteresis set point, the O.S. output goes back high. The part has an I2C (I-squared C) interface and has resolution to 1/2 degree C. The other thing is to use a temp sensor, like the LM34 or LM35, with a simple comparator: V+ ____ | | | |\ | In this circuit, the output will go |LM34|------|+ \ high when the LM34 output goes above |____| | \_____ Output the value at the comparator (-) | | / | input, which is found to be: __|__ -|- / | (V+) * (Req1) / (Req1 + Ri), where ___ | |/ | Req1 = Rs * Rf / (Rs + Rf). When the _ | | output from the LM34 goes high, it ______|___ Rf __| will then remain high until the LM34 | | output falls to a value of: Ri Rs (V+) * Rs / (Rs + Req2), where Req2 = | | Ri * Rf / (Ri + Rf). | __|__ V+ ___ _ I will leave it to you to calculate the needed resistor values. The LM34 puts out 10mV per degree Fahrenheit. -- Nick Gray Applications Engineer National Semiconductor Corporation National Data Sheets at "http://www.national.com"
Date: Fri, 19 Apr 1996 20:10:48 GMT
Original Subject: Re: Simple Thermostat