Relay Circuit Electronic ThermostatHere is a simple thermostat circuit that can be acclimated to ascendancy a broadcast and accumulation ability to a baby amplitude boiler through the broadcast contacts. The broadcast contacts should be rated aloft the accepted requirements for the heater.
Temperature changes are detected by a (1.7K @ 70F) thermistor placed in alternation with a 5K potentiometer which produces about 50 millivolts per amount F at the ascribe of the LM339 voltage comparator. The two 1K resistors affiliated to pin 7 set the advertence voltage at bisected the accumulation voltage and the hysteresis ambit to about 3 degrees or 150 millivolts. The hysteresis ambit (temperature ambit area the broadcast engages and disengages) can be adapted with the 10K resistor amid pins 1 and 7. A college amount will attenuated the range.
In operation, the alternation resistor is adapted so that the broadcast aloof toggles off at the adapted temperature. A three amount bead in temperature should account the broadcast to toggle aback on and abide on until the temperature afresh rises to the preset level. The broadcast activity can be antipodal so it toggles off at the lower end of the ambit by abandoning the locations of the 5K potentiometer and thermistor. The 5.1 volt zener diode regulates the ambit voltage so that baby changes in the 12 volt accumulation will not aftereffect operation. The voltage beyond the thermistor should be bisected the accumulation or about 2.6 volts back the temperature is aural the 3 amount ambit set by the potentiometer. Most any thermistor can be used, but the attrition should be aloft 1K ohm at the temperature of interest. The alternation resistor called should be about alert the attrition of the thermistor so the acclimation ends up abreast the centermost of the control.
