The herein article contains information related on the method of closed loop control with a sensor and an actuator and
how to apply the method to build a rudimentary On/Off heater controller (On/Off Temperature Control). The semiconductor components used to build this control circuit are thermistor, Zener diode, LED, relay, op-amp, and bipolar transistor.
Sections describe in this article are such essential elements of
the control circuit, basic thermistor sensing circuit (using an op-amp
without feedback as a comparator), thermistor sensing circuit using 2N222 transistor with 4 and output driver
stage added, controlling a real actuator with a relay, and then
finalising the circuit and improving the stability with Schmitt Trigger.
Complete read on Closed Loop Control Circuit using Sensor and Actuator can be download in pdf here (source: math.temple.edu)