Smart Heater Controller

Minuscule circuit of the
electronic heater controller presented here is built around the
renowned 3-Pin Integrated Temperature Sensor LM35 (IC1) from NSC.
Besides, a popular Bi Mos Op-amp CA3140 (IC2) is used to sense the
status of the temperature sensor IC1, which also controls a solid-state
switch formed by a high power Triac BT136. Resistive type electric heater at the output of T1 turns to ON and to OFF states as instructed by the control circuit.

This gadget can be used as an efficient and safe heater in living
rooms, incubators, heavy electric/electronic instrument etc. Normally,
when the temperature is below a set value (Decided by multi-turn preset
pot P1), voltage at the inverting input (pin2) of IC1 is lower than the
level at the non-inverting terminal (pin3). So, the comparator output
(at pin 6) of IC1 goes high and T1 is triggered to supply mains power to
the desired heater element.

Electronic Heater Controller Circuit Schematic

Electronic Heater Controller Circuit Schematic.

Note: CA3140 (IC2) is highly sensitive to electrostatic discharge (ESD). Please follow proper IC Handling Procedures.

When the temperature increases above the set value, say 50-60 degree
centigrade, the inverting pin of IC1 also goes above the non-inverting
pin and hence the comparator output falls. This stops triggering of T1
preventing the mains supply from reaching the heater element.
Fortunately, the threshold value is user-controllable and can be set
anywhere between 0 to 100 Degree centigrade.

The circuit works off stable 9Volt dc supply, which may be derived
from the mains supply using a standard ac mains adaptor (100mA at 9V) or
using a traditional capacitive voltage divider assembly. You can find
such power circuits elsewhere in this website.