Description.
The circuit is nothing but two LEDs (D1 and D2), whose status are controlled by the temperature of the surroundings. The famous IC LM35 is used as the temperature sensor here. Output of LM35 increases by 10mV per degree rise in temperature. Output of LM35 is connected to the non inverting input of the opamp CA3130.The inverting input of the same opamp can be given with the required reference voltage using POT R2. If the reference voltage is 0.8V, then the voltage at the non inverting input (output of LM35) becomes 0.8V when the temperature is 80 degree Celsius. At this point the output of IC3 goes to positive saturation. This makes the transistor Q1 On and LED D1 glows. Since the base of Q2 is connected to the collector of Q1, Q2 will be switched OFF and LED D2 remains OFF. When the temperature is below 80 degree Celsius the reverse happens.IC1 produces a stable 5V DC working voltage from the available9V DC supply. If you already have a 5V DC supply then you can use it directly.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• IC3 must be mounted on a holder.
• The temperature trip point can be set by adjusting POTR2.
• Type no of Q1 and Q2 are not very critical. Any general purpose NPN transistors will do it.

Description.
Here is the schematic of a very simple electronic thermostat using IC LM56. LM56 is a very accurate dual output low power thermostat from National Semiconductors.LM56 has various useful features like internal temperature sensor, two internal voltage comparators, internal voltage reference etc. Two stable temperature trip points (VT1 and VT2) are created by dividing the LM56′s 1.250V internal voltage reference using 3 external resistors (R1, R2 and R3). There are two digital outputs for LM56 .Output1 becomes LOW when the temperature increases above T1 and goes HIGH when the temperature decreases below (T1±Hysteresis Temperature). In the same way, Output2 becomes LOW when the temperature goes above T2 and goes HIGH when the temperature goes below (T2±Hysteresis Temperature).
By connecting a cooler as the load for relay L1 and a heater as the load for relay L2, a very simple and useful temperature control system can be constructed.

The values of R1, R2 and R3 for the required trip points VT1 and VT2 can be determined using the following equations.
VT1 = 1.250V x (R1)/ (R1 + R2 + R3)
VT2 = 1.250V x (R1 + R2)/ (R1 + R2 + R3)
where:
(R1 + R2 + R3) = 27 k Ohms and
VT1 or T2 = [6.20 mV/degree Celsius x T] = 395 mV therefore:
R1 = VT1/ (1.25V) x 27 k Ohms
R2 = (VT2/ (1.25V) x 27 k Ohms) – R1
R3 = 27 k Ohms – R1 – R2

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• Use 5V DC for powering the circuit.
• L1 and L2 can be 5V relays.Their current ratings must be selected according to the load they switch.
• Do not give more than 10V to LM56.

Description.

This simple circuit will produce an alarm whenever the temperature falls below zero degree. A thermistor is used here to sense temperature. The op-amp LM7215 is used to compare the reference voltage and voltage from the thermistor network. Reference voltage is given to the non inverting input (pin3) of the IC and voltage from thermistor network is given to the inverting input (pin4).When temperature becomes less than zero degree the voltage at the non inverting input becomes larger than the voltage at the inverting input and the output of the op-amp becomes high. This makes the transistor Q1 ON and drives the piezo buzzer to make the alarm. In the power supply section, IC 7805 is used to derive 5V from the 9V battery.

Circuit diagram with Parts list.

Power supply for this circuit.

Notes.

• Assemble the circuit on a good quality PCB.
• The thermistor used here is a glass bead thermistor, type No: KEYSTONE RL0503-5536K-122-MS (361K @ 0 degree Celsius and 100K @ 25 degree Celsius).
• The IC1 must be mounted on a holder.
• The battery B1 can be a 9V PP3 battery.