Simple Adjustable Thermostat Circuit using IC LM56

An electronic adjustable thermostat is an important part of controlling temperature and its often used in systems that heat or cool spaces.

This specific circuit uses IC LM56, which is a low power chip that can switch temperatures on and off in two different ways.

It helps control external transistors that manage the systems output loads.

The LM56 is well known for providing reliable and precise temperature control making it perfect for many electronic uses, like keeping track of temperature and protecting circuits from overheating.

We can change the temperature limits easily with adjustable variable resistors, which makes the circuit more adaptable for various uses.

Circuit here is powered by 5V DC power supply.

Circuit Working:

Parts List:

This circuit works by using temperature readings from the LM56 chip which carefully checks these readings against levels set by the user that can be changed based on what they need.

The LM56 has two built-in temperature comparators called VT1 and VT2 which compare the measured temperature to specific reference levels.

Instead of using fixed resistors, this circuit has adjustable presets VR1, VR2 and VR3 which allow for fine tuning of the temperatures at which the circuit will switch on or off.

The details for these presets are: VR1 with a resistance of 10k controls the lower threshold VT.

Preset VR2 at 50k adjusts the middle threshold and VR3 with a resistance of 100k manages the upper threshold VT2.

These presets act like a voltage divider setting the temperature points needed to turn the outputs on.

When the temperature detected by the LM56 goes above the set threshold the chip changes its output pins specifically Out1 and Out2.

Out1 connects to Q1 a PNP transistor BC557, which then activates a relay coil L1 through diode D1 1N4007 to prevent back electromotive force back-emf.

On the other hand Out2 is linked to Q2 an NPN transistor BC547, which switches another relay or load L2 using diode D2 1N4007.

If the LM56 detects a temperature above the upper threshold it turns on one relay1 to start cooling.

If the temperature drops below the lower threshold, it activates a different relay2 to start heating.

By adjusting the presets VR1, VR2, VR3 users can easily change when the circuit activates without needing to swap out resistors.

Formulas with Calculations:

Below mentioned are the formulas with calculations for Simple Adjustable Thermostat Circuit using IC LM56:

The LM56 determines temperature switching points based on the voltage divider formed by VR1 10k, VR2 50k, and VR3 100k.

Formula for Lower Threshold (VT1):

VT1 = Vref × ( VR2 / (VR1 + VR2 + VR3) )

Formula for Upper Threshold (VT2):

VT2 = Vref × ( (VR2 + VR3) / (VR1+ VR2 + VR3) )

where,

• Vref is 2.7V internal reference voltage of LM56
• VR1 is 10k adjustable preset
• VR2 is 50k adjustable preset
• VR3 is 100k adjustable preset

Calculation Example:

VT1 Calculation:

VT1 = 2.7V × ( 50kΩ / (10kΩ + 50kΩ + 100kΩ) )

VT1 = 2.7V × ( 50 / 160 )

VT1 = 2.7V × 0.3125

VT1 = 0.84V

VT2 Calculation:

VT2 = 2.7V × ( (50kΩ + 100kΩ) / (10kΩ + 50kΩ + 100kΩ) )

VT2 = 2.7V × ( 150 / 160 )

VT2 = 2.7V × 0.9375

VT2 = 2.53V

Transistor Biasing Calculation:

Base Current for Q1 PNP – BC557:

IB = ( Vout1 – VBE ) / VR2

where,

• Vout1 is 5V for LM56 output voltage
• VBE is 0.7V for base emitter voltage drop
• VR2 is 50kΩ

IB = (5V – 0.7V) / 50kΩ

IB = 4.3V / 50kΩ

IB = 0.086mA

Collector Current for Q2 NPN BC547:

IC = β × IB

where,

• β (gain of Q2) = 100

IC = 100 × 0.086mA

IC = 8.6mA

Relay Coil Current Calculation:

Irelay = ( Vsupply – VCEsat ) / Rcoil

where,

• Vsupply is 5V
• VCEsat is 0.2V for saturation voltage of transistor
• Rcoil is 200Ω

Irelay = (5V – 0.2V) / 200Ω

Irelay = 4.8V / 200Ω

Irelay = 24mA

How to Build:

To build a Simple Adjustable Thermostat Circuit using IC LM56 following steps are needed to follow:

• Assemble all the required components mentioned in the above circuit diagram
• Connect pin 1 of IC1 LM56 to center leg of VR1 and second leg of VR1 connect to pin 2 of IC1
• Connect pin 2 of IC1 LM56 to center leg of VR2 and second leg of VR2 connect to pin 3 of IC1
• Connect pin 3 of IC1 LM56 to center leg of VR3 and second leg of VR3 connect to pin 4 of IC1 and GND
• Connect pin 6 of IC1 to base of transistor Q2
• Connect a resistor R3 from base of transistor Q2 and positive supply of the circuit.
• Connect resistor R1 between pin 7 and pin 8 of IC1 and positive supply
• Connect resistor R2 from pin 7 to base of transistor Q1.
• Connect the emitter of transistor Q1 to positive supply
• Connect the collector of transistor Q1 to coil pin of relay1 .
• Connect diode D1 from coil pin of realy1 to other coil pin of relay1.
• Connect emitter of transistor Q2 to GND of the circuit.
• Connect the collector of transistor Q2 to coil pin of relay2
• Connect a diode D2 from coil relay2 pin to positive supply.

Conclusion:

The Simple Adjustable Thermostat Circuit using IC LM56 is a great way to control temperature in a smart and dependable way.

You can adjust different temperature limits for different uses through variable resistors.

With transistors and relays in the mix, this circuit can manage larger amounts of current which makes it perfect for real heating and cooling systems.

References:

3°C Dual output resistor-programmable temperature switch with fan control