Learn ON-OFF Light and Temperature Controller using 741 op-amp

I like to learn circuits about detecting temperature and light. To control various loads. Are you the same as me? Here is learning ON-OFF Light and Temperature Controller using 741 op-amps.

Some people say that learning the work of simple circuits It’s a waste of time. Because in this era, almost everything is finished. We almost don’t need to design the circuit ourselves. Just working is sufficient.

Learn ON-OFF Light and Temperature Controller using 741

But friends, have you ever thought like me? We use various things. Unnecessarily and old devices are valuable. In addition, basic learning is a challenging activity And is definitely a good foundation.

The Circuit concept

Imagine a hot day. You will be more comfortable with a ventilation fan. But when the weather is cold, we should turn off the fans because it’s not helpful.

Related: 2 ideas of Car overheat alarm circuit

And with this circuit make our lives easier. Because it will control the fan to turn on / off automatically. We don’t have to waste time doing it ourselves.

I am worried that you will not understand. Let’s look at the block diagram below. See the picture to understand how to read the characters. (Hahaha)

Block Diagram of temperature contrller using 741

Here is a step working and concept:

This circuit we use a thermistor as a temperature detection device. And use LDR as a light detection device.

Like the previous circuit, we often used a relay switch to connect the electricity to the electrical appliances (load).

Op-amp comparator

This project uses 741 op-amps as a comparator form. There is a reference voltage level for comparison.

Thermal or optical sensing will arrange the circuit in the form of a voltage divider circuit. To compare with the reference voltage level as above.

We will receive the output voltage to connect with other circuits such as the counting circuit, relay control circuit, etc.

Recommended: Diode Temperature Sensor Switch using 741

We can use an op-amp as a voltage comparison with easy and convenient.

Because one of the important characteristics is that the open-loop gain is very high (100,000 times). Without any feedback signal.

Inverting op-amp comparator

Look at the image.

Left: When we enter an input voltage(I/P) at the inverting input. And set the non-inverting input receives the reference voltage(Vref). It will compare with the signal on the input.

Right: It causes the output(O/P) to become saturated. Either between VCC and 0V.

Vref is a positive constant voltage. But Vin varies, it is waveform.

And having a reference voltage (at pin 3) causes the op-amp to operate. The output voltage changes according to the input voltage with 2 status:

  • When Vin is lower than Vref, the O/P is OV.
  • In contrast, Vin is lower than Vref. The output is VCC.

What is more?

Non-inverting op-amp comparator

In contrast, Connect Vref to inverting input. And use non-inverting as the input.

See the image below.


The output voltage will be VCC. If the waveform of the input is high as well.

The output has only 2 voltage levels, VCC and 0V. So, we can easily control it via the relay. In addition, it has logic-like conditions in digital circuits. May be applied as a count circuit or display other results

Also: Simple temperature to voltage converter circuit

Do you understand?

How this circuit works

I want to help you understand better. Almost every time I wonder. I often learn from real circuits.

Look at the circuit. See many devices. Don’t be confused.

on-off light and temperature controller
Circuit diagram of Opto-Thermo Control Relay Switch with IC 741

Here is a step by step process.

IC1 is a voltage comparison circuit, in an inverterting mode. Include many resistors. But it is easy.

With R1 and R2 determine the reference voltage(Vref) at pin 3. They are a voltage divider circuit. When both R1 and R2 are equal, the voltage at this point. It is half the power supply.

The XY connection point consists of a voltage detection circuit at pin 2. If this working. Then, the comparator circuit run, too. It makes the output is VCC.

The current flows through R4 and R5. And, has voltage across R5. It causes transistor Q1-2SC945 works. And the relay works, too.

The Diode-D3 absorbs this current spike of a relay coil. To protects the external relay driver circuit. 

Recommended: Making Simple Light-activated relay circuit with PCB

For the detection circuit on the input at pin 2. We can choose 4 types, see in the picture below.

Note: This image is old. But with a similar circuit structure. We can see that you can use it with a 9V or 12V power supply. And can be used with transistor BC549 or 2SC945 (but the shape is different)

Opto Thermo control relay switch using 741
  • Dark Mode:
    The relay will work when it is dark. Because the LDR is higher resistance. So, the voltage across it is low.
  • Light Mode:
    In contrast, the relay will work when it is light. Because the LDR is lower resistance. So, the voltage at pin 2 is low. The IC1 compares input voltage and Vref. It makes the high voltage at output to drive relay runs, too.
  • Cold mode:
    It is similar to LDR working. The relay will work when the thermistor gets a low temperature. The TH1 is a higher resistance. Then, there is a low voltage at pin 2. So, the output is a high voltage.
  • Hot Mode:
    Imagine you use this mode to control the fan. The relay will work when the thermistor gets a high temperature. We switch position TH1 and VR1. It will work in contrast.

Also, we can adjust VR1 to set sensitive of detecting in every mode.

Read Also: Automatic led night light switch

The power supply:
We use 9V to 12V DC power supply. See the circuit. They include T1, D1, D2, and C1 are 9V DC unregulated power supply.

Parts you will need

0.25W Resistors, tolerance: 5%

  • R1, R2: 12K
  • R3: 270K
  • R4: 4.7K
  • R5: 1K
  • VR1: 10K Potentiometer Trimmer
  • C1: 470uF 25V Electrolytic capacitor
  • IC1: LM741 op-amp IC
  • Q1: 2SC945, 45V 300mA NPN transistor
  • D1,D2,D3: 1N4007, 1000V 1A Diodes
  • RY1: Relay with SPDT 10A min switch. Coil Voltage 12V. Coil resistance 150-600 Ohms 
  • PCB, and others

How to build it

This project is easy. So, you may assemble it on an universal PCB. Or some want to make the PCB you can see the PCB layout and Component layout below.

The PCB layout
The components layout

I trust you are good in electronics you know how to build electronic in simple ways.

Here are a few related posts you might want to read:


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