Simple and Efficient Temperature Controller Without MCU

efy test suresh and Shiven Saigal

Modern electronics have evolved to a great extent today, encompassing computer science on one hand and information technology on the other. There is hardly any electronic project nowadays that does not involve the use of a microcontroller unit (MCU) or computer programming languages.

However, there are still projects that can achieve the same goals as those requiring MCUs and software, but with simpler and more efficient designs.

In our daily lives, temperature controllers find numerous applications, such as in rice cookers, fridges, water geysers, microwave ovens, and air-conditioners.

One device that stands out for its ability to function as a splendid temperature controller, when paired with an MCU and suitable programming, is the TMP36.

By precisely measuring temperatures and providing input to the MCU, which then converts the signal into control commands for associated devices like relays, the TMP36 simplifies temperature control.

In this project, the TMP36 controls a relay directly, without the need for an MCU or software. The circuit, composed of a few discrete components, is both precise and straightforward. Fig. 1 shows the prototype tested in the EFY lab.

Temperature Controller Circuit on Breadboard
Fig.1: Temperature Controller Circuit on Breadboard

Temperature Controller – Circuit and Working

The circuit diagram of the cute temperature sensor is shown in Fig. 2. The sensor is built around a 5V voltage regulator LM7805 (IC1), the TMP36 sensor (IC2), a 5V one-changeover relay (RL1), npn transistor BC547 (T1), PNP transistor SK100 (T2), diode 1N60 (D1), and a few other components.

DIY Temperature Controller Circuit
Fig 2: Temperature Sensor Circuit Diagram

The core of the circuit is the TMP36 temperature sensor. It possesses the following characteristics: low voltage operation (2.7V to 5.5V), an operating temperature range of -40°C to +150°C, linear operation (y=mx+c), temperature measurement directly in millivolts, uniform linearity of 20mV/°C, and a quiescent current of less than 50µA.

It also features a ramp-up of 3°C/second and a ramp-down of 6°C/second. Pin configurations of TMP36 are shown in Fig. 3.

TMP36 pins configuration
Fig. 3: TMP36 pins configuration

The project’s working principle is simple. The TMP36 sensor detects the temperature and converts it into millivolts, which are then measured. When the TMP36 senses heat, the base voltage of transistor T1 exceeds the bias limit.

As a result, both transistors T1 and T2 conduct, energizing the 5V relay, which enables the load connected across its contacts. The NPN-PNP junction (the Sziklai pair) offers an advantage over other Darlington pairs due to its lower base-emitter voltage of 0.61V, compared to 1.22V for other Darlington pairs, making low-voltage operation possible.


This is where we eliminate the need for bulky MCUs and simplify the circuit. The 4.7k pot (VR1) is a precise multi-turn type used to set the temperature setpoint. Heat the probe to the desired temperature and then adjust pot VR1 so that the relay just de-energizes. Beyond this point, if the temperature increases, the relay will energize again.

The normally closed (NC) or normally open (NO) contact of the relay can be used to disconnect/reconnect a charger, gadget, cooker, soldering iron, etc. Conversely, when the temperature decreases, the TMP36 cools down, de-energizing the relay and reconnecting the device.

Since the TMP36 has a ramp-up of 3°C/second and a ramp-down of 6°C/second, it automatically accounts for a 3-degree overlap per second to avoid oscillation over a mean temperature. Additionally, physical bodies do not dissipate heat quickly, unless fast cooling is provided.

Temperature Controller Sensor Parts List
IC1 – 7805, 5V regulator
IC2 – TMP36 temparature sensor
D1 – 1N60 diode
T1 – BC547 npn transistor
T2 – SK100 PNP transistor
Resistors (all 1/4-watt, ±5% carbon):
R1 – 1-kilo-ohm
R2 – 10-kilo-ohm
VR1 – 4.7-kilo-ohm pot
C1 – 10µF, 16V electrolyte
CON1, CON2 – 2-pin connector
RL1 – 5V, 1C/O relay
S1 – On/off switch
Batt.1 – 9-12V battery

Construction and Testing

The actual-size, single-side PCB layout for the cute temperature sensor is shown in Fig. 4, while its component layout is shown in Fig. 5. After assembling the circuit on PCB, enclose it in a suitable box.

Actual-size PCB layout of the circuit
Fig. 4: Actual-size PCB layout of the circuit
Component layout of the PCB
Fig. 5: Component layout of the PCB

Alternatively, you can assemble the circuit on a general-purpose PCB, as shown in the author’s prototype depicted in Fig. 6. Place all the components and the PCB in a suitable enclosure, and provide terminal strips for the 230V supply input and output separately.

DIY Temperature Controller without Microcontroller
Fig. 6: DIY Temperature Controller without Microcontroller

This temperature Controller has various applications:

Charging Ni-Mh or Li-ion batteries: It can be used to charge Ni-Mh or lithium-ion (Li-ion) batteries. By preventing overcharging when the battery reaches a specific temperature (45°C to 50°C for Ni-Mh batteries and 45°C for Li-ion and Lipo batteries), the project extends the batteries’ lifespan beyond normal usage.

Soldering iron tip temperature control: Use the NC connection to disconnect the soldering iron supply and position the TMP36 slightly away from the tip, where the temperature remains below 150°C.

Modulating the gas burner when water starts boiling: Use the normally open (NO) connection to regulate the shimmer knob/motor and place the TMP36 in direct contact with the boiling liquid, using a leak-proof metallic shield for the TMP36.

In a world driven by advanced technology, this temperature sensor reminds us that simplicity can be powerful. Streamlining temperature control with precision and ease opens doors to endless possibilities for enhancing our daily lives.

Download PCB and Component Layout PDFs: click here

You can also check the TMP36 Datasheet.

Somnath Bera is an electronics and IoT enthusiast working as General Manager at NTPC