Cute Temperature Controller – Electronics For You

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Modern electronics have evolved to a very great extent today such that it over spreads on the territory of computer science on one hand while under spreads on the zone of Information technology as well. Today there is hardly any electronic project which does not use the micro controller unit (MCU) and or the computer programming languages.

However, still there are projects which can strip down all these high tech MCU and computer programs and can still make life better and easier by precisely doing the same thing that were intended to be achieved using & controlled by MCUs & software.

Temperature controller – TMP36

In our daily life space, temperature controller has umpteen number of uses – rice cooker, fridge, water geyser, micro wave oven, air conditioners all uses temperature controller. TMP36 is one such device which, when associated with an MCU with suitable programming can work as a splendid temperature controller. It precisely measures temperatures ,gives input to the MCU and the MCU with the help of software, turns the signal into control command for the associated devices like relays and others. In this project the TMP36 controls the relay in the same way except there is no MCU or software is used in between. Simply and directly it controls the relay using a few discrete components yet very precise and simple.


Principles of Operation:

TMP36 has the following characteristics.

  1. TMP36 has a low voltage operation – 2.7 volt to 5.5 volt
  2. 40 Deg Cent to +150 Deg Cent operating zone.
  3. Linear operation y = mx + c
  4. Measure temperature directly into mili voltage. Uniform linearity of 20mV / Deg C
  5. Less than 50 µA Quiescent Current.
  6. Ramp up = 3 Deg C/ Second and Ramp Down = 6 Deg C / Second.

The main sensor TMP36 picks up temperature, converts it into millivolt. Which is then measured by the NPN, PNP junction – the Sziklai pair and then when the base voltage overcomes the bias limit the PNP transistor conducts and the 5Volt relay gets energized. NPN-PNP junction (the Sziklai pair ) has an advantage over other Darlington pair is that it’s base-emitter voltage is 0.61 volt unlike 1.22 volt for the other Darlington pair. Thus making the low voltage operation possible.

Schematic diagram

Fig. 1

Component List

R1 = 1K
R2 = 10K
R3 = 4.7K multi turn trim pot.

C1 = 250 MFD, 10 Volt.

L1 = 5Volt Relay

IC1 = 7805
IC2 = TMP36
TR1 = BC547
TR2 = SK100
D1 = IN60 Diode

Fig-1 shows the schematic of the circuit diagram while Fig-2 shows a typical TMP36. The whole project has a very small form factor and can be powered by a small battery of 9- 12 Volt and consumes very small current to run very long.

Fig. 2


This is where we shed off the bulky MCUs and make the circuit simple. The 4.7K trim pot is a precise multi turn type to set the temperature set point. Heat up the probe to the desired temperature and then adjust the trim pot so that the relay just de-energizes. At this point if the temperature increases beyond that point the relay will energize again. The NC (or NO) contact of the relay may be used to disconnect / reconnect a charger, gadget ,cooker or soldering iron etc. Conversely when the temperature reduces ,the TMP36 cools down and the relay gets energized again thus reconnecting the device once again.

Oscillation against a mean temperature

The TMP36 has a ramp up of 3 Deg C/Second and ramp down of 6 Deg C / Second. So automatically it takes a 3 degree overlap per second on it’s own to avoid oscillation over a mean temperature besides for a physical body the heat does not dissipate so fast unless there is fast cooling provided.


Where can you use it

Ni-Mh or Lion batteries: If you can stop the overcharging of these batteries when the battery reaches a particular temperature (for Ni-Mh it’s 45 Deg C to 50 Deg C ,for Lion & Lipo it’s 45 Deg C) the life of these batteries will extend beyond normal usage.

Author's Prototype
Fig. 3: Author’s Prototype

Tip temperature of the soldering iron: Use the NC connection to disconnect the soldering iron supply and put the TMP36 little away from the tip where temperature is below 150 Deg C.

Modulate the gas burner when the water starts boiling: use the NO connection to regulate the shimmer knob/motor and put the TMP36 in direct contact with the boiling liquid – use leak proof metallic shield type TMP36.

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