# Simple Temp Sensor Circuit using TMP36 and LM358

#### Lucky-Luka

Joined Mar 28, 2019
181
Hi all
I have to monitor the temperature of a surface for several hours per day. The circuit will be battery powered.
I already have a LM358 op-amp and a TMP36 sensor at home.
I thought 4x AA NiMh batteries (1.2V) are ok.
I just want an LED ON when the temp is over 50 degrees Celsius and another LED ON when the batteries are low (I thought 0.9V*4= 3.6V = low voltage).
Do you see anything wrong?
Any improvements or suggestions?
Thanks

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#### ericgibbs

Joined Jan 29, 2010
18,266
hi LL,
The change in battery voltage will affect the temp level comparator input, you need a voltage reference for the RV1 pot set voltage.
E

++++

Consider a tap off D1, [ decoupled] for RV1 set voltage.

#### sghioto

Joined Dec 31, 2017
5,130
You can probably use the same 3.6v reference voltage from D1.

#### Ian0

Joined Aug 7, 2020
8,955
I imagine you are sensing temperatures between 0°C and perhaps 70°C. That is an output of 500mV to 1.2V from the TMP36.
Your pot can vary the voltage from 0V to 4.8V, which is -50°C to 480°C (although the TMP36 would melt first), so the region you are interested in is a really tiny adjustment on the pot.
A 1.2V reference would be ideal as would set the upper level to 70°C. Then you might want a resistor in series with the pot to make the lower limit more sensible

#### Ian0

Joined Aug 7, 2020
8,955
You can probably use the same 3.6v reference voltage from D1.
Or perhaps not, as Fairchild is not forthcoming with the temperature coefficient of the zener, and you may find it changes with temperature by more than the output of theTMP36!

#### crutschow

Joined Mar 14, 2008
33,367
Use the cheap, reasonably stable, TL431 adjustable reverence in place of the Zener.
You can adjust its voltage to the desired trigger point.

#### sghioto

Joined Dec 31, 2017
5,130
Or perhaps not
Depending on the setup. I would suspect the sensor is located remotely.
Any improvements or suggestions?
I second crutschow's post and maybe a different battery type unless you are just trying to use what you have on hand

#### Audioguru again

Joined Oct 21, 2019
6,461
The schematic had its parts so far apart that the text was tiny and could not be seen.
I cropped and enlarged it and see that the pots are fed a diminishing battery voltage.

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#### sghioto

Joined Dec 31, 2017
5,130
I cropped and enlarged it and see that the pots are fed a diminishing battery voltage.
That would be correct for the battery monitor. For the temp sensor that was addressed back in post #2.

#### ericgibbs

Joined Jan 29, 2010
18,266
hi agu,
I don't see a tiny picture.? It fills the PC screen.
I agree the components are further apart than needed.

E
The line squared graph paper makes diagrams difficult to read.

#### Lucky-Luka

Joined Mar 28, 2019
181
I second crutschow's post and maybe a different battery type unless you are just trying to use what you have on hand
What kind of batteries would you suggest?

#### Lucky-Luka

Joined Mar 28, 2019
181
hi agu,
I don't see a tiny picture.? It fills the PC screen.
I agree the components are further apart than needed.

E
The line squared graph paper makes diagrams difficult to read.
He is the only one complaining about my schematics in different threads and even in different forums.
It does not bother me tho.
The grid is due to Proteus simulator.

#### Lucky-Luka

Joined Mar 28, 2019
181
Use the cheap, reasonably stable, TL431 adjustable reverence in place of the Zener.
You can adjust its voltage to the desired trigger point.
Isn't the voltage reference affected by the discharge of the batteries?

#### sghioto

Joined Dec 31, 2017
5,130
What kind of batteries would you suggest?
I generally use a lithium type. Should be able to run the circuit on a single 3.7 volt cell.

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#### Audioguru again

Joined Oct 21, 2019
6,461
The zener diode you selected needs a much higher current (69mA) than you feed it (5.4mA). Many 3.6V zener diodes are available that operate at 5mA.
But a low voltage zener diode makes a poor voltage regulator and its voltage drops as it warms up.

#### Ian0

Joined Aug 7, 2020
8,955
But a low voltage zener diode makes a poor voltage regulator and its voltage drops as it warms up.
Agreed. It needs to be a bandgap reference with almost zero temperature coefficient.
Even if the sensor is located remotely (post #7) the circuit is no good if the trip point varies all over the place with the local temperature. 1.2V makes a good reference as it corresponds to 70°C on a TMP36

#### sghioto

Joined Dec 31, 2017
5,130
The zener diode you selected needs a much higher current (69mA) than you feed it
Not so. At 69 ma the zener impedance is 10 ohms at 1ma it's 400 ohms. The input of a LM358 is not going to load it down.

#### Lucky-Luka

Joined Mar 28, 2019
181
Agreed. It needs to be a bandgap reference with almost zero temperature coefficient.
Even if the sensor is located remotely (post #7) the circuit is no good if the trip point varies all over the place with the local temperature. 1.2V makes a good reference as it corresponds to 70°C on a TMP36
The circuit is far from the heat source.
Can I obtain that 1.2V reference using another TL431?
So the circuit will see 2 TL431 and a lm358, right?

#### sghioto

Joined Dec 31, 2017
5,130
Just one TL431 required. You can use the same 1.2v reference for the battery monitor.

#### Ian0

Joined Aug 7, 2020
8,955
The circuit is far from the heat source.
Can I obtain that 1.2V reference using another TL431?
So the circuit will see 2 TL431 and a lm358, right?
TLV431 perhaps, but there are plenty of 1.2V references out there.
You can use a single reference for both parts of the circuit.