# Thermistors and LEDs

Discussion in 'General Electronics Chat' started by Non-Sequitur, Oct 27, 2014.

1. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
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I am looking to use Thermistor to light an LED when the temp exceeds 85C. the input voltage is 5volts and is pulled from a USB connector.
I only understand the basics, and though ive seen several diageams, none actually specify the value of the fixed resistors.
I would need the thermistor to be SMT/SMD, as with all other part, including the LED. Looking for a blue or white LED.
Can anyone provide direction?

2. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
0
I also have a 3.3v supply I can tap instead of the 5v if necessary... one last thing, I'm the last"S" in the KISS concept of operations

Last edited: Oct 27, 2014
3. ### MrChips Moderator

Oct 2, 2009
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3,454
Your circuit will need a temperature sensing device, (e.g. thermistor, RTD, LM35), bias circuitry, analog comparator, LED driver or circuitry.
You need to provide more information on the thermistor, i.e. part number. The specific resistor values will depend on the actual thermsitor and the circuit topology.

Are you asking for someone to design a circuit for you or do you already have a circuit in mind?

4. ### pwdixon Member

Oct 11, 2012
488
56
You could use a comparator with a resistor divider consisting of the thermistor and a fixed resistor on one input and a fixed resistor divider on the other input. The output of the comparator driving the LED through a resistor. The fixed resistors in the two dividers are selected by knowing what the thermistor resistance is at your target temperature. So perhaps make the resistor with the thermistor equal to the thermistor value at target and the other divider is just two equal resistors giving a half power rail comparison level.

5. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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Yes, my apologies, I am actually asking for someone to provide/design a KISS circuit. I don't have the part (Thermistor) if that is the simplest solution, but I do have a handful of TMP302B devices that I can use.
Also, I have a handful of LB N91E-AADA-35-1 blue LEDs from OSRAM which have a forward current of 20mA and forward voltage of 3.4V.
I'm not trying to over-achieve here, just looking to provide a reliable 85C trigger to turn on an LED

6. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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I understand about 50% of what you are saying here. I can implement whatever design is necessary to reach my goal.
But beyond that I'm not as smart as the others in the room.
Simply not my background.

7. ### pwdixon Member

Oct 11, 2012
488
56
If you have TMP302B's then there's nothing to talk about really just wire it up ala the datasheet and stick an LED/resistor on the output, TI have done everything for you already, apart from the value of the LED resistor what more do you need?

8. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
If you already have TI TMP302B then why not use that. Connect pins 1 and 2 to Vs, pin-4 to GND. Connect the cathode of the LED to pin-3 and the anode to Vs via a 33Ω resistor. Note that the maximum supply voltage is 3.6V.

You can also use an PNP output driver transistor to power the LED from the 5V supply. Increase the LED series resistance to 470-1kΩ.

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9. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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The value of the LED resistor would be very helpful, and a short explanation of who it is derived. So I don't have to ask questions at this so basic level.
I assume the resistor would be after the LED, with het Anode connected to the LED and the Cathode to ground?
Also, as I read the data sheet, the OUTPUT goes low at the trip point, doesn't go high.
Does that mean that the LED would be lit until the trip point and then go off when the temp is too high?
If so, that is backwards from my goal.
Which is why I was considering a thermistor.
I can get my hands on these devices in pretty short order.

10. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
0
Sorry, I just saw this reply.... I hate smart phones, they make me feel stupid. I will draw this out on a chow hall napkin and see if this is something I can get done... Thanks!
My only other hesitation is the footprint of the TMP302... small for hand soldering. But it is what it is.

11. ### pwdixon Member

Oct 11, 2012
488
56
Is the transistor really needed as this is an open drain part.

12. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
OP wants to light a blue or white LED. TMP302 is spec'd at 3mA, max Vs is 3.6V.
You don't want self-heating of the device. That might affect the hysteresis.

13. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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What about Pin 6, the Trip 1?
I'm assuming that is also connected to Vs so that Trip0 & Trip1 = Vs means default + 15C to equal 85C...

14. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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The SIGINT guy has wiped out the supply of the blue LEDS.
I have only LR H9GP-HZKX-1 at my disposal (red)... this has a 2.1V forward voltage and 350mA forward current... My supply is 3.3V
Can someone please recomputed the required resistor or explain how the math is done?

15. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
You are not going to get 350mA output from the TMP302.

Let's assume the LED Vf =2.1V @1mA
and the voltage OUT = 0.4V
Then the series resistor has to drop the remaining voltage, 3.3 - 2.1 - 0.4 =0.8V
The required resistor value = V/I = 0.8V/1mA = 800Ω.

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16. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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Got to tell you, this is actually fun. Thanks so much!

17. ### MrChips Moderator

Oct 2, 2009
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Just to let you know that this is just an example.
The final value chosen will depend on the actual LED, its color and efficiency, desired brightness, supply voltage and how much heating you will allow in the TMP302 driver chip.

I used 1mA as a simple example. With Vs = 3.3V you will find that a suitable resistance value could be somewhere between 0Ω and 1kΩ. Experimentation will guide you to a suitable value.

Glad you're having fun. That's the important thing.

18. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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I understand. In fact the 800 Ohm resistor may be hard to come by, we don't have a lot of variations, I think I can come up with a 750 and some others in that range.
I will play around with it. But at least I have the basics down.
Now to get the SIGINT guy off of the workbench!
If I hear SIGINT rules one more time I'm going to show him that IMINT rules with a picture of the inside of his colon from my boot's perspective!

19. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
Sorry, I made a typo. I meant pins 1 and 6 to Vs for a 85°C trip-point.
Pin-2 is GND and pin-5 is Vs.

20. ### Non-Sequitur Thread Starter New Member

Oct 27, 2014
23
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So for a instant replay... Pin 1, 6 and the Anode of the LED to Vs. Pin 2 and Pin 4 to GND. And the Cathode of the LED to Pin 3.