Thermistor for simple breath detector

Thread Starter

sneakingcactus

Joined Oct 4, 2020
3
Hello,
I'm a very-amateur electronics enthusiast. I generally assemble my own desktop computers and I build my own (simple) circuits maybe every couple of years or so.
I'm unsure if I have the right place or if this is too basic a question, but I would like to add a feature to a Halloween mask where a couple of red LEDs light up when I exhale. The sensor will be fairly close to my nose. I'd like the LEDs to be on when I exhale and then turn off almost immediately. I'm OK if there's a little flicker during exhale.
The specs for the thermistors that I've seen have measured response times around 10 seconds. Is there a thermistor that would work? Is there maybe another way to do this? Any suggestions / perspectives welcome.
 

crutschow

Joined Mar 14, 2008
25,686
If you use the smallest physical thermistor you can find, it may respond fast enough to give a detectable change in the time-frame you need.
10 seconds is the time to settle close to the final reading, not for just a detectable change.

The largest signal will be with a constant current drive for the thermistor.
This can be done by using it as a collector load for a BJT (transistor) with a current bias of the transistor appropriate for your supply voltage (such as a 9V transistor battery) and the nominal thermistor resistance.

Do you have an oscilloscope so you can look at the signal?
 
Last edited:

joeyd999

Joined Jun 6, 2011
4,434
An option is to run the thermistor at constant temperature, and measure the power required to maintain that temperature.

The total power will be a function of the difference between the thermistor and ambient temperature, and the amount of airflow past the thermistor. A setup like this will respond quickly to airflow changes.
 

ericgibbs

Joined Jan 29, 2010
11,162
Hi Dick,
I wasnt suggesting the user develop individual nostril control, in using a pair in a bridge, just when he when 'nosed out', either one or both of the therms would generate an output.

Ha ha, you made me smile Thank you, ;)

E
 

jpanhalt

Joined Jan 18, 2008
10,531
An option is to run the thermistor at constant temperature, and measure the power required to maintain that temperature.

The total power will be a function of the difference between the thermistor and ambient temperature, and the amount of airflow past the thermistor. A setup like this will respond quickly to airflow changes.
That is the principle of thermal conductivity detection for GLC (gas-liquid chromatography). Water vapor or organics will respond differently from simple changes in airflow (e.g, from a slight breeze).
 

joeyd999

Joined Jun 6, 2011
4,434
That is the principle of thermal conductivity detection for GLC (gas-liquid chromatography). Water vapor or organics will respond differently from simple changes in airflow (e.g, from a slight breeze).
Yes. It is also the principle for a lot of other gas-property related measuring techniques.
 

Ian0

Joined Aug 7, 2020
768
Drive it with a constant current source as suggested, then put a series R C across the thermistor, say 1M and 1uF. A comparator with its inputs connected to either side of the 1M resistor will detect any sudden change in the thermistor resistance and ignore any long term variations.
 

Janis59

Joined Aug 21, 2017
1,232
1) RE:""An option is to run the thermistor at constant temperature, and measure the power required to maintain that temperature"" That is the way how Pirani vacuum meter is organized. In the result, from 10 sec it arise to 10 milicec.
2) Other option is to diminish the mass. For example digital i2c thermometerfor 16 bits TSYS-02 (~1 dollar) taking the 2x2 mm in 1 mm thick. Sense the proximity just from the skin at about half meter in the last digit. The breath in first digit. Month ago I enjoyed that.
 
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OBW0549

Joined Mar 2, 2015
3,566
An option is to run the thermistor at constant temperature, and measure the power required to maintain that temperature.

The total power will be a function of the difference between the thermistor and ambient temperature, and the amount of airflow past the thermistor. A setup like this will respond quickly to airflow changes.
(emphasis added)

That's what I observed when fooling around with an anemometer design a few years back, using a de-enveloped 7 watt tungsten lamp as a self-heated airflow sensor:
anemometer.png
As I recall, I sized R1 to keep the lamp filament at about 200 °F. Response time was very short, since the operation of the circuit did not depend on the filament's mass having to actually change temperature in response to changing airflow.
 

joeyd999

Joined Jun 6, 2011
4,434
(emphasis added)

That's what I observed when fooling around with an anemometer design a few years back, using a de-enveloped 7 watt tungsten lamp as a self-heated airflow sensor:
View attachment 218879
As I recall, I sized R1 to keep the lamp filament at about 200 °F. Response time was very short, since the operation of the circuit did not depend on the filament's mass having to actually change temperature in response to changing airflow.
https://forum.allaboutcircuits.com/threads/constant-current-hot-wire-anemometer.63204/post-432202
 

Thread Starter

sneakingcactus

Joined Oct 4, 2020
3
@sneakingcactus We could use some guidance on what kind of restraints you have on (spatial) volume and circuit complexity.
Thank you all for your ideas and tips!

@DickCappels , as to circuit complexity, I'm not wanting to spend a lot, and as I say I don't build circuits very often. Nor do I have much to salvage from. What I build I usually find a diagram for online. So, the simpler the better. Spacially: I'd like the circuits to sit between the cheeks of the mask and my own cheeks, though I could include a mall box that at on the strap behind my head. There will be a second circuit (already found) that will light up the mouth when I talk. Mounting a sensor for each nostril is do-able, if that's more effective. A board that's 4cm x 5cm would be easy to hide, though I'd be tempted to solder wires together, test the circuit and then mount it inside the mask as a (tangled) string of components. Is that helpful?
 
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