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Just want a pulsing led

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Thejcnr

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20190524_193606.jpg
I'm an electrician but not great with small electronics so I could use some help designing a circuit. This is what I would like to accomplish: When a thermistor temp is raised by my hand an LED slowly increases brightness. When my hand is removed the LED dims to off. I measured the thermistor. At room temp it is 10k ohms, when touching my hand it lowers to 7k. The LED has a forward V of 3-3.2V.
I believe the schematic would work but I can't figure out the appropriate resistor values. I would prefer it runs on a 9V battery but that can change
 

Tony Stewart

Well-Known Member
Most Helpful Member
118481

This seems to work with some added features.
When it starts to blink at a slow heart rate speed, it speeds up as it gets warmer.
 

eTech

Active Member
View attachment 118471
I'm an electrician but not great with small electronics so I could use some help designing a circuit. This is what I would like to accomplish: When a thermistor temp is raised by my hand an LED slowly increases brightness. When my hand is removed the LED dims to off. I measured the thermistor. At room temp it is 10k ohms, when touching my hand it lowers to 7k. The LED has a forward V of 3-3.2V.
I believe the schematic would work but I can't figure out the appropriate resistor values. I would prefer it runs on a 9V battery but that can change
Hi

Suggestion...

Its up to you....but it seems like it would be difficult to see the difference of intensity. Wouldn't it be more useful if the LED turned steady on, or steady off, or blink, when some temperature threshold was reached?

eT
 

AnalogKid

Well-Known Member
Most Helpful Member
It is not clear from your schematic which part represents the thermistor, or what the function of the left-facing transistor is.

ak
 

Thejcnr

New Member
eTeck - I think you may be right. There may be a small amount of fading just because the nature of a thermistor and transistors anyway. I assume I can just make a voltage divider circuit to trigger the transistor?
 

Thejcnr

New Member
AnalogKid - the thermistor is the potentiometer and 10k resistor in the top left. Doing the best I can with my program. The backwards transistor is to fade out the LED. Otherwise the cap discharges too quickly to notice the effect
 

Thejcnr

New Member
Tony Stewart - thank you for the time spent designing that. I think the not gates may be beyond my ability to create. Unfortunately I'm also limited on supplies and space. The circuit needs to fit in a 1" copper pipe. And I'm in Small Town Iowa. There isn't an electronics store for 100 miles.
 

eTech

Active Member
eTeck - I think you may be right. There may be a small amount of fading just because the nature of a thermistor and transistors anyway. I assume I can just make a voltage divider circuit to trigger the transistor?
Perhaps we can change the design and use a comparator as level detector to light an LED. The LED could be a one that flashes on its own, or lights steady on. Your choice. :)

How do you plan to build the circuit?

eT
 

Thejcnr

New Member
Uh oh. Looks like I'll be googling comparator. The circuit is to light up the head of a cane when my hand is on it. So I would prefer the light to be steady on
 

eTech

Active Member
Uh oh. Looks like I'll be googling comparator. The circuit is to light up the head of a cane when my hand is on it. So I would prefer the light to be steady on
Hi

This is a first pass just to show the concept.TempSensor.png
The comparator does what it sounds like.. :) It compares a voltage on the (+) terminal with the voltage on the (-) terminal.
When the voltage on the (+) terminal is less than the (-) terminal, the output turns "on" (meaning it "grounds" the output).
The LED turns on when the output of the comparator is at "ground".
Potentiometer P1 sets the threshold for turning on the LED. Its set to about 3.7 volts so as to turn on the LED when the thermistor resistance decreases to below 7k. The threshold is adjustable.

Let me know what you think..

eT
 
Last edited:

AnalogKid

Well-Known Member
Most Helpful Member
Re-reading your posts, things might have changed. What do you mean by "pulsing"? Do you want the LED brightness to

a) ramp up and down with changes in temperature

b) snap on and off as the temperature crosses a threshold

c) go from dark to flashing as the temperature crosses a threshold

d) other

ak
 

eTech

Active Member
Do you want the LED brightness to

a) ramp up and down with changes in temperature

b) snap on and off as the temperature crosses a threshold

c) go from dark to flashing as the temperature crosses a threshold

d) other

ak
See post #11
 

Thejcnr

New Member
AnalogKid- ideally I want the light to fade on when the temp is above a threshold and fade off when it is below the threshold. However I'm a beginner when it comes to electronics and I'm limited on the parts I can get. So I'm looking for something simple. A light that simply turns on above a temp threshold is good enough for now.
 

Thejcnr

New Member
eTech. Thank you. That's simple and adjustable. Perfect. Couple questions. I'm assuming "RTH" is the thermistor? What is "RS"?
 

eTech

Active Member
eTech. Thank you. That's simple and adjustable. Perfect. Couple questions. I'm assuming "RTH" is the thermistor? What is "RS"?
Rs is a current limiting resistor. It prevents the thermistor from overheating which would affect its accuracy.
Do you have a part number for the thermistor your using?

eT
 

Thejcnr

New Member
Rs is a current limiting resistor. It prevents the thermistor from overheating which would affect its accuracy.
Do you have a part number for the thermistor your using?

eT
Unfortunately I do not. It came with another kit I got quite a while ago. Is it safe to solder directly to the LM339N or should I get a socket for it?
 
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