Temperature Comparator/Display

[WayneVT]

I'm a little new here, but read your post on Yoshi (from last year) about a temperature display that would sequentially light up as temperatures rose. I am working on a similar problem. I am looking to build a simple comparator to display one LED if the temperature is over X degrees, and also light it up if it falls below Y degrees. I want to have a second light that lights whenever the first is off. I figure I need two comparators (I'll be using an LM339), but I can't figure out how to connect the outputs to get the LED logic I want. Any help?

Thanks.
Wayne

[Rob]

Welcome to the boards Wayne.

Check out this thread: http://forums.linear1.org/index.php/topic,828.0.html

and if you still have questions, holler up. There's even a diagram in there by JustDIY.

[WayneVT]

Thanks, that link was very helpful. I'm sorry, though, that I don't understand how you "swap the setpoints". Does that mean to reverse the two LED's? I know it must be pretty trivial, but I would appreciate that last bit of clarification.

Thanks,
Wayne

[cpemma]

I'm a little new here, but read your post on Yoshi (from last year) about a temperature display that would sequentially light up as temperatures rose. I am working on a similar problem. I am looking to build a simple comparator to display one LED if the temperature is over X degrees, and also light it up if it falls below Y degrees. I want to have a second light that lights whenever the first is off. I figure I need two comparators (I'll be using an LM339), but I can't figure out how to connect the outputs to get the LED logic I want.

Rather than use a true comparator, use an op-amp as a comparator. LM358 is a cheap common dual. If the voltage on the + (non-inverting) input is greater than that on the - (inverting) input the output goes high, otherwise it's low. (A true comparator's output  is low or floating) Then you can put LED A - Resistor A -junction - Resistor B - LED B in series across the supply, with the junction to the opamp output.

On bargraph thermometers, I came across this 20-LED thermometer recently using cascaded LM3914 bargraph chips.

[justDIY]

Thanks, that link was very helpful. I'm sorry, though, that I don't understand how you "swap the setpoints". Does that mean to reverse the two LED's? I know it must be pretty trivial, but I would appreciate that last bit of clarification.

what Rob meant by swap the setpoints was instead of having one led turn on after a certain point, have the other led turn off .... this allows a range where both leds are on, instead of one or the other.   That bit was specific to my application as a "volt meter", however, your temperature display will essetially be the same thing... throw is thermister in series with VR3 and you'll have yourself a thermometer.

Build it up on a breadboard and experiment, I think you'll be able to adapt that circuit fairly easy.

[Rob]

Thanks, that link was very helpful. I'm sorry, though, that I don't understand how you "swap the setpoints".

Yeah, that was just directed at how to get the yellow light to come on in that specific case, nothing for you to worry over.

I agree, get the breadboard out!