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Can someone please explain how this circuit works?

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I was digging thru some old files left here by a former employee and found this.  There was a text file with it that said it was a temperature alarm that was supposed to have a green light when the temp was between 50 and 100F, but a red light at any other temperature.  I can play with it and see that the lights change, but how do I verify the temperatures?  Also, how does this circuit work?  I have never done anything like this on my own but I can see that it is looks to be a comparator being fed by two OPAMPS.  Can one of you please shed some light on this so I can prove it works?  I have a use for this but I need to figure out if this will work as it stands or if I need to make some tweaks.  

 

 

 

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Hi, 

 

have you tried to run it on your computer? it compares the value from the thermistor to show an alarm using the LEDs at the end. It looks like it campares the signal on a window parameters if it is too low that gives you the alarm (red LED on). The same thing if it is to high. Other case if the temp is ok the green led should be on. 

 

Regards. 

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Solution
Accepted by topic author ssnapier

While that circuit might work in simulation it has several flaws which would cause problems if implemented in hardware.

 

First, thermistors typically have a nonlinear inverse relationship to temperature. In this case that is not a huge deal since temperatures too high or too low are both supposed to turn on the red LED. However, setting the comparator thresholds at 1/3 and 2/3 of Vcc2 are not very likely to correspond to the desired temperatures.

 

Next, the LM324 has a totem-pole output circuit so R4 and R5 are rather meaningless.  The output circuit can source or sink tens of milliamperes so the base emitter junctions of Q1 and Q2 may be vulnerable to damage.

 

When both Q3 and Q4 are off as shown in your image, the current through the green LED is 30 mA. Indicator LEDs are usually rated for 20 mA or less.

 

When Q3 or Q4 is on, the current through the red LED is limited by the transconductance of the transistors or the V-I characteristic of the LED. In either case the values are almost certainly in the "let the magic smoke out" of the LED range.

 

Although not destructive, Q3 and Q4 are not necessary.  The logic can be changed by reversing the inputs to the comparators and perhaps changing from OR to AND on the transistor connections.

 

Lynn 

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