# How does this temperature alarm circuit work

Casn you please help me understand how this temperature alarm circuit works? I am a bit over my head here, but I would very much like to understand this.

If you would like to look at the multisim file it is here: http://www.3sixty-solutions.com/mystery.zip

It is supposed to trigger alarms (red LED) for temperatures above 100F or below 50F. I have no idea how to prove this, but I can move the slider on the thermistor and see that from 34% thru 68% the green LED is activated.

• Looks like it's using LM324 op-amps as comparators. Honestly, redrawing the circuit yourself without the MultiSim XMM nodes might help you considerably. The thermistor being connected like a potentiometer makes me suspicious because most thermistors are a single variable resistor, but their are three-terminal thermistors (multiple resistors). Jul 17, 2016 at 3:43
• Agreed. Leave that image there but add a plain schematic using the button on the editor toolbar. Jul 17, 2016 at 8:42
• Also, if you expect folk to download a zip file forget it. Jul 17, 2016 at 9:07
• It is not a large zip file, it is 199kb. Jul 17, 2016 at 10:50
• @ScottN. It's not the file size that's the problem, it's the extra step for every one of us that reads it (and as a result that won't be many). If you do the work and include the information in your post it only has to be done once. The other concern is virus payload. We barely know you! Aug 16, 2016 at 13:11

Most thermistors are just temperature dependent resistors. They are combined with a normal resistor to form a voltage divider with temperature dependent output.

In your schematic, a potentiometer is used to simulate the voltage output.

The resistors R1, R2, R3 also act as voltage divider, giving two fixed voltages used as threshold references.

The two operation amplifiers act as comparators and give 5V when the level at the +-terminal is higher than that of the --terminal, otherwise they give 0V.(If they are rail-to-rail types. Otherwise they don't reach 5V or 0V exactly, but this doesn't matter here.)

For example, the upper OpAmp gives 0V when the output voltage from the thermistor exceeds the threshold. This opens Q2 and this in turn closes Q4. So, the left side of the LEDs is connected to GND, and since the right is at VCC2, the red LED is on.

The lower OpAmp does the same, but turns the red LED on when the voltage is below the threshold.

When the voltage is between the threshold levels, Q3 and Q4 are open, so the left side of the LEDs is connected to VCC3=12V, and the green LED is on.

To avoid confusion: I treat transistors like switches. They must be closed to conduct current (Others see them like doors: must be open)

Finally, I agree with the comments: Remove all multimeters and clean up the schematic. The circuit is quite simple, but the schematic is confusing.

EDIT:

where would I measure it to prove that? Also, I am still not clear as to how I prove what the temperature range is where the circuit would trigger the green LED. Is that specific to the thermistor or is there a formula I could use to figure it out? e:

As said, typical thermistors are just resistors with temperature-dependent value. One class of thermistor is the NTC, and their behavior is described for example by the B-formula:

$R_{NTC}=R_0\cdot e^{B\left(\frac{1}{T}-\frac{1}{T_0}\right)} \quad \Leftrightarrow \quad T=\frac{B\cdot T_N}{B+\ln(R_{NTC}/R_0)\cdot T_0}$

$R_0$ is their resistance at $T_0$. $T_0$ is given in Kelvin and typically is 25°C=298.15K. B is a value typically in the range of 2000...5000. See the datasheet for this values.

Next, the thermistor typically forms a voltage divider with a normal resistor. You should be able to calculate the output voltage for a given temperature / the temperature for a given output voltage.

And the OpAmps compare that voltage to the voltage at the node between R1 and R2 as well as R2 and R3.

We can't tell you more, because the schematic doesn't give a hint how the voltage depends on temperature.

• Here is the circuit stripped of meters. i.imgur.com/WnaeExX.png I appreciate the explanation above, but where would I measure it to prove that? Also, I am still not clear as to how I prove what the temperature range is where the circuit would trigger the green LED. Is that specific to the thermistor or is there a formula I could use to figure it out? Jul 17, 2016 at 11:25
• One more question, is there a way to limit this? The chart I am looking at puts this thermistor outside of the range I need. According to this chart the percentages where the green LED turn on correspond to a range of 94 - 127 degrees. I need the lower end of the to be 50 degrees, but according to this chart that would be 19.9k which makes no sense on a 10k device. Jul 17, 2016 at 14:59
• What chart are you looking at? I can't see it. The potientometer is just a voltage divider. A 100k poti would work, too. And adjustment of the thresholds is done by choosing the right R1,R2,R3 Jul 18, 2016 at 8:37