How does the thermistor activate/not activate a circuit?

Question

I have this schematic which is temperature-dependent and would only make a sound once the heat detected is above the threshold of the NTC thermistor.

Can I get some help with circuit analysis?

How does the thermistor in this schematic stop/not make the buzzer buzz when the heat is below its threshold? Does the Zener diode have a play in this or is it just simply about the op-amp in terms of turning on?

My take is that it when the resistance is high (the threshold heat is not achieved) there will be less current passing through so it won't be enough to conduct the diode. I am unsure with these so please correct me.

Just as an aside -- that LM741 is being used as a comparator. It's usually not a good idea to use an op-amp as a compartor like that. There are perfectly good compartors out there, that are just as cheap as the LM741 and come with a lot less built-in grief for applications like this. — TimWescott, May 15 '23 at 21:53
This group discourages recommending parts -- just find out what you can get where you are, and look at the specifications to see what fits into your circuit. — TimWescott, May 16 '23 at 17:25

score 1 · Answer 1 · answered May 15 '23 at 21:37

1

The IC U2 is an op-amp used to compare two voltages.

One voltage depends on thermistor resistance (i.e. temperature) and the other voltage depends on the potentiometer resistance (i.e. setpoint).

The comparator will output high or low depending on which input voltage is higher, and this in turn controls the NE555 to stop or start running the tone to speaker.

answered May 15 '23 at 21:37

Justme

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what does the diode do here? – Hezekiah Derilo May 16 '23 at 05:09
@HezekiahDerilo I don't know why it is there and why someone who made the drawing put it there. You can simulate the circuit with and without the diode to see what it does. But it has nothing to do with comparing the voltages as it's on the output. – Justme May 16 '23 at 05:29
one last thing would be possible to replace the potentiometers with a resistor? in reference R4, can I replace it with a resistor with resistance that can complement the thermistors resistance? – Hezekiah Derilo May 16 '23 at 07:09
I reckon D1 was included to ensure that the 741's high output (which could be only 10V or so) will not accidentally invoke reset, but without a pull-up resistor on the 555's reset line, it's a liability. It certainly wouldn't be necessary when using a proper comparator. – Simon Fitch May 16 '23 at 07:11
@HezekiahDerilo Of course you can replace R4. It must be of course be replaced with two resistors to be able to choose the output voltage. You can't replace it with a single resistor. – Justme May 16 '23 at 07:16

How does the thermistor activate/not activate a circuit?

1 Answers1