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Thermistor - comparator circuit doesn't work as it should

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shachar85

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Hi

I'm trying to build a circuit which will alert me when the temperature rises above -4 deg.(in the fridge...of course).

I built the circuit, as the image attached, just that I used LM358 instead of LM311.

at first I used the IRF530, but it didn't work, I don't know why.
so I used a c945 instead, and it works. just that when I measure volts on the 33Kohm (even without the buzzer,if it matters). I get about 2.3V instead of 4.5V as I thought I should.

Can anyone tell me why is that? and why doest the IRF530 didn't work?

Thank you
 

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Hi,

Looks like you dont have enough drive voltage. At 4.5v supply, you're lucky if you get 3.5v out of the LM358. You probably need a 'logic level' MOSFET rather than a run of the mill part.
If you like, you can measure the output of the LM358 to verify that it is very low. Even the full 4.5v might not do it. A logic level device would work though.
 
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I measured and got about 3.5V.
I tried to use 12V before, but I think it ruined the first LM358,cause it doesn't properly anymore.
will 6.5V be enough?
and why do I get only 2.3V using the c945?
 
Looking back to your original circuit the LM311 would have been the best choice as it is a comparator. Using an operational amplifier like the LM358 with the low VCC of 4.5 volts will give you at best an output high of VCC - 1.5 volts or around 3 volts. That is not going to drive the gate of the IRF530 MOSFET. This is why MrAl suggested the use of a logic level MOSFET that is designed for a low gate voltage (around 5 volts). I have no idea why you had problems using 12 volts which would still likely not get the MOSFET into saturation.

How did you derive the 33 KΩ resistor in series with your buzzer? With a VCC of 4.5 volts that 33K seems high but then again I don't know your buzzer specifications?

If you want to remove and replace the MOSFET with the C945 NPN transistor and use the operational amplifier LM358 I would increase the VCC to 12 volts and try a 2.2 or 3.3 KΩ base resistor. Then choose your current limiting resistor for the buzzer based on the buzzer specifications. The buzzer specifications will determine the collector current for the transistor which will in turn determine the correct value for the base resistor.

Overall I would use a LM311 comparator as your original drawing and remove and replace the MOSFET with a NPN transistor.

Ron
 
oh...the 33Kohm is not calculated yet. I'm gonna put there led as well. I'll calculate it once everything else works.

I don't know why, but the circuit didn't work at all with the LM311. maybe I didn't connect everything right. I switched to the LM358 quickly.

I wonder if I could change the transistors with a relay. I think it will make everything much more simple... won't it?
 
If I can trust my memory the LM311 is one of those copmparators that needs a pullup resistor on its output (see my post).

The LM311 does indeed require a pull up resistor.

This link gives some very good examples of using comparators to do what you are looking to get done. You may want to give the link a read and scroll through it.

Ron
 
Hi shachar85,

Replacing the transistor with relay will only complicate issue on ground. Leave the ckt as it is, increase the applied voltage and make sure ur R5 value is calculated base on the buzzer ratings (voltage and current).

Good luck
 
Hi again,


I really dont think the LM358 is the real problem here, the problem is the low voltage power supply. It wont drive a standard MOSFET properly.
As Ron was nice enough to point out too, the 33k resistor for the buzzer really does seem kinda high. Also, we dont know what kind of buzzer this is. Is it really a self contained buzzer (with internal oscillator) or is it just a piezo? piezo would require an external oscillator...just thought i would mention that just in case it is. DC operated buzzers most likely take more current that a 33k resistor would allow, and so you should check the required drive voltage for the buzzer if it isnt really a piezo. A DC operated buzzer could take anywhere from 10ma to 100ma or even higher at a specified voltage like 5v, 6v, 10v, 12v, etc. We have to know the spec's of the buzzer to recommend something more precise.
 
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Hi guys
I finally got to check the circuit today. I changed to input to 6.5V and used the IRF530 and everything worked great!
I stay with the LM358 because its working and its already there.
I used a 200ohm resistor with the buzzer and its fine. even 1.5Kohm was too much.
Now all I have to do is add a led and solder everything up. peace of cake.
thanks for all the help!
 
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