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Circuit digram

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Gregory said:
The two Thermistors are for heat detiction to turn the 555 off at the set temperature.
But they connect pin 2 of the 555 to the positive supply all the time, so the 555 won't do anything.

Why do you have two thermistors in parallel?

Instead of the 555 you should use an opamp wired as a comparator. Its output would be continuous when the temperature exceeds the set amount. If the sound from the piezo buzzer is annoying then it can be turned off temporarily with a flip-flop circuit and your pushbutton.
 
Why do you have two thermistors in parallel?

he asks this because a thermistor is a resistor whose resistance is based on temperature.
Placing two in parallel cuts the resistance in half, and therefore, double the current will enter pin 7.

This will be a problem if the resistance is too low.

It might be better to remove one thermistor and connect a potentiometer, in series with one thermistor.
 
MStechca,
The paralleled thermistors connect pin 2 (not pin 7 as it looked like) of the 555 to the positive supply. They are wrongly marked "100k" in the upper left of the schematic.
Pin 2 of a 555 is the base of a PNP darlington transistor so the thermistors won't have any current and the 555 will do nothing when the resistance changes.
The thermistor has such a low resistance that a pot in series to make a voltage divider would smoke and the massive current through the thermistor would heat it much more than the ambient ever will.
 
This circuit is a mess. If you want to use trigger comparator you must know that it has trigger level set to 1/3*Vcc. When pin 2 voltage goes below this level the output goes high.
To use this thermistor in this circuit it will be neccessary to connect a 50ohm resistor from pin 2 to ground for a trigger temperature of 20°C. Output will go high when temperature drop below 20°C.
But, current of such potentiometer composed of thermistor and resistor is about 80mA at 20°C and 190mA at 60°C. This current will heat up thermistor (Pth=300mW at 20°C) too much and regulation will be lost.
 
bloki said:
This circuit is a mess.
we agree.

I think that we should start in sections, so that troubleshooting is easier.

To start, go to:

**broken link removed**

and go to the 555 timer tutorial.
When you have the 555 timer working, tell us, and then we can proceed to the next section.
 
Hi,
The buzzer is connected reversed too. The red wire is normally + and the black is -.

A comparator circuit with hysteresis should work better for this application.

TOK ;)
 
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