What is the role of NE555 in this fire alarm circuit?

Thread Starter

christw16

Joined May 31, 2018
21
Here is the diagram of fire alarm
when I use lighter to burn at the thermistor, the speaker rings

In this circuit, what is the role of ne555?

Thank you!


upload_2019-6-1_12-23-52.png
 

wayneh

Joined Sep 9, 2010
16,114
It provides a tone to drive the speaker and make noise. The transistors enable the timer, allowing it to run.
 

absf

Joined Dec 29, 2010
1,942
T1 is NPN transistor BC548 and
T2 is PNP transistor BC558.

Both will conduct when the thermistor is heated.
 

DickCappels

Joined Aug 21, 2008
5,938
Like absf said in post #4, T1 is an amplifier while T2 switches on when current from T1 is high enough, enabling the NE555 to oscillate, producing the tone that wayneth mentioned in post #1,
 
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