Police siren NE555 heats, strange schematic

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mensbufo

Joined Jul 4, 2022
2
I found this police siren schematic on the internet. At first it seems really strange to me that on the first ne555 pin 2 and 3 are not marked, but I build something similar in falstad, connected pin 2, 3, 6 on the first ne555 and it worked??? But when I build it in real life, it doesn't work, just squeal and two ne555 got really hot. Is there problem with this circuit, because it seems strange. I really tried to check all possible misconnections and all the details before I solder it. Thanks for help!
 

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sghioto

Joined Dec 31, 2017
4,203
Add R2 and C5 as shown below. What is your supply voltage? Chips should not even be warm.
Siren sound is a High-Low tone.
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Last edited:

Audioguru again

Joined Oct 21, 2019
5,820
Before the siren was am odd buzz with the sawtooth waveform from the 555 capacitor slowly raising then lowering the loudness with PWM. Edited.
Now the siren is a European high, low, high, low frequency.

The 1uF output capacitor has a very low capacitance then drives only high frequency squeaks into an 8 ohms speaker. But a 555 cannot drive an 8 ohms speaker anyway.
 

AnalogKid

Joined Aug 1, 2013
10,308
I found this police siren schematic on the internet.
What are you trying for? Do you want a European-style siren that switches back and forth between two steady frequencies, or an American style that sweeps up and down between two frequency limits?

Also, please post a link to the original schematic page.

ak
 
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