EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: mensbufo on February 16, 2023, 07:11:05 pm
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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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It should work. The voltage on C1 controls the frequency of U2.
Treat 555 circuits as two separate oscillators.
What's the waveform on pin 3 of U1? If you don't have an oscilloscope, connect pin 3 to a 1k resistor in series with an LED to 0V. The LED should flash.
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On the first 555, pin 2 is connected to pin 6 (pin 2 is just not marked). Pin 3 should be left unconnected.
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If you are using a bipolar NE555 rather than one of the CMOS equivalents, then it's important to have bypass capacitors connected very close to the supply pins of each device. 100n should suffice.