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Help with calculating Hz in a siren curcuit

SaintAralor

Oct 9, 2016
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Hi, this isn't really homework, but this seemed to be the best place to post it :)

Could anyone help with how to calculate Hz in a siren system? Here is a picture of the system: http://imgur.com/a/N9E6Q

The Hz of the first astable 555 circuit is supposed to be 1 Hz and the other system is supposed to be 1kHz.

I used this formula: http://imgur.com/a/SZipc (the one on the top called "to calculate frequency")

I tried to put 3.3u as the c value and 0.01u (converted to Farad of course). I also tried to put both in series and in parallel, but i couldnt get 1Hz!

What am i doing wrong here?

All help much appreciated, and thanks for your time!
 

Alec_t

Jul 7, 2015
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What am i doing wrong here?
Putting those caps in series or in parallel is wrong. Check the schematic.
Welcome to EP!

Edit: Replacing the 1meg resistor with 270k will get you closer to 1Hz.
 
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SaintAralor

Oct 9, 2016
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Yea figured as much, but i just tried it anyway because i couldnt get it to work ;)

The schematic as is should get those reults of 1hz and 1khz, and i just don't see how they got that
 
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Alec_t

Jul 7, 2015
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Not off-hand. Have you tried this calculator? It agrees with the LTspice simulation.

Googling '555 timer formula' will answer your question.
 
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SaintAralor

Oct 9, 2016
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Yea thats what i get on my own as well.. Does this mean that my book is wrong? seems weird
 

SaintAralor

Oct 9, 2016
7
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Ive asked on a few other forums as well, and it seems that most agree that it should be 2 and 0.5k, so i guess im rolling with that, and just assuming the book is wrong. Thanks a lot for your help, ive really been pulling my hair out over this ;)
 

Alec_t

Jul 7, 2015
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Not a good book then :(.
 
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