# Help with calculating Hz in a siren curcuit

#### SaintAralor

Oct 9, 2016
7
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
3,641
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.

Last edited:

#### SaintAralor

Oct 9, 2016
7
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

Last edited:

#### Alec_t

Jul 7, 2015
3,641
The schematic as is should get those reults of 1hz and 1khz
The frequencies are 0.36Hz and 327Hz to 1kHz according to LTspice.

#### SaintAralor

Oct 9, 2016
7
The frequencies are 0.36Hz and 327Hz to 1kHz according to LTspice.
Hmm, do you know what formula you would use to calculate that?

#### Alec_t

Jul 7, 2015
3,641
Not off-hand. Have you tried this calculator? It agrees with the LTspice simulation.

Last edited:

#### SaintAralor

Oct 9, 2016
7
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
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
3,641
Does this mean that my book is wrong?
Looks like it. Can you post the formulae that the book uses?

#### SaintAralor

Oct 9, 2016
7
Thats the problem, the book doesnt show what formulae it uses.

#### Alec_t

Jul 7, 2015
3,641
Not a good book then .

Last edited:

Oct 9, 2016
7
Agreed haha

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