# Help with calculating Hz in siren curcuit

Discussion in 'General Electronics Chat' started by SainAralor, Oct 9, 2016.

1. ### SainAralor Thread Starter New Member

Oct 9, 2016
2
0
Hi! Could anyone help with how to calculate Hz in a siren system? Here is a picture of the system:

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:

(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!

Last edited by a moderator: Oct 9, 2016
2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
6,424
1,017
481 Hz central frequency, it will sweep up and down at 1hz rate...

3. ### SainAralor Thread Starter New Member

Oct 9, 2016
2
0
Could you explain that a bit? Sorry i'm quite new to this

4. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
3,070
1,819
Please provide a 'clean' link! -- Speaking for myself, I have no desire to join another site nor expose myself to their spam, etc. merely to view your images/'outside' post{s}! Advice and instruction are offered gratis here - inasmuch as basal courtesy (on your part) costs nothing!

With regards
HP

Last edited: Oct 9, 2016

Apr 5, 2008
16,986
2,966
Hello,

@Hypatia's Protege ,

Bertus

Hypatia's Protege likes this.
6. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
3,070
1,819
@SainAralor
For the benefit of other readers I've done your work and posted the image (below):

What would you have us clarify? The second stage's 'control voltage' input (and, hence, output frequency) is 'swept' by the first stage...

With regards
HP

7. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
3,070
1,819
Thanks @bertus -- I made my second response prior to seeing your amendment to the OP

Best regards
HP

8. ### Aleph(0) Member

Mar 14, 2015
423
592
HP that brings back memories for me cuz when I was a kid I used switched DC levels on pin 5 of astable 555 for FSK generator and it ran error free at 19.2k baud with just ps osc cap but I had ten pole RC LPF on output! Now everyone is probably saying I was super industrial strength stupid for not just using XR2206 with onchip digital keying circuit and sine output! So I was just having fun building RS232 to FSK translator w/o special chips

9. ### Dodgydave AAC Fanatic!

Jun 22, 2012
6,424
1,017

The main frequency astable is centerd on 481hz, without the 1 hz astable it would make a continuous tone, the second astable is clocking at 1 hz and increases and decreases the 481hz, so it may sweep from 400hzto 600hz due to the control voltage on pin5.

Mar 1, 2015
3,070
1,819
Go figure!

TTFN
HP

11. ### ci139 Member

Jul 11, 2016
343
39

blaah, blah , . . .