Siren schematic: What controls the cycle speed?

[cf8]

I found http://www.uoguelph.ca/~antoon/circ/wailing.htm" schematic for creating a siren sound, but I can't figure out which component could be altered to change the cycle time of 6 seconds.

If I had to guess, I'd say either C1 or C4, but I'm really lost as to how this schematic works, so I don't know. Also, am I correct in thinking that the smaller the value of the key capacitor, the shorter the cycle time?

 

[vk6kro]

You would change R1, R2 or C1. making any of them smaller makes the period shorter.
If you change the resistors, you should keep them in about the same ratio (about 1 to 10 ) if you want the same up to down ratio as it has now.

I would make the capacitor smaller if you want the alarm rate to be quicker. 6 seconds does sound a bit slow.
25 uF would give about 1 second up and 1 second down so 2 seconds total.

There are lots of calculators for doing this:
http://ourworld.compuserve.com/homepages/Bill_Bowden/555.htm
seems OK.
They use the formula
Frequency = 1.44 / ( (R1+R2+R2) * C) where resistor values are in OHMS and capacitor values are in FARADS. So, 25 uF is 25 E -6 FARADS on a calculator.

 

[cf8]

Wow, that's lots of information; it should be just what I need. Thanks!

 

[Bob S]

You could simulate the entire circuit in LTSpice; they have the NE555 timer under misc components.

 

[cf8]

You could simulate the entire circuit in LTSpice; they have the NE555 timer under misc components.

Interesting. I might have to try this the next time I boot into Vista. (I primarily use Ubuntu.)
I was originally looking for some way of using an Arduino, but I couldn't find anything.