Siren schematic: What controls the cycle speed?

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Discussion Overview

The discussion revolves around modifying a schematic for creating a siren sound, specifically focusing on which components can be altered to change the cycle speed of the siren, currently set at 6 seconds. Participants explore theoretical and practical aspects of the circuit design, including the roles of resistors and capacitors in determining the cycle time.

Discussion Character

  • Technical explanation, Exploratory, Debate/contested

Main Points Raised

  • One participant suggests that either capacitor C1 or C4 could be altered to change the cycle time but expresses uncertainty about the schematic's operation.
  • Another participant asserts that changing resistors R1, R2, or capacitor C1 to smaller values would decrease the period of the cycle, recommending a specific ratio for the resistors to maintain the up to down ratio.
  • A claim is made that reducing the capacitor value would lead to a quicker alarm rate, with a specific example of using a 25 uF capacitor to achieve a total cycle time of about 2 seconds.
  • Participants mention the use of calculators and formulas for determining frequency based on resistor and capacitor values, providing a link to a resource for further calculations.
  • One participant suggests simulating the circuit in LTSpice, noting the availability of the NE555 timer component.
  • Another participant expresses interest in using an Arduino for the project but indicates difficulty in finding relevant information.

Areas of Agreement / Disagreement

Participants present multiple viewpoints on which components affect the cycle time, with no consensus reached on the best approach or specific values to use. The discussion remains unresolved regarding the optimal modifications to achieve the desired cycle speed.

Contextual Notes

Participants do not fully agree on which components should be altered, and there are varying opinions on the implications of changing specific values. The discussion includes assumptions about the circuit's operation that are not explicitly detailed.

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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?
 
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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.
 
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Wow, that's lots of information; it should be just what I need. Thanks!
 
You could simulate the entire circuit in LTSpice; they have the NE555 timer under misc components.
 
Bob S said:
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.
 

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