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Project Notes

#045 SimpleSiren

Classic timer circuit producing a two-tone oscillation.


This is a build of a circuit published by electronicshub.

It is allegedly a “ding dong” bell.. sounds more like a siren to me!

It operates with two oscillating 555 timers chained together.

The first 555 provides the low frequency oscillation between high and low output states. The frequency is determined by the values of R1, VR1 and C1. With R1=2.2kΩ, VR1 at 22kΩ and C1=47μF this runs at 0.7Hz and 52% duty cycle.

The pin 3 output of the first time switches the control voltage of the second 555 timer. In practice, with R3=1kΩ in place, it switches from 1.57V to 7.06V, as measured with a multimeter.

The oscilation of the second 555 drives the output peizo, and the frequency is modified by the control voltage.

1.57 high 7.06 low

When Vcontrol = 1.57V, the frequency of the oscillation is 566 Hz - see the actual calculation on WolframAlpha

When Vcontrol = 7.06V, the frequency of the oscillation reduces to 248 Hz - see the actual calculation on WolframAlpha.

The derivation of the fomula considering a control voltage is explained in a question on EE.SE. The formula with variables as named in this circuit is:

f = 1/( C2.(R2+VR2)ln(1 + Vcontrol/(2(Vcc - Vcontrol))) + C2.VR2.ln(2) )


The Breadboard

The Schematic

Breadboard Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.