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Project Notes

#201 555 Timer Doorbell Kit

Another two-tone doorbell using a 555 timer oscillator

The Build


I picked up this kit on special for $0.66. It even came with extra components, so a bit of a bargain impulse-buy.

The kit turns out to be a simplified version of the TwoToneDoorbell circuit I’ve built before.

It lacks the power control circuit, and draws about 4mA when idle - so if you actually use it, expect to be replacing batteries on a regular basis.

The basic operation is similar. When S1 pressed:

  • 555 timer reset (4) is pulled high and charges the R4/C1 RC circuit
  • it bypasses R1 (via D2) so that the 555 a-stable behaviour is governed by R2/R3/C2
  • speaker is driven with a 102.128 Hz square wave

When S1 released:

  • R1 is added to the 555 a-stable biasing
  • speaker output modultes to 76.596 Hz

The duration of the “dong” depends on the R4/C1 RC time constant (𝛕), 470ms. When the voltage on pin 4 falls below the reset threshold (typically 0.5V), the oscillation stops.


Here are some traces of the circuit in operation with 3 channels on a scope:

  • CH1: monitoring the C2 charge (pin 2, 6 input)
  • CH2: monitoring the pin 4 reset charge/discharge
  • CH3: monitoring the pin 3 output

Here’s a trace of the initial trigger shwoing the reset line coming high, and the oscillation kick-off:


And on a wider scale - though hard to see the details - here’s a complete “ding-dong” cycle. The main point to note is the reset discharge over about 1.2s before the 555 shuts off:




The Schematic

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.