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Project Notes

#059 Two-tone Doorbell

A basic two-tone doorbell using a 555 timer oscillator, with n-channel FET for power conservation.

The Build

Here’s a quick video of the circuit in action:



When the button is pressed, it triggers three actions:

  • 555 timer reset (4) is pulled high and charges the R4/C2 RC circuit
  • it bypasses R1 (via D1) so that 555 astable behaviour is governed by R2/R3/C1
  • it pulls Q1 gate high and charges the R5/C4 RC circuit

While the button is down (“ding”), the frequency of 555 astable oscillation is governed by R2=47kΩ, R3=47kΩ and C1=33nF, which results in a frequency of about 309Hz.

When the button is released:

  • R1 comes into play, reducing the 555 astable oscillation
  • 555 remains triggered while C2 drains through R4

The “dong” frequency of 555 astable oscillation is governed by R1+R2=94kΩ, R3=47kΩ and C1=33nF, which results in a frequency of about 232Hz.

The duration of the “dong” depends on the R4/C2 RC time constant (𝛕), 220ms. When the voltage on pin 4 falls below the reset threshold (typically 0.5V), the oscillation stops.

The circuit remains “live” while Q1 permits drain-source current flow. Q1 will remain on while C4 discharges and maintains gate voltage above the gate threshold voltage.

A couple of bypass capacitors minimize the impact of voltage spikes and noise on the operation of the 555 timer, especially during transitions of the timer’s output transistors:

  • 100nF (C5) between control pin (5) and ground
  • 100nF (C6) across the power supply


The Breadboard

The Schematic

Testing on a breadboard:


Protoboard Build

OK, this is a pretty crappy doorbell, but just for the heck of it I put the circuit on a protoboard and used as a short-term replacement for a corroded/failed bell.

Protoboard build

Protoboard layout

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.