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Project Notes

#252 555Timer/MessageWaiting

Two-LED flasher with a 555 and minimal components - turned into a “message waiting” indicator with some free-wired SMD construction.

Here’s a quick video demo:



This is a variation of a bi-polar LED flasher circuit that uses two single LEDs. Since that’s not very interesting, I free-wired it with surface mount components to make a little “message waiting!” indicator. Putting a card in the slot turns on the LED flasher.

The BI-POLAR LED DRIVER Circuit published by 555-timer-circuits described how to flash a bipolar LED with a 555 timer chip.

I haven’t come across a true bipolar LED in a very long time, so I first simulated it with two LEDs back-to-back on a breadboard. But due to the 220Ω + 220Ω bridge across the power supply, it drew over 20mA.

By replacing the bipolar LED with two LEDs, and spliting the in & out paths, the effect is the same, but current draw is 25% less (~15mA).

Circuit Modifications

  • replaced the RC timer circuit with 1µF & 220kΩ. This slows the flashing (was a little fast) and means I can use a small SMD ceramic capacitor instead of an electrolytic package.
  • tap the green LED with 1kΩ, to even the apparent brightness of the red and green LEDs.




Testing on a breadboard:


Free-wire/SMD layout and construction:


Finished “Message Waiting” gadget. Putting a card in the slot turns on the flasher.


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.