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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

About LEAP#252 LED555 Timer
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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, 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 (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.