#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:

Notes

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.

Construction

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

• BI-POLAR LED DRIVER Circuit - 555-timer-circuits
• LM555 Datasheet
• Bi-ploar LED parts - at parts.io
• ..as mentioned on my blog