Let's Make Robots!

2 LEDs on only one output

So, this challenge is for anyone!
From analog guys to digital guys.
Only thing you should make is get leds turn on and off alternately, when you switch on and off the power.
You can do it from MCu or from power source with button.

Output off.
Led 1 turns off. Led 2 turns on.
Output on.
Led 1 turns on. Led 2 turns off.

1. It's your solution! Use ready IC's or your own circuits.
2. Only one MCu pin or button.
3. You can make it to breadboard, or make a PCB.
4. Make a video which shows your circuit in action, and how is it connected.
5. No mechanical switches allowed on solution!

Here's some pics:

Analog solution

Digital solution

So nice time to solve this challenge!

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That's the one I'd go for.

That would be ideal for 2 LED control and if you turn the pin into an input you could turn them both off . . . nearly.  You'll get both on dimly if LED1 Vf + LED2 Vf < 5V , but the problem didn't specify they had both to turn off so that's fine. (An extra diode or 2 would probably sort it.)

A simple inverter would do the trick.

Something like a 7404 IC. 


Can you post a schematic or a video for what you have in mind with this?

Simplest solution:SPDT relay.

Yes, you can use what you want, but everyone doesn't have relays.

Wouldn't using a relay violate rule 5....No mechanical switches?


So I guess using a relay would be ok then?   I know it's academic at this point, since it's been solved, but I'm just trying to get a handle on your definitions.......seems like mechanical is mechanical, switch or relay.


The button in the initial diagrams is more of a mechanical switch than a relay.
No, but a relay is a more common component than an AVR or a 74/46 series inverter chip. What is the target of this thought experiment anyway? Does it have a practical application for you or were you just looking for the range of solutions offered to a problem?