Minimalist LED Light Detecting Candle


Our flickering LED circuit combined two known circuit, and certainly wasn’t graceful because of it. [sprite_tm] saw quite a few areas where the circuit could be reduced. He ended up taking it down to just two LEDs, a battery, and an ATtiny13. The first step was getting rid of the current limiting resistors. The datasheet shows that with a 3V supply the AVR will limit the current well below the maximum current. The light sensor was removed next. [sprite_tm] referenced an earlier post on sensing with LEDs. He measures the voltage across one of the LEDs while it is off to see how much light is hitting it. The current draw while on is 10mA and 50uA while off.

17 thoughts on “Minimalist LED Light Detecting Candle

  1. Really slick! Nice work. Add a timer to the micro so that the LED’s turn off ~4 hours after dusk and it would make a great internal replacement for controlling solar garden lights. These things run all night and are usually dead the next morning. Your attiny 13 with a timer would leave energy in the battery for another night of accent lighting despite a cloudy day.

  2. How is the LED sensing performed here ? , all the material I’ve read on it requires that the LED is switched on , shutoff and then timed which doesn’t seem to be the case here.

  3. what the heck are lead’s? Those look like L.E.D’s to me.

    Why is it that most hobbiests cant pronounce the acronyms? It’s pronounced. Ellll EEEEE DeeEe
    not lead.. See what happens when you guys dont go to college?

  4. @franklyn: LEDs are essentially photodiodes. They put out a small current when they’re exposed to light. Sprite is just using that fact in this project. The light-then-wait approach uses a different characteristic of the LED, something more akin to a CCD.

    @fartface: It’s spelled “hobbyist”.

  5. Now _this_ is an LED hack.
    Go go go!

    1. Using the same LEDs as photosensitive devices (yes I know it’s a well known old idea (though many of us would be still be surprised such a thing could happen))
    2. The minimalism of the circuit – the way it makes you smile when you look at the circuit diagram.

  6. Fartface: I’m Dutch, and in Dutch, LED (or any pronounciation of it) doesn’t have a special meaning aside from the semiconductor photon emittor thingies. We all pronounce it as ‘led’ here (as one word) and I probably used it in my English explanation by habit.

  7. Just a quick note: High-Brightness LEDs typically show only a very small voltage as a light detector. Maybe not even enough to perform an A/D reliable on. I’m guessing that is why you see the older type LEDs in this demo. Test your LEDs before use. Of course a cheap old LED that is only used as a light detector is still much cheaper than a cDs cell, transistors, etc.

  8. Made this exact same circuit for having blinking white LEDs all over my house for Christmas. During my tests the 2032 lasted about 6 mo with using the WDT in interrupt mode and having a 6s interval. So a year is pushing it but still pretty good. I don’t have to buy 2032s every year.

  9. If we’re going minimalist, we really only need one LED, right? It’d be easy enough during the flickering to test the ambient light, right?

    I pronounce LED like “lead”, as in pencil lead…

    Though I wouldn’t ever write out lead to refer to a LED… :-)

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