Circuit Design Question - Photo Sensitive Led with fade out timer
  • I'm not technically a noob, just been away from this craft for a while and want to get into IOT designing.   I've seen the designs for the 'Photosensitive LED Throwables' and I was curious about what it would be needed for a circuit that would, using a battery like a CR2032 turn on an LED in the dark, have it lit for a few minutes, then fade out.  A slight variation would be to also have it (when dark) randomly light up (slow fade) for a few seconds then fade out.  I'd like to accomplish this with the smallest footprint and simplest circuitry.  Any advice?
  • A low power microcontroller -- possibly a 6 or 8 pin PIC or AVR is probably your best choice for this, if you only need one input and one output. 

    That said, "simplest" is relative. The circuit is very simple (a chip, the LED, the battery, and the sensor-- possibly with a resistor added), but the microcontroller would require programming, and you would need a programming adapter that can connect to it. (This can be as simple as extra blank locations in the circuit board, designed to mate (temporarily) to a programming adapter.)  

    There are other ways to do things like this as well-- for example a "one-shot" (monostable multivibrator) circuit based on a low-voltage 555, such as the LMC555. No programming required, but you'll need additional analog components to configure it, you'll have less output headroom to drive the LED, and you'll potentially spend more power when the LED is off.
  • I'm a bit late to the show on this one but this was my solution to a similar problem:

    It's a throwie consisting only of an LED, an ATTiny85 and a CR2032.  The trick is that the LED can also act as a photodiode and the ATTiny has an analogue voltage sensor so you can read the ambient light level without any additional parts.

    Message me on instructables (or here if the facility exists - looks like it might) if you want a hand with code that could fade up & down as you were looking for.