Simplest Project Ever? Fairy lights without a series resistor!

Engle

Active member
OK, I know it goes against all best practise, but I still wanted to experiment(play). I’d previously noticed a fair amount of voltage sag on PICAXE outputs under load. So why not check if the forward voltage of LED fairy lights would help the outputs stay within the maximum 25mA sink/source?

I was converting 1 Solar and 4 battery LED strings to mains and each string required about 2.7V AC. The eventual projects were always going to use H-Bridges, which provided a little more oomph, but it was fun to see a 08M2 successfully drive a string without a series resistor.

I can’t bring myself to recommend this approach, but for anyone who may be interested my thoughts and findings for internal resistance of outputs, voltage drops and LED failure modes are shown in the schematic.

The PICAXE managed to raise the current for a set of fairly dim ex-solar power lights from 13mA to 20mA. At 8MHz the 08M2 toggled 2 outputs at 114Hz and in the eventual project the program execution time provided 140us deadbands to prevent shoot-through current on the L9110 PDIP-8 H-Bridge.

Attached are all the files and the master copy is held here .Fairy_lights-Simple_Experiment.png
 
... but it was fun to see a 08M2 successfully drive a string without a series resistor.

I also did this, in one of my projects...

I'd decided I wanted to make the LEDs 'pulse', rather than 'blink' and so I PWM'ed them. I've no idea what thought process, led me to believe that PWM meant no series resistors were required :unsure:, but the PCB was made with no provision for them, so I had to leave them that way.

Three years later, everything's still working - though I can't really recommend the practice.

The funny thing is, said project lives in a cupboard, where the LEDs are not visible anyway! :rolleyes:
 
I also did this, in one of my projects...

I'd decided I wanted to make the LEDs 'pulse', rather than 'blink' and so I PWM'ed them. I've no idea what thought process, led me to believe that PWM meant no series resistors were required :unsure:, but the PCB was made with no provision for them, so I had to leave them that way.

Three years later, everything's still working - though I can't really recommend the practice.

The funny thing is, said project lives in a cupboard, where the LEDs are not visible anyway! :rolleyes:

Good story, nice to think things can be a little awry and still work! Should have done the lottery that day! 😀

At least you're using a PCB, recently I almost finished soldering a biggish stripboard and realised a quarter of it was 1 track out. 😂
 
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