Interesting LED switching circuit

[syc]

I came across this circuit today while reading another list:

(From http://kmt.hku.nl/~pieter/HARD/bicycle-electronics/led-switching-array.jpg )

Apparently it is designed to serve as a current regulator for the upper pair of LED's, and when the voltage goes above a configurable level, the lower pair of LED's get powered on.

No idea how well it works, just thought I'd post it here in case anyone wants to experiment with it.

 

[Bandgap]

I quite like that.

It is for small leds,

The top two transistors form a constant current source.

The bottom two transistors short out the bottom two leds with low input voltage, then turn off and allow the regulated current to flow through the bottom leds when enough voltage is available.

There is a feedback resistor to ensure crisp switching.

Probably really only of use when voltages from 0 to 6 are being handled as above this all four will be on and a simpler circuit could be used.

with four transistors to play with, you could also make a switching design.

Steve

 

[Steve K]

That's an interesting circuit, although I'm trying to understand the original intent (I tend to view everything as if it was intended for dynamo use).

The current limit is about 45mA, so it was to be used with 5mm LEDs. The power supply range goes up to 25v, so it's not powered by AA NiMH cells! I can only assume that the LED switching feature is to get light when the battery voltage has dropped quite a bit.

Or maybe it really is a dynamo circuit, but just uses the rectified output? Still not sure why you'd only use 45mA, though. Could this be a taillight?

It's an interesting circuit, and a chance to make the brain work a bit to figure it out. Just not sure that it's especially useful, though.

Steve K.

p.s. nice to see Mr Bandgap in the neighborhood again!

 

[pe2er]

Or maybe it really is a dynamo circuit, but just uses the rectified output? Still not sure why you'd only use 45mA, though. Could this be a taillight?

It is...

http://kmt.hku.nl/~pieter/HARD/bicycle-electronics/#array

 

[epa]

Thanks who ever this posted and grabed from a nother article.
I was finding it.:thumbsup: