16 January 2009

Breathing a LED down to zero

Finally I got something broken to fix. Actually this time was something to improve.

I helped the amateur astronomer of my optical experiments with a circuit to give a "breath" effect to the eyepiece grid LED.

He built the circuit off a diagram I found on the web right away on the perfboard/veroboard. The LED throbbed/breathed, but not all the way to "off".

All similar circuit uses probably do not need to turn off the LED, but he did. The op-amp has a virtual ground at Vcc/2: what about retouching it?

Pulling it higher, say at 2/3*Vcc, does the trick. This requires that the two 100k (or 47k) resistors become 100k on the Vcc side and 47k on the GND side. Or something like that. A 100k linear trimmer allows for finer adjustment.

[Picture: nice wiring, isn't it?]