"Victory" wrote in message
...
Thanks for your help. Great to hear it worked. I don't want to go
above the brightness limit of the LED, so that shouldn't be a
problem,.
Now all of the parts you just listed, are they some of the things I
can find at a local Electronic store, Radio shack or something I need
to find online? Also, what kind of cost am I looking at here?


OK. I just tried it with a few bits that came to hand. White LED. 200k
pot.
82k resistor. 2N3904 transistor. 12v supply. LED current adjustable from
0 -
25mA. You could also do it using an adjustable regulator as a constant
current source.

Arfa


Parts values are not that critical. Any linear taper pot of around that
value (150k, 200k, 220k, 470k), any resistor around that value (68k, 82k,
100k, 120k, 150k). Might be worth you getting a few resistors of around that
sort of value, and just experimenting to see which one gives you best
adjustment range with the pot value that you finish up using. The value of
this resistor, along with the gain of the transistor, will determine the
full on current through the LED. Start with the highest value resistor, and
the pot turned right down to the ground end, and measure the current through
the LED as you slowly turn up the pot. You don't really want more than 25mA
through a standard white LED. The brightness did not increase much above
about 18mA, with the example I tried. As far as the transistor goes, any
general purpose NPN small-signal silicon transistor with a collector rating
of 100mA or so, will be fine, so 2N3704 or whatever you can lay hands on.
Any electronics component store should have these parts on the shelf. Prices
are cents.

In case you don't know how I hooked this up, it was fixed resistor top end
to +12v, bottom end to the clockwise (from the front) tag on the pot.
Anticlock tag of the pot to ground (supply "-") together with transistor
emitter. Transistor base to centre tag of pot. LED anode to +12v. LED
cathode to transistor collector. Measure current in LED by inserting
milliammeter in series with LED ( meter "+" to +12v, meter "-" to LED anode.

Arfa