From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


The easiest thing is to just setup the tests in a row on top of the tester.
This will mean that with a little bit of lateral motion, you can change
from one test to another.
As to the light sources, the same system of a LED can work for each of them
without problems.
I tend to work a lot with the ol' wall warts (little power supply thingies
that you plug in the wall for a lot of the low power electronic stuff) and
have a 9V one for my tester which was setup for a 9V battery. The current
for the LED is usually about 20ma (1/1000 amp = 1ma) max although some of
the newer ones will take more current.  Then find the voltage at which they
run at (often something like 1.8V or the such) and you then take those
numbers and figure out how much voltage you have to drop across the
resistor at the current you want to run the LED at. The simple formula - E
= I * R  - where E is the voltage desired, I is the current through the
resistor and R is the resistance of the resistor.  There are standard 20%,
10% and 5% values (this % value is the tolerance for the actual resistance
vs. what it is marked for with the cheap easy to find resistors) for the
ordinary resistors. Resistors are so cheap (I buy resistors in quantity for
less than $.0001 each at work) that shipping will be a lot more than the
cost of the resistor so finding the surplus shop or Radio Shack in your
area is the best place to buy them.  Usually the 1/8th watt size is good
enough as the current just isn't that high.  If you want to have the
ability to reduce the LED brightness, calculate what the resistance needed
for 1/2ma of current and choose a pot that has at least that much
resistance but not much more and place in series with the resistor
selected. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

--- BBBS/NT v4.01 Flag-4