ATM batteries for LED

[Mark Holm <mdholm@telerama.com>]

I wonder why people use 9 volt batteries to power a single LED?  The forward 
voltage of an LED is usually around 2 volts (except perhaps for those blue 
LEDs).  A single carbon-zinc or alkaline cell has a voltage about 1.5 volts. 
Two in series gives 3 volts.  That gives 1 volt for the resistor to drop.  If 
the forward current is 20 mA then the resistor is about 1/.020 = 50 ohms.  Radio 
Shack carries 22 and 33 ohm resistors in their stores.  One of each connected in 
series is 55 ohms, close enough.

The green LED I am using must have a slightly higher forward voltage.  I can't 
find the spec right now, but I wired it with just a 33 ohm resistor and I know I 
calculated that when I had the data.

Using lower voltage means less heat from the resistor.  Even an AAA size cell 
has more capacity than the tiny cells stacked together in a 9-volt battery and 
9-volt batteries are comparatively expensive.  In a Foucault tester, C-sized 
cells are convenient.  Radio Shack sells cheap battery holders for them.  They 
will last a long time at 20mA.  I don't even need a switch in the circuit.  I 
just lift one end of one of the cells out of the holder when I want to turn the 
LED off.

Mark Holm
mdholm@telerama.com