Physicians and repair
engineers often use small light pens for visual examination Spurposes.
Rugged and expensive as these pens may be, their weak point is the bulb,
which is a ‘serviceable’ part. In practice, that nearly always equates
to ‘expensive’ and / or ‘impossible to find’ when you need one. LEDs
have a much longer life than bulbs and the latest ultra bright white
ones also offer higher energy-to-light conversion efficiency. On the
down side, LEDs require a small electronic
helper circuit called ‘constant-current source’ to get the most out of
them. Here, T1 and R1 switch on the LED. R2
acts as a current sensor with T2 shunting off (most of) T1’s base bias
current when the voltage developed across R2 exceeds about 0.65 V.
The constant current through the white LED is calculated from R2 = 0.65 / I LED
With some skill the complete circuit can be built such that its size is
equal to an AA battery. The four button cells take the place of the
other AA battery that used to be inside the light pen.
Author: Myo Min – Copyright: Elektor July-August 2004