Here's one that is baffling me.
The original problem was that when I tried to build a multiplexed LED display the LEDs were too dim to be visible under normal lighting.
Here is a little test circuit that I'm playing with around an IL-1 opto-isolator.
The circuit is fed with a 5V 333Hz square wave 1mS (ON) 2mS (OFF).
Now Ic(max) of the IL-1 is 50mA (cont) and 400mA (t(on) < 1mS).
So ignoring Vce of the IL-1 I tried R = 220R and measured the voltage across R with a 'scope. V=1.5V so I=7mA.
A lot lower than I expected.
OK I thought lets try R = 100R. V=0.7V so I=7mA.
Weird.
OK I thought lets try R = 47R. V=0.34V so I=7mA.
Weird.
I'm missing a trick here somewhere but I can't figure out where.....
The original problem was that when I tried to build a multiplexed LED display the LEDs were too dim to be visible under normal lighting.
Here is a little test circuit that I'm playing with around an IL-1 opto-isolator.
The circuit is fed with a 5V 333Hz square wave 1mS (ON) 2mS (OFF).
Now Ic(max) of the IL-1 is 50mA (cont) and 400mA (t(on) < 1mS).
So ignoring Vce of the IL-1 I tried R = 220R and measured the voltage across R with a 'scope. V=1.5V so I=7mA.
A lot lower than I expected.
OK I thought lets try R = 100R. V=0.7V so I=7mA.
Weird.
OK I thought lets try R = 47R. V=0.34V so I=7mA.
Weird.
I'm missing a trick here somewhere but I can't figure out where.....