11 December 2007

Driving a LED from TTL/CMOS


It's the first thing you learn about TTL/CMOS chips: they cannot (should not) drive a LED directly. A transistor driver is needed, and is shown at the right in its simplest form.
When the gate output goes high the BJT saturates and current flows through your LED. A limiting resistor is needed to control the LED current.

The table below shows R values vs supply voltage vs LED current.




Vcc R ohm R_ohm R_ohm
3,6 45 30 22,5
4,8 105 70 52,5
6 165 110 82,5
7,2 225 150 112,5
8,4 285 190 142,5
9,6 345 230 172,5
10,8 405 270 202,5
12 465 310 232,5
13,2 525 350 262,5




I_LED [A]
0,02 0,03 0,04
V_LED [V]
2 2 2
V_CE [V]
0,7 0,7 0,7

I will need these in my hi-brightness red LED optical MCW transmitter.