ECE 102
Engineering Computation
Chapter 10
LabJack LED Interface
Dr. Herbert G. Mayer, PSU
Status 9/2/2015
For use at CCUT Fall 2015
Syllabus

Light Emitting Diode

Connect LED to digital FIO Channel

Connect LED to DAQ Channel
Light Emitting Diode


A diode is a semiconductor device that primarily
conducts current in only one direction.
Symbol:
anode (+)
cathode (-)
anode (+)
Light Emitting Diode
(LED)
Standard Diode

cathode (-)
I-V relationship (non-linear):
i(t)
v(t)
i(t )  I 0e
kT
v t 
q
2

A diode is created when a semiconductor is doped
with impurities to create a p-n junction.
Courtesy Wikimedia Commons
• A difference in energy exists at the
junction (called the band gap).
• When charge carriers recombine at the
junction, they can release energy in the
form of photons.
• The band gap determines the
wavelength (and color) of light emitted
by the diode.

When fully conducting, the voltage drop across the
diode is a fixed value ( ≈ VD ).
3
Connect LED to digital FIO Channel
Type #1: Source current from FIO
LED specs: VD = 2 V, IDmax = 20 mA
Is an external current limiting resistor
needed?
–(3.3 V) + VR + VD = 0
VR = (3.3 V) – (2 V) = 1.3 V
Rmin = VR / IDmax = 65 Ω
Internal circuit – You do
not need to build this.
R
3.3 V
VR
FIO: R = 550 Ω
FIO
ID
VD
Since R > Rmin, the internal resistor is
enough to limit current by itself.
EIO/CIO: R = 180 Ω
LabJack
Write FIO:
0 → Turns LED OFF
1 → Turns LED ON
4
Connect LED to digital FIO Channel
Type #2: Sink current to FIO
LED specs: VD = 2 V, IDmax = 20 mA
VS = +5 V
Internal circuit – You do
not need to build this.
R
GND
VR
FIO: R = 550 Ω
VD
FIO
ID
Is an external current limiting resistor
needed?
– VS + VD + VR = 0
VR = VS – VD = (5 – 2) V = 3 V
Rmin = VR / IDmax = 150 Ω
Since R > Rmin, the internal resistor is
enough to limit current by itself.
EIO/CIO: R = 180 Ω
LabJack
Write FIO:
0 → Turns LED ON
1 → Turns LED OFF
5
Connect LED to DAQ Channel
LED specs: VD = 2 V, IDmax = 20 mA
What is the safe VDAC range?
VRmax = R∙IDmax = (50 Ω)(0.02 A) = 1 V
– VDACmax + VRmax + VD = 0
VDACmax = VRmax + VD = (1 + 2 ) V = 3 V
Internal circuit – You do
not need to build this.
R
VDAC
VR
DAC: R = 50 Ω
DAC
ID
VD
Hence, 0 V ≤ VDAC ≤ 3 V will ensure
that 0 ≤ ID ≤ 20 mA.
LabJack
6