PHYS 373: Instrumentation and Interfacing
Example Photo Circuits
Dr. Pogo
100 Ω
Build an LED Emitter:
+5 V
Ground
10 kΩ
Build a Digital Photodetector:
Output
+5 V
NPN 222A
Ground
Build a Digital Photodetector with latch and reset:
Output
10 kΩ
14 13
VCC
C
+5 V
Ground
CB
11
10
9
8
5
6
GND
7
E
7400
NPN 222A
B
E
1
Reset
12
2
3
4
PHYS 373: Instrumentation and Interfacing
Example Photo Circuits
Dr. Pogo
Analog Photodiode Detector, Version 2:
This circuit generates an output voltage as follows:
“Dark” 5.00 volts.
“Bright” 0.05 volts.
+5V
+5V
RC
RB
VA
To DAQ
VB
For this circuit, the numerical value of RB is pretty
unimportant. I’ve tested it with values between 100Ω and
10kΩ.
For this circuit, the choice of RC affects the kind of
illumination you can easily detect.
VC
VE
CB
E
If RC is large (e.g, 10 kΩ), then generic room lighting creates an output voltage of about 1.0 V.
If RC is small (e.g, 1 kΩ), then generic room lighting creates an output voltage of about 4.0 V.
You should adjust RC (possibly using a potentiometer) so that the bulk of the illumination range
you want (whether darker levels than background, or brighter) occupy most of the output voltage
space. So, if you want to measure differences in light that is generally pretty bright, then use a
small RC. If you want to measure differences in light that is generally pretty dark, then use a
larger RC.
Parts List:
1.
2.
3.
4.
5.
Photodiodes:
Bright Red LEDs:
100 ohm resistors:
10 kohm resistors:
NPN Transistors:
Mouser # 638-PD333-2C/HOL2
Mouser # 638-333-2SURCS5306
Mouser # 791-RC1/4-101JB
Mouser # 791-RC1/4-103JB
Mouser # 610-PN2222A
6. Alternate Photodiode: Mouser # 512-QSD2030
7. Alternate Bright LED: Mouser # 638-333-2USOC/S530-A6