LM741 1 2 2 3 3 4 8 7 7 +15 V 15 V 4 6 6out 5 + - Indiana University

(a) Photodiode: Use the supplied phototransistor (feel free to peel back some of
the rubber coating if not enough light is getting into the fiber optic lead) as a photodiode
in the circuit above. To determine the pins, try using the DVM to distinguish them as
learned previously. Examine the output signal (if the DC level is more than 10 V, reduce
the feedback resistor to 1 MΩ).
If you see fuzz on the output (oscillations), put a small capacitor (~100 pF) in
parallel with the feedback resistor.
What is the average DC output level, and what is the percentage "modulation"?
This can be quite large due to the fluorescent lights in the lab... What input photocurrent
does the output level correspond to? Try covering the phototransistor with your hand.
Look at the "summing junction" (the inverting input pin) with the scope, as the output
voltage varies. What should you see?
(b) Phototransistor: Now connect the device as a phototransistor, as shown
below (the base is to be left open, as shown). What is the average input photocurrent
now? What about the percentage modulation? Look again at the summing junction.
light
+15 V
Phototransistor
2
LM741
3
+
100k (or more)
If you are curious and/or interested, try out the "game" on pg. 182 of your
manual!
!
6
out