If you need an electronic device that can detect rapid changes in light, a photodiode makes a good choice. It's similar to a standard semiconductor diode, except it has a window that exposes its insides to light. To make the most of the photodiode, you need an amplifier to convert its small current to a more useful voltage. As with many other linear circuits, an inexpensive op amp will fill this need. You can put together a photodiode amplifier circuit with a handful of inexpensive parts.
- Skill level:
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Things you need
- BS120 photodiode
- LM741 op amp integrated circuit (IC)
- Solderless breadboard
- 100-megohm ¼-watt resistors, 2
- .1-microfarad 50-volt capacitor
- 22-gauge hookup wire
- Wire strippers
- 9-volt battery
- 9-volt battery clip
- Desk lamp or flashlight
Orient the LM741 op amp on the breadboard so its legs straddle the horizontal groove in the board. Gently press the op amp into the board, checking that its pins go straight in.
Insert the diode's cathode lead into the board so it connects to the IC's pin 2. Connect the anode lead to pin 4. Place one lead of the capacitor so it connects to pin 4. Connect the other lead to pin 3. Place one 100-megohm resistor so one lead connects to pin 4 and the other to pin 3. Place the other resistor so one lead connects to pin 2 and the other to pin 6.
Cut two 6-inch lengths of wire and remove ¼-inch of insulation from both ends of each. Connect the end of one wire to pin 6. Connect the end of the second wire to pin 4.
Insert the red wire of the 9-volt battery clip so it connects to the IC's pin 7. Connect the black wire to pin 4. Snap the battery into the clip.
Aim the light from a nearby desk lamp or flashlight at the photodiode.
Turn the multimeter on and set it to read DC volts. Touch the red probe to the wire connected to the IC's pin 6. Touch the black probe to the wire connected to pin 4. Read the voltage on the multimeter's display.
Move your hand over the photodiode to block the light and observe that the multimeter shows reduced voltage.
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