5.4.1: LED as Detector
1. Most LEDs can be used as light detectors. In photovoltaic mode, the output potential is a function of the light level and the make-up of the device (i.e., typically its color). Insert a yellow LED into a protoboard with nothing obstructing it. Place a DMM across it and measure the resulting DC voltage, recording it in Table 5.3.1 under “Normal”.
2. Shade the LED so that minimal light strikes it and measure the resulting voltage. Record the value in Table 5.3.1 under “Dark”.
3. Using the pen light, illuminate the LED from a distance of approximately 10 centimeters, measure and record the voltage in Table 5.3.1 under “Bright”. Also, slowly vary the distance of the pen light from a few centimeters to 20 or so and note what happens to the voltage.
4. Replace the yellow LED with the blue LED and repeat steps 1 through 3.
5.4.2: IR Emitter/Detector Pair
5. Figure 5.3.1 shows an emitter/detector pair. These devices will emit and detect light at the same wavelength and tend to not produce or detect light at other wavelengths. This aids in avoiding interference. The detector is configured in photoconductive mode. Its current will increase with increasing light level. This current also flows through Rload meaning that Vload will be proportional to light level.
6. Build the circuit of Figure 5.3.1 using E = 7 volts, Rs = 470 \(\Omega\) and Rload = 33 k\(\Omega\). The emitter diode is denoted with a yellow dot on its case while the detector diode shows a red dot. It is very important that the pair properly be aligned. The bubbles should face each other and cases should be at same height, effectively aiming one bubble at the other. Further, they should only be a few millimeters apart. Finally, the short leads indicate the cathodes.
7. Energize the circuit. Because this pair operates in the infrared, nothing will be apparent to the human eye. Verify that the emitter is operating by measuring the voltage across it. It should be in the vicinity of 1.1 volts.
8. Measure Vload and record the value in Table 5.3.2.
9. Slip an opaque card such as a thin piece of black plastic or cardboard between the emitter/detector pair. Measure and record Vload in Table 5.3.2.