What's the Missing Component?by Forrest M. Mims III
The circuit in Fig. 1 is a spectrally selective photometer that detects a narrow band of red light emitted by a red traffic light, the brake light on a vehicle or the beam from a red diode laser. The circuit totally ignores near-infrared like that emitted by IR remote control devices. When a few microwatts of red light arrive at the detector, the operational amplifier produces enough voltage to switch a transistor or a comparator. This, in turn, can trigger an alarm or trip a counter.
A white light source includes red light. Thus, the circuit will respond to sunlight and most artificial room lighting. But the circuit will totally ignore monochromatic light other than red wavelengths.
Figure 1. Simple monochromatic light detector.
What is the component concealed behind the puzzle piece? Before revealing the answer, here are some hints:
The op amp and R1 form a transimpedance amplifier that transform the photocurrent from the unknown component into a voltage. R1 determines the gain of the amplifier.
The missing component is clearly some kind of photodiode or photoresistor. But what kind?
Cadmium sulfide photoresistors detect a rather broad band of visible light while ignoring near-IR. But they detect green better than red.
Silicon photodiodes detect red light rather well, but they detect near-IR even better.
A red-transmitting optical filter placed before the silicon photodiode will solve the puzzle, but this means there are two components behind the puzzle piece when, in fact, there is but one.
Various kinds of silicon photodiodes with a built-in red filter will work, but they generally cost much more than the mystery detector.
Certain specialized photodiodes made from III-V compounds detect red light while ignoring near-IR. But they also detect other visible wavelengths, and they cost much more than the mystery detector.
Do you now know the component behind the puzzle piece?
Click here for the answer.