Electronics Puzzler Solution:
Monochromatic Light Detector

By Forrest M. Mims III

The Mystery Electronics Component: Light Emitting Diode

The mystery component is a red light-emitting diode. A high-brightness AlGaAs LED will work very well in this application. It will detect a narrow band of red light centered at about 625 nm. In this role it is a spectrally-selective photodiode.

Since 1972 I've tested numerous LEDs, and most work well as spectrally-selective photodiodes that detect wavelengths of light slightly below their emission wavelength. A major exception is white LEDs, which are actually blue LEDs in which the chip is coated with a phosphor that transforms the blue emission into white.

How It Works

The circuit below shows the circuit with the hidden component displayed. The LT1001 is a dual-supply op amp that will not work in this single supply circuit. Nevertheless, the Puzzler circuit does produce an output voltage when illuminated with light having a wavelength near that which would be emitted by the LED in its emission mode. How? The output voltage is the photovoltaic voltage from the LED, and it reaches the output through R1.

Light Detector The mystery component is a light-emitting diode (LED).
Note that the op amp does nothing in this circuit. The photovoltaic voltage from the LED is simply coupled to the output through R1.

For the circuit to be used in a practical application, the negative 9-volt supply shown in the circuit below is required. When the LT1001 is connected to a +/- 9-volt dual supply, light of the proper wavelength (e.g., red for a red LED) striking the LED generates a photocurrent that is amplified by the op amp. Capacitor C1 suppresses oscillation. The output voltage range of the practical circuit in the circuit below is much greater than that of the photovoltaic in the circuit above..

Dual polarity Supply Practical version of the circuit shown with a dual-polarity supply.

The LED used in the prototype was a LEDTech Electro UT1813-83 red LED encapsulated in water-clear epoxy. Most other red LEDs will also work, and those encapsulated in clear epoxy will provide more signal. While I used a low noise Linear Technology LT1001CN8 op amp, most op amps should work for demonstration purposes.


During experiments in college in 1966, I first learned that a pn junction can both emit and detect near-IR. When LEDs were commercialized, I learned that they too, could both emit and detect light. I also found that laser diodes can both emit and detect light.

My first application for LEDs as both detectors and emitters was a variety of lightwave communication systems. Beginning in 1990, I began seriously using LEDs as detectors in a variety of sun photometers that measure haze and the total column amount of water vapor in the atmosphere. My first such instrument has been used to make measurements every day the sun shines since February 4, 1990.

Going Further

Others have also experimented with LEDs as spectrally selective photodiodes. You can find some of their work by entering relevant keywords into www.google.com.


Back to the Electronics Puzzler >>

Forrest M. Mims III, Light Emitting Diodes, Howard W. Sams & Co., 1973, pp. 118-119.

Ibid., Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics, 31, 33, 6965-6967, 1992.

Ibid., An inexpensive and stable LED Sun photometer for measuring the water vapor column over South Texas from 1990 to 2001, Geophysical Research Letters 29, 20-1 to 20-4, 2002.

Ibid., Five years of photosynthetic radiation measurements using a new kind of LED sensor, Photochemistry and Photobiology 77, 30-33, 2003.

Forrest M. Mims III and David R. Brooks, Validation of remote-sensing satellites using inexpensive, ground-based instruments (GLOBE Annual Meeting, July 2002).