By Forrest M. Mims III
Light-emitting diodes can double as sources and detectors of light. This capability makes possible free-space and optical fiber communication links with a single LED at each end of the link. Also possible is a bidirectional optoisolator made from two similar LEDs. To the best of my knowledge, no such devices have been made commercially available since I first described the idea in back 1979 (1) . But that's not a problem, since you can easily make one yourself.
A two-way optoisolator is unnecessary when only one-way operation is required. But experimenting with two-way operation just might provide some new ideas.
How to Make a Two-Way Optoisolator in 60 Seconds or LessFor best results, use LEDs of the same type to make a two-way optoisolator. The LEDs should both be encapsulated in clear epoxy. I've had very good results with red AlGaAs super-bright LEDs and AlGaAs and GaAs:Si near-infrared LEDs. All that's required is to point the LEDs at one another and secure them in place with black electrical tape or heat shrink tubing. When using tape, cut a 1.25-inch length of tape and place it on a work surface with its sticky side facing up. Then place the two LEDs across the tape so that they point directly at one another. For best results, rotate the LEDs so that all their leads are in the same plane. Then simply roll the free tape over the LEDs and over onto itself. Figure 1 shows the end result. (See Going Further below for more options.)
Figure 1. Two LEDs form an optoisolator when wrapped with black electrical tape or held together with heat shrinkable tubing.
Simple Demonstration Circuit for a Two-Way OptoisolatorVarious circuits can be used to make sure the dual-LED optoisolator is working, and one is shown in Figure 2. The circuit lights an LED when a forward current is applied to one of the two LEDs in a two-way optoisolator. Each half of the circuit is electrically independent of the other to establish full electrical isolation.
Figure 2. Simple demonstration circuit for a two-way LED optoisolator. (For permanent applications, be sure to ground all the unused inputs of the 4011)
How It WorksThe circuit in Figure 2 applies a forward current to LED1 of the two-way optoisolator. R2 pulls down the input of the inverter formed by 1/4 of a quad NAND gate. This keep indicator LED3 off. When LED2, which is reverse biased, is illuminated by LED1, the input to the gate is pulled high, the output at pin 3 of the 4011 goes low and LED 3 glows.
V1 and V2 can range from 3 to 12 volts. R1 and R3 are assigned nominal values of 1K each. The prototype was powered by two 9-volt batteries. This provided around 7.5 mA to the LEDs, which is adequate for this circuit. For optimal results, select the resistance of R1 and R3 according to R = (V - VLED)/ ILED), where VLED) is the LED forward voltage and ILED) is the desired forward current.
Note that V1 (and ground 1) and V2 (and ground 2) are electrically independent. This assures that there is no electrical connection between the two halves of the circuit.
Parts You Will Need
The following Jameco parts were used to assemble a breadboard version of the circuit:
|Part Description||Mfr. Part No.|
|IC1 - 4011 quad NAND gate||CD4011|
|LED 1, LED 2 - Identical AlGaAs red or
near-IR LEDs (clear epoxy)
|LED 3 - Red or LED 3 - Green||UT9C13-86-UDC2-R (red) or UT4J23-4D-UBC2-S11 (green)|
|R1, R3 - 1K||CF1/4W102JRC|
|R2 - 1M (Resistor value 1M to 2M)||CF1/4W105JRC|
Additional resistor values for reference:
Resistor - 1.1M
Resistor - 1.2M
Resistor - 1.3M
Resistor - 1.5M
Resistor - 1.6M
Resistor - 1.8M
Resistor - 2M
Miscellaneous: Solderless breadboard, two 9-volt battery, two battery connector clip and wire jumpers.
Note: While the components listed above were used for the prototype, substitutions can be made.
Assemble and Test the CircuitThe test circuit was assembled in a few minutes on a solderless breadboard as shown below in Figure 3.
Figure 3. The circuit in Figure 2 assembled on a solderless breadboard.
The dual LED optoisolator was made by wrapping two LEDs with black electrical tape as described above and shown below in Figure 4.
Figure 4. Close up view of the assembled circuit showing the 2-way LED-LED optoisolator.
Check your wiring and then test the circuit. First, connect a 9-volt battery to V2. LED 3 should not glow. Then connect a second 9-volt battery to V1. LED 3 should glow to indicate that the optoisolator has switched the 4011 output from high to low. Remove and reinsert into the breadboard the red clip lead from B1 to switch LED 3 off and on.
Figure 5 is an animated GIF that shows what happens when I tested the circuit.
Figure 5. LED3 glows when a forward bias is applied to LED1 of the two-way optoisolator.
Going FurtherThe basic concept presented here can be used to provide a bidirectional, optical interface between two data transceivers. Other applications are also possible and may come to mind as you experiment with the basic circuit described here. For permanent circuits, heat shrink tubing is better than tape. The do-it-yourself optoisolator can even be potted in black epoxy if a straw or similar tube is first placed over the LEDs to keep them pointed at one another and to keep epoxy from flowing between them during the potting process.
For many years I've experimented with and written about the dual nature of many solid state devices that emit and detect light. During high school in 1962, I found that a cadmium sulfide photoresistor designed to detect light would emit a soft green glow and bright green flashes when excited by an automobile spark coil. On March 14, 1966, I connected a 2-transistor pulse generator to a silicon solar cell. An identical solar cell nearby detected infrared pulses emitted by the first cell and converted them into a pulsed photocurrent that was amplified and transformed into an audio tone by a simple amplifier. In 1969 and again on October 18, 1972, I used an IR image converter to observe IR emission from forward biased silicon diodes (1N3070 in the latter experiment). During 1972-73 I experimented with a wide variety of light wave communication systems in which a single LED served as a dual-purpose light emitter and detector at each end of both free space and optical fiber links. More details about the use of LEDs as dual-function emitter-detectors can be found in references 1-4.
1. F. M. Mims III, Bidirectional Optoisolator, Electronics, 127, May 10, 1979.
2. Ibid., Light Emitting Diodes, Howard W. Sams & Co., 1973, pp. 118-119.
3. Ibid., Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics, 31, 33, 696 5-6967, 1992.
4. Ibid., LED Sun Photometry, Optics & Photonics News 20, 32-38 (2009).
About Forrest M. Mims III