Electronic components are expensive and the more I tinker the more I need, so I salvage as many components as I can. Visible light LEDs are easy to test, however Infrared light LEDs, Ultraviolet light LEDs, Pin diodes, and Phototransistors are not as easy to test as they ether do not emit visible light or they need the right light source to test them. What makes salvaging them difficult is many of them look the same and have no markings to tell them apart, standard transistor testers don’t work on phototransistors. Small enough to fit in an Altoids box, I found myself building a pocket sized tester just for photo components and LEDs.
Step 1: Light
First let’s talk about Light; light is used in many fields of industry, medicine, electronics, and nature.
Name Wavelength Frequency (Hz) Photon Energy (eV)
Gamma ray less than 0.01 nm more than 10 EHz 124 keV - 300+ GeV
X-Ray 0.01 nm to 10 nm 30 EHz - 30 PHz 124 eV to 124 keV
Ultraviolet 10 nm - 380 nm 30 PHz - 790 THz 3.3 eV to 124 eV
Visible Light 380 nm - 750 nm 790 THz - 405 THz 1.7 eV - 3.3 eV
Infrared Light 750 nm - 1 mm 405 THz - 300 GHz 1.24 meV - 1.7 eV
Microwave 1 mm - 1 meter 300 GHz - 300 MHz 1.24 µeV - 1.24 meV
Radio 1 mm - 100,000 km 300 GHz - 3 Hz 12.4 feV - 1.24 meV
Other than Microwave and Radio waves there are three forms of Electromagnetic radiation (EM radiation or EMR) commonly found in the home and home electronics, Infrared light, Visible light, and Ultraviolet light, and the electronic componets these three lightforms come in are, LEDs, pin diodes, sensors, and phototransistors.
Step 2: Salvage
I don’t throw circuit boards in the garbage, I salvage everything reusable I can. LEDs phototransistors and everything else I might have a use for I take them off the circuit board test and sort them.
Step 3: Visible Light LEDs and Testing
Testing visible light LEDs is easy on a multimeter, set the meter to test for continuity; when the LED is conected forwards bias (the cathode is connected to ground) the LED lights up. My multimeter is set for 200 ohm.
Step 4: Pin Diodes IR LEDs and Phototransistors
Pin Diodes IR LEDs and Phototransistors do not test well on a multimeter you cannot see Infrared or Ultraviolet and many of them look alike. This makes a specialized tester nessary for testing and sorting.
Photo transistors are made with wavelength sensitivity and spectral range of sensitivity as well as LEDS are made to emit lights of specific wavelengths and spectral range. You do not need an oscilloscope with this tester to match LED to phototransistor.
Step 5: Building the Tester
I started with a broad spectrum IR pin diode for testing remotes and other IR emitters as well as I knew the specs of this pin diode since I bought five of them while working on another project.
LEDs only need five to ten milliamps to work and Phototransistors work well at low voltages and current.
I had a choice of circuits to choose from so I decided to use a circuit that will work with pin diodes LEDs and phototransistors; I used an op amp circuit just for the practice of working with op amps.
This circuit can be wired so the test LED turns on with the photo component being tested or turns off when the component being tested is on.
I decided to go with test LED on with photo component being tested on to avoid confusion.
Other than the components I had, I got the rest of my tester parts from my local Radio Shack The Source,they have a nice matched IR LED and Phototransistor for testing.
Step 6: Testing in the Tester
Visible light LEDs
Visible light LEDs can be tested in the probe socket, when forward bias each press of the test button lights up the LED being tested.
Forwards bias is when the cathode is connected to ground and the LED lights up.
Or they can be tested in the test socket.
Testing Diodes in the Test Socket
All diodes test the same in the test socket of the tester, when inserted in the test socket forward bias, the green LED lights up. When inserted in the test socket reverse bias, the green test LED does not light up.
Pin diodes when inserted in the test socket reverse bias and exposed to the right form of light, light up the green test LED.
Testing Phototransistors in the Test Socket
All phototransistors test the same in the tester, the green test LED does not light up until the emitter of the transistor is to ground and the transistor is exposed to the correct form of light. Some IR & UV receivers look like phototransistors, but are built with internal band pass filters. These will only work with IR and UV emitters of the correct carrier frequency.
Step 7: Three Pin Sensors
This tester is not built to test three pin sensors; most sensors are multi internal components with band pass filters mosfets and transistors. The three pins are Vcc, ground, and output. So three pin sensors need a circuit like this and the right IR or UV source.