A LED or a phototransistor?!

IR LED and a photo-transistor - which is which? (click for high-res)

Among the electronic components I’ve ordered recently were a Kingbright L-53F3BT infrared LED and a Kingbright L-53P3BT infrared photo-transistor (PT). All was fine, until the salesman at the local shop told me:

We’re having a little trouble with our inventory – these components are from the two different storage bins, but they seem to be identical. Probably someone has misplaced them, when restocking. Can’t tell you which is which. We can get you the right ones tomorrow, if you’d like to wait…

Anyway, I purchased them. Now I had either two LEDs, two PTs, or a LED + PT (preferably!). It was time to do some tests and measurements

A bit of theory

LEDs and photo-transistors can be wired in a similar way – using a current-limiting resistor in series. The schematic on the right shows a LED and a PT, both of them forward-biased. Under certain conditions, they will conduct electricity. If we swap the LED’s anode and cathode leads and PT’s collector and emitter leads, the two components will be reverse-biased and will not conduct electricity.

When forward-biased, the LED will conduct (and light up) when VCC is greater than its forward voltage Vf. The current passing through it will be equal to (VCC – Vf) / R. Unfortunately, it’s hard to tell if an infrared LED is on, with a naked eye.

When forward-biased, the current through the photo-transistor is proportional to the amount of light. In darkness it’s be next to zero; When the PT is exposed to infrared light, it becomes conductive and the current starts to rise.

This leads us to our criteria – the current through a forward-biased LED is a constant; the current through a forward-biased PT varies with light.

Test setup, expected values

Here’s a table with the important LED specs, taken from its datasheet:

Forward voltage Vf 1.2V
Reverse voltage (max) Vr 5V
Forward current (max) If 50mA

And similarly, for the photo-transistor:

Collector-to-Emitter Breakdown Voltage Vbr_ceo 30V
Emitter-to-Collector Breakdown Voltage Vbr_eco 5V

In order not to damage any of the components, regardless of how they are biased (forward / reverse), VCC should be below 5V. To drive the LED, it has to be above 1.2V. A VCC of 3~4V should do fine.

The value R of the resistor is usually chosen in the 200 ~ 300R range, thus limiting the current to 10~20mA (I = VCC / R). The total dissipated power will be below 80mW (U = VCC * I), so a 1/4W resistor will be perfectly OK.

Doing the actual measurements

I decided to use a 220R resistor (which measured 228R) and a pack of three NiMh AA batteries, with a total output of VCC = 3.8V. Re-doing the math with these particular values gives the expected current for the LED: I = (VCC – Vf) / R = (3.8V – 1.2V) / 228 = 11.4mA. The current for the PT would vary between 0mA and VCC / R = 16.67mA.

The final step was to take a digital multi-meter (DMM), set it to current measurement, 20mA range and complete the circuit:

Battery+ -> DMM Red Lead … DMM Black Lead -> Resistor -> “Specimen” -> Battery

 Setup Ambient light Flashlight
“Specimen A”, direction 1  10.96mA  10.96mA
“Specimen A”, direction 2  0.00mA  0.00mA
“Specimen B”, direction 1  0.00mA  0.03mA
“Specimen B”, direction 2  0.34mA  3 ~ 11mA

Well, “Specimen A” behaved exactly the same, regardless of the amount of light – it was a LED, forward-biased when in direction 1.

“Specimen B”, when in direction 2, conducted between 0.34mA and 11mA of current, depending on the presence and proximity of the light source. It was a photo-transistor.

I got lucky with my purchase, after all :-)


Posted on May 21, 2011, in Electronics and tagged , . Bookmark the permalink. 9 Comments.

  1. Not sure if your confusion about which component is which got resolved. I bought today. The LED is blue, the phototransistor is clear. You have 2 LEDs in your picture.

    • These are not two LEDs, that’s for sure – some physical differences can easily be seen on the full-size picture. One of the “cathodes” is striped, the other one is dotted, for example.

      The L-53F3 LEDs come in two varieties – with clear lens (L-53F3C) and with a blue-transparent lens (L-53F3BT). I think it’s the same with the photo-transistors – there are L-53P3C and L-53P3BT ones.

  2. Chanced upon this post, and while the proper testing of the components to electrically differentiate the two is useful knowledge, but thought it worth mentioning: if you have a device you expect is an IR led, you can use a digital camera (such as on a typical cellphone) and see the IR emission, as typical CCDs are sensitive to that wavelength.

    This is a useful way to verify that an IR remote control is in fact transmitting. Check your camera against a known working IR remote control, and you’ll know that your camera sees IR.

    The diode check setting of a half-decent DMM will sufficiently bias a LED to illuminate it, even if only dimly. Point a camera at it and see if you see light (which on a DMM diode check won’t be particularly bright, but on a properly powered remote, should illuminate clearly).

  3. Thanks for writing this up! Photo diodes have always confused me a bit.
    One interesting trick for checking an IR LED. Use your smart phone or camera. The image sensors pick up IR!

  4. will the phototransistor light uo?

    • No, it won’t. You can’t see the light from the IR LED with a naked eye, either

      • Yes it did (checked with a camera). Both pt and led (as with photodiodes) are composed of pn junctions and should theoretically light up when forward biased to greater than their turn-on voltage

      • Not really Ike. A bipolar transistor is a NPN or a PNP device while a diode is a PN device. They work differently. When a LED is forward biased, it emits light while the transistor switches and amplifies the colector-emitter current if the base-emitter junction is forward biased. So a LED will light and a phototransistor will not. (And sorry for the a’s and the’s -this is not my native language).

  5. How would you go around this issue but with a phototransistor and a photodiode?

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