A LED or a phototransistor?!
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:
|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–
|“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 :-)