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Author Topic: IR diode and phototransistor  (Read 9257 times)
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Germany
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Hey there!
In my project I'd like to use the following emitter-detector pairs:



http://www.pollin.de/shop/downloads/D120592D.PDF

Now I've got some "small" issues deciphering the datasheet. It says under "Basic characteristics" that the emitter typically uses 1.25V and max. 1.6V at 50mA.
For the detector, there's two different values, "collector-emitter voltage" (min. 70V) and "emitter-collector voltage" (min. 7V).

If I get this right, I can hook up a minimum of 7V to the emitter line on the detector and get.. well, where can I see how much I get out of it again? Do I get either the full load or zero, depending on the base, which is the IR-sensor itself?
Also, does this mean I can't hook up the detector to my +5V line of the arduino to control digital input pins, but I'd need another transistor in between the power line and the detector to up the voltage to +7V?
Oh, and have I read all the data correct? I'm not sure if anything I read about the emitter concerning power hookup was correct (e.g. the typical 1.25V at 50mA)

Would be glad if someone could answer my questions.

Greets,
Philipp
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Katowice POLAND
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i suggest to change both elements smiley find photo-transistor for 5v and IR diode of wave length accepted by phototransistor

You will get a voltage level something between 1024 and 0 depending on ir level
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So I'm right assuming this phototransistor only works with +7V and above? Damn, that sucks.. they were so cheap to get smiley-grin

Hm, that is, if I run it to an analog input. But since you mentioned 1024, which equals 5V on the analog input, I think I would simply get a HIGH on any digital input, is that correct?
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No, I read it as 7v max, not 7v min. They should work.

The circuit to connect it up would be similar to "Test circuit Detector" on the data sheet, except that the emitter resistor should be about 500 ohms (unless you are using a pulse driver, which you probably aren't) and you can experiment with increasing the detector resistor (dont decrease it below 100 ohms) until you find something that works.

If they are at close range and shielded from ambient light you might get it working on a digital pin. Otherwise you may need special pulse drivers and amplifiers and detector circuitry to get it to work reliably.
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Thanks a lot for the advice! Did you by chance find any min. values? (Though I assume you found some below 5V since you say they should work)

Okay, I will try that, thanks! So the only reason they might not work in digital pins is that the digital pin can only take on a HIGH or LOW state? In that case I can surely wire it up to an analog input and check the input values.
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The minimum voltage is VCE(sat) which is 0.3v. So if you had it in the circuit I mentioned previously you could get a theoretical high output level of 4.7v which is plenty for a digital pin.

In practice, whether you'd get a voltage that high depends on how close the IR diode is to the detector. Starting off wiring it to the analog inputs is a good idea.
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Just one quick question before I get to build the circuit.. how did you calculate the 500 ohms needed for the emitter resistor? If I calculate the resistor using 5V as my supply, 1.25V as forward voltage and 50mA as my used current, I get a value of 75 ohms.. Also, how exactly do I connect the detector to the circuit? The image looks like I would connect the emitter to +5V and the collector to the resistor and to my arduino input.

[EDIT]

A quick note on the resistor values.. I tried the emitting diode with two 220 ohms resistors in series and am receiving a very, very dim IR-light (using a digital camera to "see" the light). Did you by chance mean the "emitter" part of the phototransistor? smiley-grin Cause that might make some more sense..
« Last Edit: January 29, 2011, 02:44:36 pm by Christoph680 » Logged


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I was just erring on the safe side, you can increase the current and lower the resistor. I prefer to start with a current too low, then if it isn't enough, increase it by lowering the resistor (having too often accidentally blown LEDS through miscalculation the other way!). Clearly you've found that the emitter isn't emitting enough light, from the digital camera. Remember though that you will never get it to emit as bright a flash of light as a remote control would, since the remote control uses a pulse driver. If it's that dim though you might have to start around 100 ohms.

For the detector it's collector to +5v, emitter to resistor and arduino input pin, other end of resistor to 0v. They suggest 100 ohms for the resistor but I think you will need something higher, you will have to experiment to find a good value (don't go below 100 ohms).
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Germany
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Thanks a lot! Turns out I've measured the wrong wires with my multimeter so I always got an output of around 2V, no matter how the transistor was connected. It appears to work just fine now smiley-wink (0 on analog if it's off and around 670 if it's on).. might not be enough for a digital pin, but I ain't running out of analog pins in the near future, so one less doesn't matter.

Greets,
Philipp
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Methinks some people haven't the first clue about interpreting data sheets.....

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Might be true smiley-wink Though how should you know if it's one of the first times you ever worked with datasheets?
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Nice generic circuit for a IRLED / phototransistor pair.  I was refering to the poster with a hangup with voltages, In semiconductors, voltages are usually ratings between pins, not the voltage they are capable of switching.  I'd set the resistor for the LED (IRLED) to a value to limit the current to close to its maximum, and then suck it  and see using a DMM for the resistor on the phototransistor side. It will vary on many different factors, especially the optical path between the two devices.  Increasing the resistor on the phototransistor will increase its high output level with the above circuit (when the LED is off or obscured), but its a trade off with noise and the low level when the LED is On or the optical path is open .  
« Last Edit: January 30, 2011, 11:14:12 am by pluggy » Logged


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Thanks man smiley-wink That's pretty much exactly like the circuit I came up with in the end. It's working really well. I'm surely gonna need to measure the returned values after I assemble the circuit inside the machine I'm building again, but that should work out fine anyway smiley-wink

Greets,
Philipp
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