infraredphototransistorpulse
I want to detect an IR pulse with a phototransistor. I did the following schematic, using the voltage comparator LM339:
The problem is that this will work if in the dark, but if there is some light, the OUT LED will be ON anyway. What I want is a way to automatically subtract the "constant" ambient light, and have the OUT LED ON just during a pulse of light to the phototransistor.
Thanks a lot in advance for your help.
The main difference is that the passive circuit has an output impedance roughly equal to the resistor value. The active circuit has very low output impedance. That could mean you can get much more gain from the active circuit before the response time gets to be limited by the capacitance of whatever load is receiving the circuit output.
Also, in the passive circuit the photodetector bias will vary as its output current changes. The bias on the photodetector in the active circuit will be constant. This could be important if you care about the capacitance of the detector changing.
Both of these effects might mean that you can use a higher resistance value in the active circuit, which will reduce the Johnson noise contribution from that resistor, which gets to your question about accuracy. In either case you'll want to add a capacitor in parallel with the resistor in your circuit to limit the noise bandwidth of your receiver. Op-amp vendors have app notes describing how to optimize the noise in transimpedance amplifier circuits (here's one from National)
If you go with the active circuit, you'll also want to pick an amplifier with low thermal drift of the offset current and low flicker noise (assuming your main goal is to measure the dc light power).
I'm not much familiar with phototransistors, but I noticed the datasheet you linked shows nearly 2-to-1 change in collector current for temperature varying from 0 to 100 C. This is probably going to be the limiting factor in your sensor accuracy. Using a photodiode instead of the phototransistor, and making up the gain using the transimpedance amplifier circuit, and carefully selecting the op-amp and resistor you use, you ought to be able to achieve much better accuracy (I'm thinking on the order of a few % over 0 - 100 C, but I may be forgetting some critical issue).
Edit - One more thing
One minor difference (that might be important depending how you use the circuit) is that because its an inverting amplifier the active circuit will require both positive and negative power supplies (or some possibly accuracy-limiting hoop-jumping to avoid them), whereas the passive circuit may be easier to work in a single-supply environment. But even with the passive circuit you'll have to be pretty careful with the next amplifier circuit down the line to keep its accurate when the current is small.
You need to introduce some hysteresis (positive feedback) into the circuit.
This is done, below, by connecting the collector of Q2 to the base of Q1 through the network R1 R2 R3. You'll probably have to fiddle with the resistor values in order to make it work with your phototransistor.
I simulated it with LTspice, and if you want to play with it, the circuit list is here.
U1 is an opamp functioning as a comparator with PT1 and R1 used to set the voltage on U1- to half of the supply when PT1 is illuminated enough to make its resistance equal to 1.2 megohms. R3 R4 and R5 are used to set PT1's low illumination trip level, with R3 and R5 limiting R4's range from about 2 to 3 volts, and R7 is used to provide hysteresis around the comparator and set the low and high switch points for the illumination incident on PT1.
The circuit has been simulated, appears to work nicely, and the LTspice circuit list is here.
Best Answer
The secret of successful IR transmissions in ambient light conditions, with it's combined many mixed frequencies, is modulation. The transmit source produces an on/off carrier frequency and then "information" is introduced on this carrier by switching the carrier itself on and off at some lower frequency with a recognizable pattern.
The receiver uses a band pass filter tuned to the transmitter carrier frequency that is then able to recognize the "information" modulation and output a waveform that represents that information.
The whole concept is very similar to how a radio channel can work despite the fact that the ether is full of thousands of radio signals.
The IR transmission scheme I describe is exactly the same as common TV remotes work. You could use an old TV remote as your IR source and then get an IR receiver module that has all the receiver logic built into a single chip. Check out something like a TSOP1738 and other similar devices.