[Laser] further experiments with the SFH203 and the E700STO

[stuart.wisher at talktalk.net]

I have found several other devices that emit long wavelength IR radiation,  the photo diode BPX25 also does, but the BPW21 does not.


Interestingly, some glass encapsulated zener diodes, particularly the 400mW BZX55C, both the 5.1V and 6.2V types emit similar radiation, as does the BZX85C 1.3W 12V device.


By far the most powerful emitter is the original device tested, the SFH203.


I have also now used a pyroelectric detector, the E700STO and found that I cannot detect the photodiode radiation with it, it seems that it is either too insensitive, or the radiation from the photo diode is outside the pass band (5nm - 14um) of the detector, or I am doing something wrong. I did cover one of the sensing elements with black PVC insulation tape.


I then moved on to using the 6V 40mA filament lamp as a source which was detected by the pyroelectric detector (with the tape over one sensor which improved the response)) at a range of a few centimetres, but only at very low frequencies, in the range 0.2Hz - 2Hz. A useful way to detect the presence of rectangular pulses in this frequency range is to connect the detector output to an oscilloscope and observe the trigger LED, which flashes on positive and/or negative edges when the trigger selector is set to LF. I could see the LED flashing in step with the function generator output.


I returned to the previously reported set-up which involved using two SFH203 photo diodes, one on transmit and one in the receiver. Reducing the value of the series resistor on the transmit side increased the output greatly and I ended up with strong signals over a 1m distance on the test bench, but this is greatly exceeded by the use of the filament lamp as a transmitter instead, which I have demonstrated to be able to carry a speech bandwidth quite well. The IR output from the lamp could be filtered and collimated to communicate over a much longer range than possible on the test bench.


Returning to the photo diode radiation, I then coupled the SFH203 and 1k resistor to one of my AM opto TX circuits and confirmed that the radiation is indeed capable of carrying a speech bandwidth, as the microphone signal came out of the receiver loudspeaker clearly, with no sign of hf roll-off, at least in the 300Hz - 3kHz range that the system is designed for.


Stuart