Week 9. Input devices: Synchronous laser detector

This week, my goal is to apply synchronous detection to a laser diode and phototransistor, for the detector of the confocal 3D scanner.

Production of FabFTDI cable

I followed the tutorial by Prashant to make my own FTDI cable! I used the same process as in the electronics production week. Here is an image of the board with GND, VCC, and polarity of Zener diodes indicated. It took me some time to correlate the schematic and the traces.

FabFTDI board Testing FabFTDI with echo board

I programmed the board with the cdctiny45.hex file provided by Prashant, using the same process as in the embedded programming week. As hoped, the FabFTDI appeared as USB-232 in About This Mac → System Report ... → Hardware → USB. Note that System Report ... window does not update if a cable is connected or disconnected, one needs to open and close the window to reflect recent changes.

As recommended by Prashant, I next tested the board on Neil's hello FTDI echo board from embedded programming week. In hello.ftdi.44.echo.c I changed the bit delay time to 208.3 since the baud rate of the FabFTDI is 4800.

Picture of button Tried to make my own FTDI cable following instructions from Prashant. It worked on Ubuntu, but did not work on Mac. Some note about (explain). Copy FTDI information here
  • Electronics

    • Synchronous detection board

    I modified Neil's synchronous detection board to drive an external laser diode rather than a surface mount LED. In particular, I replaced the copper pads for the LED package with pads for a pair of header pins, and I increased the current for more power from the laser diode.

    Phototransistor considerations

    Neil's page links to the phototransistor OP580DA. So DA stands for Darlington. The datasheet is here: http://optekinc.com/datasheets/OP580DA.pdf Photo darlington devices are normally used in applications where light signals are low and more current gain is needed than is possible with phototransistors. At 1 mW/cm2, OP520 gives 0.2 mA, whereas OP580A gives about 22 mA. So OP580A gives about 100x more current than OP520. Note that OP580DA is matched to the LED OP280. The datasheet for OP520 is here: http://optekinc.com/datasheets/OP520-521.PDF The OP520 and OP521 are mechanically and spectrally matched to the OP250 series infrared LEDs. Go for OP520 which has optical filter to block visible light, and OP250 as LED. Went for OP522 which is clear, and a red LED. Checked the LED orientation with a multimeter. the green line adjacent to the cathode, same as with the Zener diode Darker green mark is the collector.

    Modification of the traces with GIMP

    I modified Neil's synchronous detection board to drive an external laser diode rather than a surface mount LED. In particular, I replaced the copper pads for the LED package with pads for a pair of IDC header pins. I used GIMP to modify the design. I learned a few GIMP hotkeys

    Select rectangle R
    Select → Float Control + Shift + L
    Fill with foreground (black) Control + ,
    Fill with background (white) Control + .

    and to increase the size of the image used Image → Canvas size.

    More power from laser diode

    In order to increase the power from the laser diode, I decreased the resistance of the current limiting resistor. An illegible sticker on the laser diode indicates

    Voltage 3V DC
    Current 19 mA (?)
    Power 3.64 mW (?)
    Wavelength 660 nm

    Using this calculator, I calculated the resistance of the current limiting resistor. Since the supply voltage is 5 V (Sam noted that FTDI cables do 5 V power and 3.3 V logic), the voltage drop across the LED is 3 V, and the desired current is 3.6 mA, the resistance should be 555 Ohm (instead of the original 1 kOhm) and the resistor dissipates 7.2 mW. I used a 560 Ohm SMD resistor. Note the upper bound on a USB port is 100 mA, so we should be far from that upper bound. Also here is some background on laser diodes.