Nuts and Volts - November 2009

In Practice

■ FIGURE 3. The transceiver board. The transceiver circuitry is in the middle, mounted above the Arduino. Wiring — rather than pins and headers — was used to maintain greatest flexibility possible. On the left is the laser, mounted in a showerhead and secured to the transceiver board with wire. On the right is the photodetector, mounted with electrical tape. See text for other mounting options.

practical and theoretical information from this very friendly, extended family. Finally, from a practical level, the Arduino uses a USB virtual serial port with an FT-232RL chip to provide the serial translation. A full featured USB port on a computer guarantees five volts, 500 mA to an attached device for the device’s own use. This project uses much, much less power than that so no other power source is required.

Some may be concerned that the original Arduino boards only have one hardware serial port. This project requires two serial ports and an older soft serial library didn’t work well. While this is true, a newer software based serial port (NewSoftSerial) is very robust. Code is provided online at www.nutsvolts.com for the original Arduino (one hardware, one software port) and the Arduino Mega (two hardware ports).

If you are not familiar with the Arduino, check out Smiley’s Workshop that runs in NV each month. It will introduce you to this system. To use the communicator without further modification, build the board, install the Arduino software, let Windows install the virtual serial port, and choose the target board. Hook up the target board. Copy or enter the code, press compile, and then press download to board. Software setup is less than 30 minutes.

The system requires two adjustments in order to function. The first — and by far the most difficult — is aiming the laser to hit the phototransistor square on. I do not have a foolproof method for doing this. I mounted my communication units on some scrap lumber (Figure 3), then I mounted the laser unit within a ball joint type plastic shower head for even more control (Figure 4). Consideration could be given to mounting the unit on a camera tripod for even greater aiming control.

The other adjustment is setting an offset for the comparator. Depending upon ambient light, the baseline current of the phototransistor may vary considerably, and depending upon the distance from the laser to the detector, excitation may vary considerably. The negative input of the comparator is attached to a potentiometer which permits setting the comparison voltage, which enables extraction of the data. This is a simple adjustment.

Building It

■ FIGURE 4. Close-up of laser mounted in showerhead. The showerhead articulates in a ball and socket fashion. This particular model of showerhead had a gasket which permitted a press fit of the laser unit without further modification. Other heads may require glue or creation of a sleeve around the laser to allow a press fit. Wires follow the water path.

The electronics are easy. I recommend point-to-point wiring on a piece of perf board. The component count is so small that the work involved does not justify a printed circuit. If you wish to be particularly careful (which I heartily recommend), attach a five volt source to Vcc and Vgnd and sniff for smoke before hooking up the Arduino. Apply a five volt signal to the base of Q1 and make sure the laser is working. If you have a multimeter, set the negative input of the comparator at approximately 1-1.5 volts. Even with the laser on, total current draw should be well under

■ FIGURE 5. Close-up of photodetector consisting of hacked CNY65 optocoupler. With chip text correctly oriented for reading, as you face the chip the left side of center houses the emitter and the right side contains the detector. Using a tiny burr or milling tip, carefully start on the left of center of the chip and remove the black plastic until a white matrix is identified. On the left side of the matrix, continue to remove material until a small translucent cube is identified. This is the emitter. The detector looks almost exactly the same. Now that you know what to look for, carefully remove as much white matrix from around the detector as possible without destroying the detector plastic. This is actually quite easy, and the end result should be similar to the photo here. Only the two right side pins need to be connected. The upper right pin is high. Other optocoupler chips likely have a similar mechanical construction, but I cannot confirm that (follow schematic for appropriate hookup).