Nuts and Volts - November 2013

cable for the connection to the 3. 5 mm audio jack for the Audio IN. Connection of the programming port requires a specific connection to the jack as described in the PICAXE manual. The four AAA battery pack is connected directly to the board and doubles as the power switch for the circuit. If you like, you can substitute or add a 5V supply such as from a USB port or other rectified source to power the circuit without modification to the electronics. The NiMH supply will last about a week before needing to be recharged.

Figure 7 is an example of the assembled components. The display driver connects to the back of the display board in a shield-like fashion using SIP headers. With the driver connected to the display board, the entire assembly mounts to the upright panel of the document holder, also shown in Figure 7. The two eight-pin male headers located on the driver board supply the display board with 15 I/O lines and a positive supply by way of the two headers. Threaded standoffs are used to attach the display and driver shield to the frame of the document holder.

The Discriminator Software

Software for the circuit is pretty straightforward. We are basically using the PIC as a low-level resolution frequency counter. I began writing this software using the COUNT command to actually measure the literal frequency. This would work fine if we were just measuring static tones, but for dynamic music it turned out to be a little too slow. The alternative is to use the PULSIN command which measures the length of the incoming pulses and produces a corresponding number between zero and 255. This is less graceful but at the end of the octave sampling day, achieves the same net result. You just have to be willing to translate your target frequency range initially into their corresponding 256 bit

■ FIGURE 6. Wiring connections to the driver board.

values, and then use that translated value in your conditional statements. This is actually easier than it sounds. To translate the output using the PULSIN command, start a DEBUG window and input a frequency to the PIC using the following example code:

DO ; begin loop
PULSIN C.6, 1, W0
 
  ; measure the frequency on pin C.6 and put
  ; result in variable W0
DEBUG W0 ; start DEBUG monitor

■ FIGURE 7. Display and driver board connecting to the front of the document holder.

The DEBUG window will show a list of all your variables. If the input frequency was 2,400 Hz, the translated value of 19 is what got stored in W0 as shown in Figure 8. Instead of using the actual frequency in Hz for the octave range, you would record the translated value for both the low and high end of the octave range and then use those values in your conditional statements.