Nuts and Volts - October 2010

PICAXE PRIMER

and error” approach to the problem.

Before I discuss my method and the results I obtained, I need to mention an important factor to keep in mind. Standard 1/4 watt resistors have a 5% tolerance rating which means that a 10K resistor can actually measure anywhere between 9.5K and 10.5K. This is why it’s important to be able to produce a wide range of analog voltages. If the ADC readings for two adjacent keys were too close to each other, variations in actual resistor values could result in misidentifying the specific key that has been pressed.

In order to make my trial and error approach as painless as possible, I set up a simple Excel spreadsheet to compute the ADC values that would result from a specific combination of resistors and then tried various combinations until I found one that worked. When I used the 256 levels provided by the readadc command, some of the ADC values for two adjacent keys were so close (differences of 4 or 5) that I was concerned that errors could result. Switching to the 1024 levels provided by the readadc10 command greatly simplified the task.

The resistor values that I finally chose are shown in Figure 4. Each of the 16 “key” positions includes two pieces of relevant data: the total resistance that is connected in series with the 10K base resistor when the corresponding key is pressed; and (in parentheses) the resulting value produced by the readadc10 command.

If you would like to experiment with different resistor values, the formula you need is the basic voltage divider rule. In English: The ADC reading is to the maximum ADC value (1023 for readadc10) as the base resistance (10K) is to the total resistance. Figure 5 presents the same thing algebraically if you prefer it that way.

If you double-check some of my computations, you’ll find that they are sometimes off by a small amount. That’s because I didn’t round anything up; I truncated all my results because that’s what the 08M and all PICAXE processors do. Finally, I need to emphasize that these are theoretical results; your specific ADC values will almost certainly be somewhat different. We’ll confront that issue in the next section when we actually construct and test our breadboard circuit.

CONSTRUCTING AND TESTING THE BREADBOARD CIRCUIT

■ FIGURE 4. Specific resistor/ matrix values.

The schematic for our breadboard circuit is shown in Figure 6. As I mentioned earlier, the eight connections to the keyboard that I’m using are not logically ordered, but it really doesn’t matter much. The important thing is to make sure that the connections for each resistor are the same as the ones presented earlier in Figure 2. If the pinout is different for the keypad you intend to use, simply rearrange the connections appropriately. The parts list for our breadboard circuit is too simple to warrant a separate sidebar; just a matrix keypad with a male header, the seven resistors shown in the schematic, and a PICAXE-08M processor are needed.

My breadboard layout is shown in Figure 7. To save some space on the breadboard, I’m using the AxMate-FT programming adapter which is also supplying power to the breadboard. The small red printed circuit board (PCB) attached to the AxMate-FT is the 5V version of SparkFun’s latest FTDI Basic Breakout board (DEV-09716) which I have reconfigured to work correctly with PICAXE processors (as we discussed back in the June Primer). I really like SparkFun’s new board because they have moved the six-pin female header underneath which makes this the smallest FTDI-based board that I have found so far.