Nuts and Volts - December 2010

■ FIGURE 8. Breadboard setup for Experiment 4.

■ FIGURE 9. Bottom view of modified touchpad.

Download TouchMoment.bas to your breadboard setup. Whenever you touch the on key, the red LED should light; as soon as you withdraw your finger, it should go out. When the circuit is functioning correctly, try sliding your finger back and forth between the on key and the off key, and watch the LED as you do so. You will most likely find that you need to slide your finger entirely off the on key before the LED goes out. In fact, you may even be able to light the LED before your finger is actually touching the on key at all.

Next, change the value in the if…then statement to 150 and slide your finger back and forth on the keypad again to see the difference it makes. This little experiment should highlight the importance of properly calibrating any touchpad sensors you implement in a project. (Also, don’t forget that each pin may require a slightly different calibration.)

EXPERIMENT 4: IMPLEMENTING TWO LATCHED TOUCH-SENSITIVE KEYS

For our last experiment, we’re going to modify the previous experiment so that it implements two latching keys. In order to do so, just add a green LED (and a current-limiting resistor if necessary) to pin C.0 on your breadboard setup (see Figure 8) and modify the

TouchMoment.bas program by adding two additional constant definitions (LED_G = C.0 and offKey = B. 2), one additional variable definition (offVal = b0), and editing the main do…loop so that it reads as follows:

do
  touch onKey, onVal
  touch offKey, offVal
if onVal > 100 then
  high LED_R
  low  LED_G
endif
  if offVal > 100 then
 low  LED_R
 high LED_G
  endif
loop

Save the modified program as TouchOnOff.bas, download it to your breadboard setup, and test it out. You may also want to experiment with adjusting the calibration values in the two if…then statements to see what happens. For example, suppose the two switches were controlling a device for which an accidental on condition would be much more serious than an accidental off condition. How would you want to calibrate the two switches in that situation? Also, what changes would you make if the reverse were true, i.e., an accidental off condition had serious consequences?