Page 282 - Making PIC Microcontroller Instruments and Controllers

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2A2 BUILDII{G A SIIIIPLE EIGHT.BUTTOII TOUCTI

The operation of a touch inteface is very similar to the scanning of a keyboard
except that the wires and switches are replaced by infrar€d light beams and, of course,
instead of making a contact with a switch, we break a light beam and detect the eflect
on a phototransistor. The problem is that light beams scailer aid can illuminate mole
than one detectot while the wires go 1o one button only. making them easier to use This
to be successful.
means the scanning hasto be more device-specifio We need to turn on
one IR emitter at a time and look al the one phototransistor opposite it (ignodng all
others). You will more clearly see what I mcan by this as we go along
Port usage: PORIA (two linet, PORTB (six lines), ard PORTC (six linet will be
useal lor the project. Let's use PORTB (six lines) for connection to the six phototran_
sistors, and use PORTC'S (six linet to power the six infrafed LEDS one at a time. Wc
will use the two lines (A.2 andA.3) on PORTA lbr th.3 two LEDS. The wiring diagram
for the touch screen as we will wire it is shown in Figure 20.4.

A
rOUCH PANEL lAS FOF CONTBOLLINC
SMALL DC MOTOF W fr TNE TOUCH PANEL)
wking ctiagram. ihe 16F877A,
|tiatti€:?e{i1 Touch panel schemaiic (on PoRTD
and PORTE are being used forlhe LCD.)