Page 99 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

5.24 Schematic of IR DTMF transmitter circuit

78 of the transistor. Although the IR LED may be connected
directly to the output of the 5089, the power output
would be small. The NPN transistor allows additional cur-
rent to power the LED.

Figure 5.25 shows the front end of the IR receiver. An IR
phototransistor is coupled to a CMOS op-amp. This com-
bination of components allows the receiver chip (8870) to
lock in on the IR radiation from a distance of several feet.
Remote control
Using the IR link, you should be able to press a number on
the keypad and see the corresponding number displayed
on the digital display. Test the IR link at this point for
maximum distance and direction. You should be able to
increase the distance by placing the IR LED and photo-
transistor in their own reflectors. The light reflector from
an old flashlight will work well.
The remote control begins by adding a 4028 IC. The 4028
is a BCD-to-decimal decoder, meaning it reads the binary
number (remember the four LEDs from Fig. 5.22) and
outputs a single line equal to the decimal equivalent. The
4028 has 10 (0 to 9) output lines. Whatever 4-bit binary
number is placed on its input lines, the 4028 outputs a
high signal on that output line (see Fig. 5.26).
Chapter five Team LRN