No special tools are
needed other than a soldering
iron and a drill. The board files
and schematics are generated
by ExpressPCB software, which
is available for free at www.
expresspcb.com. Preprogrammed code matched
microprocessors are available,
along with boards from the
Nuts & Volts webstore. Go to
the article link for part
numbers, sources, extra
schematics, “Hints and Tips,”
PCBExpress board files, etc.
Figure 1 shows what’s needed
for our You Have Mail app.
The transmitter uses a Microchip PIC16F690 for
processing several types of signals. A Linx TXM-418
transmitter is used which has a frequency of 418 MHz. A
rotary BCD switch allows for 16 programs to be run; refer
to Figure 2. It is important to remember when building a
particular application that other parts can be omitted, e.g.,
if only the photodiode is used, you don’t need the op-amp
or its resistors or R5. However, more than one application
can be run on the board since the additional parts will not
interfere with each other. Keep in mind, the more
applications, the more power the circuit will draw. The
one exception is that only one light type sensor can be
used at one time.
The transmitter microcontroller goes to sleep and is
only awakened when a pin change takes place. It turns on
the transmitter and sends a 256 code that activates the
receiver. The method for modulating the frequency is bit
banging. Any switch — whether normally closed or
normally open — that changes will cause a transmission.
The temperature and amplifier circuits are on the
comparator microprocessor inputs. If they are above or
below the voltage set by the reference voltage (R5), it will
trip the micro. The above or below settings are
determined by the rotary switch. I designed the transmitter
software not to alarm when resetting its condition.
D1 provides a . 8 volt diode drop to reduce the
transmitter voltage to 3. 3 volts.
The receiver consists of a Microchip 12F508 and a
Linx receiver RXM-418 chip; refer to Figure 3. The
microprocessor decodes the transmitter codes and will
alarm if it receives a signal matching its key code. It
October 2013 29
■ FIGURE 1.
■ FIGURE 2.