The Microcontroller Idea Book - Jan Axelson's Lakeview Research

through the LEDs. If you prefer an audible indicator to announce a valid transmission, you
can replace LED5 with a piezoelectric buzzer. Instead of the LEDs, you can connect just
about any digital inputs to D6-D9.
Power-supply Options
Wireless Links
Figures 12-1 and 12-2 show the transmitter and receiver powered at +5 volts. Recommended
supply voltages for U1 and U4 are 4.5 to 18V, for U2 and U5, 2 to 6V, for U3, 2 to 15V, and
for MOD1, 4.7 to 5.3V. This means that usable supply voltages for the transmitter circuit
are 4. 5 to 6V, and for the receiver circuit, 4.7 to 5.3V.
Each circuit draws only a few milliamperes, though the test LEDs add about 5 milliamperes
each when on. Any regulated 5-volt supply that can output 100 milliamperes is suitable for
the transmitter or receiver.
Chances are that you’ll want to operate the transmitter, receiver, or both, from batteries. Four
NiCad cells in series create a reasonably stable source at around 4.8V. Using unregulated
alkaline cells is less desirable, since their voltage drops quite a bit as they discharge (from
1.5V to around 1V per cell), and there is no series combination of 1.5V cells that meets
MOD1’s supply-voltage recommendation.
A regulated supply is another option. When the supply voltage varies, U3’s output frequency
and MOD1’s frequency response also vary slightly. A regulated supply eliminates these
concerns.
Figure 12-5 shows a 5V supply that uses five or six NiCad or alkaline cells and National
Semiconductor’s 2931T-5.0 low-drop-out 5V regulator. The regulator requires an input of
just 5.6V for a 5V output at 100 milliamperes. You can also use a 9V alkaline NiCad battery
to power the regulator, but due to the low capacities of this type, you’ll get fewer hours of
use.
Figure 12-5. The LM2931T-5.0 voltage regulator creates a stable +5V supply
with an input of 5.6V or greater.
The Microcontroller Idea Book 207