LM1872 National Semiconductor, LM1872 Datasheet - Page 16

• Manufacturer Part Number: LM1872
• Description: Radio Control Receiver/Decoder
• Manufacturer: National Semiconductor

Applications
EXPANSION TO SIX ANALOG CHANNELS
Still greater analog capacity can be obtained with an out-
board auxiliary decoder The LM1872 a simple comparator
and an 8-bit parallel-out serial shift register comprise a six
analog channel receiver decoder (Figure 12) The one tran-
sistor comparator reconstructs the detector output of the
LM1872 from the sync timer waveform and feeds it to the
clock input of the shift register The channel 1 output then
loads a ‘‘one’’ into the register and the clock shifts the
‘‘one’’ down the line of analog channel outputs in accord-
ance with the time information from the detector output
Note that the reconstructed detector waveform lags the
channel 1 output very slightly ( j 10
slope of the sync capacitor discharge edge This delay is
very important as it insures that channel 1 is high when the
clock strikes initially (thus loading a ‘‘1’’) and low for each
subsequent positive clock edge (thus preventing the loading
of extraneous ‘‘1’s’’)
(Continued)
s) due to the finite
FIGURE 12 Deriving Six Analog Channels
b) Six Channel Auxiliary Decoder
a) Six Channel Timing Diagram
16
CONVERTING AN ANALOG CHANNEL
TO A DIGITAL CHANNEL
Either analog channel can be converted to a digital channel
with the aid of a low cost CMOS hex inverter (Figure 13)
The internal 10k resistor and external capacitor C1 set a
time constant (1 ms) that falls between a short (0 5 ms) and
a long (2 ms) transmitted pulse option For pulses longer
than 1 ms the first inverter will pull low momentarily once
each frame Repetitive discharges of C2 prevent it from ever
reaching threshold (V
stant is set longer (70 ms) than the frame period With the
inverter input below threshold Q1 will energize the load For
analog output pulses shorter than 1 ms the first inverter will
back bias D1 allowing C2 to ramp past threshold and Q1 to
go off For extra output drive the remaining inverters in the
package can be paralleled to drive Q1 Alternatively for light
loads Q1 can be eliminated altogether
a
2) because the R1 C2 time con-
TL H 7912 – 24
TL H 7912– 23