BY PAUL J. WEIJERS
Use your oscilloscope and
check the PPM signal from the
transmitter. Use a voltmeter and
check the presence of the battery
voltage, typically 9. 6 volts. The PPM
signal for a six channel system is
shown in Figure 2.
The pulses P0 to P6 are negative
going in some systems, in others they
may be positive going. The time slots,
T0 to T5, will vary from one to two
milliseconds. As an example, if the
aileron control, T0, goes to maximum,
T0 will be two milliseconds and all the
pulses in the frame move up, but the
time intervals T1 to T5 remain the
same. After the last pulse, we see
an extended period with no pulse
present. This period is at least four
milliseconds or more.
The Missing Pulse
The first section starts with a dou-ble-pole single-throw switch. Because
the trainer switch on the transmitter will
be permanently set to the trainee
position, the ACU operator must be able
to relinquish full control immediately to
the pilot. The switch has two functions:
it connects the battery supply to the
ACU circuit and connects pin 2 to pin 3.
This has the same effect as the trainer
switch on the pilot’s transmitter.
You can now wire in IC1, a CD4538.
This IC contains two re-triggerable multivibrators used as the missing pulse
detector. R1, 6.8K ohm protects the
input of IC1A. R2 and C1 are the timing
elements causing the multivibrator
UC1A to output a pulse of approximately 2.2 milliseconds — slightly
longer than the longest timeslot in the
pulse train. However, if a trigger pulse
comes along before the multivibrator
times out, the timing sequence starts
again. Pin 7 of IC1A will show a negative pulse slightly longer than the total
pulse train. During the rest period in
the pulse train (see Figure 3), pin 7
will go positive again. This causes the
second multivibrator, IC1B, to trigger.
Its timing is controlled by the R3,C2
pair causing a positive output pulse of
300 microseconds on pin 10 of IC1.
■ FIGURE 2. Six-channel PPM signal
the hex inverter are not needed. Make
sure to ground their respective inputs
to avoid unwanted oscillations. The 0.3
millisecond pulse from IC1B resets
the counter. Run a test with the oscilloscope to make sure we have got this
right. Check the stages Q0 through Q5
for a six channel system, or Q0 through
Q7 for an eight channel system.
The Trigger Stage
The Building Process
The easiest way to approach this
circuit is to build it in sections. Since
there is no feedback in the system you
can build a section, test it, and then
go on to the next section. Use Figure
5 as a guide to check your signals.
Note that everything is referenced
to the input signal on pin 2 of the
connector. See Figure 3 for the
complete schematic of the ACU.
IC4, CD4093 is a quad two-input
NAND Schmitt trigger. IC4A is used as
an AND gate. One input is from IC2, Q5.
This signal goes high at the fifth pulse
in the pulse train. The second input is
the clock pulse for IC2. The output of
IC4A goes high for the duration of the
clock pulse and provides the trigger for
the PPM regenerator, to be described
later. The last stage of the trigger
section is a differentiator C3 and R5.
The positive pulse from IC4B produces
a positive pulse coincident with the
start of the pulse and a negative pulse
coincident with the trailing edge of the
pulse. IC4C inverts the negative pulse
into a positive pulse for the next stage.
Select a Box
Of course, we need a box to
contain the circuit board and the
controls. I used an old single stick
transmitter and removed everything
except the stick assembly. The potentiometers on this assembly were
5K ohm. Because of the limited
movement of the stick, we have a
range of about 1K ohm. With a slight
adjustment of the potentiometers,
we have a minimum resistance of 1K
ohm and a maximum resistance of 2K
ohm. The reason for this will become
clear when we get to the regeneration
stage. The photo in Figure 4 shows
the assembly I used.
The Decoder Stage
IC2 is a divide by 10 counter,
CD4017, used as a serial to parallel
converter. IC3 is a hex inverter, a
CD4049. The PPM pulse train is used as
a clock signal for the counter which we
use to decode the pulse train, i.e., find
the pulse we need to split the pulse
train in two sections. Because the clock
pulse must be positive, we invert the
PPM signal with one stage of the hex
inverter. The remaining five stages of
JUST A LITTLE NOTE
To understand why IC7 acts as a
non-inverting comparator and IC8 as
an inverting comparator consider the
1. Current will flow through the open
collector when the voltage at the PLUS
input is lower than the voltage at the
2. Current WILL NOT flow through the
open collector when the voltage at the
PLUS input is higher than the voltage at
the MINUS input.
I am indebted to Ron Paisley for this
concise and illuminating statement. His
website has some excellent explanations
of comparators and operational amplifiers. You’ll find him at http://home.cogeco.
January 2006 33