Elenco Radio Controlled Car Soldering - Manual (Page 5)

reduce power to the Motor and degrade the receiver’s ability to filter, amplify, and decode commands from the
Transmitter.
Radio Receiver: (refer to the schematics and block diagram on p.31 as needed)
The car antenna collects radio energy and converts it back into electrical energy; the energy here will always
be much less than the energy originally applied to the transmitting antenna. If the car is turned on then the radio
receiver in the car is continuously monitoring the electrical energy from its antenna. The first stage of the
receiver is basically a filter which is tuned to amplify any energy around 27.9MHz and block energy the antenna
picks up outside this region. If the Remote Control Transmitter is sending commands then its radio signal will
be picked up by the receiver and converted back into the original pulse sequence. Decoding circuitry then
determines which commands were sent by measuring the number of received pulses in the sequence. Signals
are then sent to the motors to execute the commands.
Take a closer look at the receiver schematic. The sub-circuit centered around transistor Q1 filters the antenna
output, if an RCC-7K transmitter is operating nearby then the 27.9MHz burst signal may be visible at its
collector. Inductor L1 is tuned so that the circuit amplifies around 27.9MHz while rejecting all other frequencies.
But we really want the pulse sequence that is hidden in the 27.9MHz signal, so then C10 is used to filter out the
27.9MHz from the burst signal we received. This result is applied to pin 14 of the SCRX2BC integrated circuit.
Inside SCRX2BC the signal is amplified and filtered in two stages between pins 14, 15, 16, 1, and 3. Pin 3 (DI)
is the output pulse sequence that was picked up by the receiver; this is used as the input to the decoder. The
SCRX2BC scans for the 4 long (synchronization) pulses and then counts the number of short pulses after them
to determine which command was sent by the transmitter. The gain of the SCRX2BC stages is high enough to
produce a pulse sequence at pin 3 even if no signal from a transmitter is present (it amplifies random noise),
but the resulting sequence will seldom be identified as one of the transmitter commands. Note from above that
there are 4 long pulses and 10 - 52 short pulses for each command, less pulses could have been used but then
the car is more likely to activate on random noise.
Pins 4 and 5 of SCRX2BC are a 100 kHz (±30%) oscillator that is used as a reference by the decoder.
Car Steering Mechanism: (refer to the schematics on p.31 as needed)
When a command is received to turn left, the SCRX2BC creates a voltage at pin 7 which turns on transistor Q9.
This then turns on Q11 and Q14 and current flows from the batteries through Q11, then through the steering
motor, and then through Q14 to ground. This current through the Motor creates a magnetic field. Inside the
motor is a small magnet which is connected to the gear you see on the outside of the motor. The magnetic field
turns the magnet in the motor, which turns the gear. The “teeth” on the gear grab the Steering Bar and pull it
to one side. Since the Front Wheels are connected to the Steering Bar, the car will turn.
To turn right, the SCRX2BC creates a voltage at pin 6 instead of pin 7. This turns on Q10, Q12, and Q13, and
current flows through the steering motor in the opposite direction. In turn this causes the steering gear, the
steering bar, and the car to turn in the opposite direction.
Car Drive Mechanism: (refer to the schematics as needed)
The Driving Mechanism works the same as the Steering Mechanism. When a command is received to go
forwards the SCRX2BC creates a voltage at pin 11 which turns on Q2. This then turns on Q5 and Q8 and
current flows from the batteries through Q5, then through the driving motor, and then through Q8 to ground.
Similarly to go backwards the voltage is created at pin 10, and Q3, Q6, and Q7 are turned on. The small gear
on the Motor drives the Middle Gear, which drives the gear on the rear wheels axle, making the wheels move.
Note that the gears on the Motor and the rear wheels axle rotate forward and the Middle Gear rotates backward
to drive the car forward, this is because interlocking gears spin in opposite directions. Also notice that between
the Motor gear and the Middle Gear and again between the Middle Gear and the Rear Wheels axle gear, the
number of “teeth” is increased by 4:1 and 5:1 respectively, for 20:1 overall. The Motor must rotate 20 times to
rotate the rear wheels once. The reason for this is that if the Motor were to drive the wheels directly then the
RCC-7K would be very hard to control.
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