by Bill Donofrio
Project
Build an RC Car on the Cheap
Versatile Circuit Allows for Personal Customization
Radio control racing can be a fun and rewarding
hobby. Over the years, I have designed numerous
electronic projects, but this one was the most fun to
build and use. While there are many advantages to building
your own RC car or vehicle from scratch, the bottom line is
that you can modify it to suit your needs. This multiple function car was built on the cheap with inexpensive electronic
components and a body from a wire-controlled car that
can be purchased from any toy store for about $10.00. The
wire-controlled cars are those with the joysticks connected
to the cars by trailing wires. While you may only want to use
this circuit to design RC cars, you can also use it in boats,
planes, and robots, or even to open your garage door.
The car will go forward, stop, and reverse, and the
steering is time proportional. With time proportional steering, the longer you hold down the joystick, the farther the
wheels will turn left or right. This allows for precision in
wide or tight turns.
Circuit Description
and a couple of momentary switches, I was able to change
the frequency of a 555 timer. The tone generator circuit is
relatively simple: capacitors C1, C2, and C3 determine the
frequency of the 555 timer. Since they are connected in parallel, disconnecting any capacitor from the circuit will change
the frequency of the timer. If all three switches are open, the
total capacitance seen by the 555 is .0127 uF, and a 751 Hz
tone with an approximately 50% duty cycle is generated. The
exact value of the tone generated may vary somewhat from
these values due to the tolerances of the resistors and capacitors used. When SW1 is depressed, C1 is disconnected from
the circuit and an 1,192 Hz tone is generated. If SW2 is
depressed, C1 and C2 are disconnected from the circuit and
a 2,029 Hz tone is generated. When SW3 is depressed, the
timer is reset and no tone is generated.
The output of the 555 tone generator is connected to
the electric microphone of the two-way communicator
through a 10 uF capacitor and 100K resistor. If none of
the switches are depressed, the two-way radio puts out a
continuous tone of 751 Hz.
When I designed this RC circuit, I didn’t want it to have
dual power supplies, or analog filters (high, low, or band
pass). I wanted to keep the analog circuitry to a minimum.
The circuit also needed to operate on a single frequency
and have long range. I have seen too many radio
controlled cars go off into the wild blue yonder after they
lost communication with the transmitter. It also had to be
inexpensive to build and have reasonable battery life.
Receiver
Transmitter
At first I didn’t think I could meet all of these criteria, but
with a little thought, things started to come together. After
designing a couple of transmitters, I decided they either didn’t have the range I needed or they were
too bulky. So I opted to modify a commercial two-way radio. A two-way radio can be
purchased for as little as $10.00, is crystal
controlled, and has good range. The first
thing I needed to do was build a tone generator that could produce three frequencies in the audio range. Since most two-way radios have a limited audio frequency
range — from about 500 Hz to 3,000 Hz —
the frequencies chosen would be limited
to this region. Figure 3 shows a block
diagram of the transmitter and receiver.
Using a 4066 quad bilateral switch
SEPTEMBER 2005
The tones are received at the other two-way radio, and
the output from the speaker is connected to the tone decoder
circuit through a 10 uF capacitor and a voltage divider consisting of 1K and 100 ohm resistors. From there, the signal
transformed from an audio AC signal to a square wave DC
signal by amplifying it through a LM324 Quad Operational
Amplifier with a gain of 1,000. The square wave signal is
measured by a relatively simple frequency counter consisting
of a 4017 Decade Counter/Divider and one half of a 556 100
Hz oscillator. This part of the tone decoder is really the heart
of the circuit. When a tone is received by the clock input of
the 4017, it’s counted until
Figure 1. View of remote control car.
Figure 2. Transmitter for
remote control car.
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