that the 4011 NAND gate is configured as an AND gate.
The steering logic directs which half of the H-Bridge will
be enabled. When the 2,029 Hz steering tone is transmitted,
pins 2, 4, 7, 10, 1, 5, 6, 9, and 11 on the 4017 counter will go
high, causing both halves of the 4528 to go high. This, in turn,
causes the AND gate — consisting of two gates of the 4011
NAND to go high, turning on Q1 and Q4. Depending on how
the motor is wired, it will turn either left or right. If the 1,192
Hz steering tone is transmitted, pins 2, 4, 7, 10, 1, and 5 on
the 4017 will go high, causing only ne half of the One Shot to
go high. On the One Shot, pin 6 will be high while pin 10 will
be low. This will cause the Exclusive Or to go high and turn
on Q2 and Q3. The motor will now go in the reverse direction.
Forward, stop, and reverse are controlled by the 751
Hz tone. Since this tone is transmitted continually, absence
of the tone changes the direction of the car. One pulse
moves the car forward, two pulses stops it, three pulses
reverses it, and four pulses stops the car again.
When pin 10 of the 4017 counter produces a low output,
the inverter — consisting of 1/4 of a 4011 NAND gate — produces a high output, triggering the retriggerable One Shot
which consists of transistor Q5 and 1/2 of timer LM556. As
long as a continuous tone is sent, the output of the One Shot
remains high, and no pulses are counted by the 4017 counter. When no tone is received, the One Shot goes low for . 11
seconds and a pulse is counted by the counter. The two
relays attached to the direction motor act as an H-Bridge.
They were used instead of a transistor H-Bridge to extend battery life. Even though transistors have only a few ohms resistance when they are on, they draw considerable current when
attached to a low-resistance device, such as a motor. In this
case, good old fashioned relays work better than transistors.
At this point, you may ask why anyone in his right mind
would transmit a continuous tone when a momentary tone
would use less power. The rationale behind this was to solve a
problem associated with two-way radios. Every time you press
the transmit button on the communicator, it sends out a tone
to alert the other party that you are calling, much like the
ringer on a telephone. Since tone would be counted, it would
act as a trigger tone, possibly activating the steering or direction of the car. To solve this problem, I decided to transmit a
continuous tone. While this does waste some extra power, it
doesn’t amount to much because every few seconds you are
sending steering or direction commands to your car anyway.
Construction
The transmitter and receiver were constructed on perf-board. The ICs were mounted on IC sockets and soldered
together using 30-gauge wire wrap wire. This produces a
low profile circuit, which in many ways works as well as a
printed circuit board. I used to wire wrap all my circuits, but
found this mounting technique to be easier and faster. It’s
also less cumbersome when you need to modify or repair
a circuit. All you have to do is desolder a connection and
move the wires around.
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