IC1is an encoder whose job it is to send out a signal that tells a decoder
on the receiver what to do. Pins 4, 6, and 7 are inputs to the encoder
(inputs 3, 2, and 1, respectively). When a normally open (NO) pushbutton
switch (B1, B2, B3)tied to one of these pins is closed, the encoder modu-
lates the 38 kHz carrier wave that tells the decoder IC in the receiver
exactly which button has been pressed. This signal goes out through Pin
5 and then through a 150 ohm resistor (R1). The resistor limits the cur-
rent to about 22 milliamps; that’s so you don’t burn out the LED. The
signal then goes through the IR LED, which generates an infrared signal.
X1is a 4 MHz ceramic resonator. This works with components within
the encoder to generate timing signals that help generate the 38 kHz car-
rier wave and runs an internal clock used to generate a signal that identi-
fies which pushbutton switch has been closed.Receiving what the transmitter sends .............................................
Just like your TV receives the signal from a remote control telling it to flip
over to MTV, something has to receive the transmitter signal to make the kart
go. The receiver circuit is shown in Figure 11-3.Note what’s going on in this schematic:The IR detectorcontains a photodiode; when the infrared signal reaches
the photodiode, it produces an electrical signal. This electrical signal
goes into Pin 4 of the decoder (IC1).
IC1 then decodes the electrical signal sent by the transmitter. Pressing
and releasing a pushbutton on the transmitter causes the decoder to
switch around the state of the corresponding output; Pin 7 is output 1,
Pin 6 is output 2, and Pin 5 is output 3. So, if the output pin is high, (5
volts), it will be changed to ground; if the output pin is low (ground), it
will be changed to high (5 volts). The output stays at that voltage until
another signal appears to change it.The resonator (X1)drives an oscillator within the decoder to generate
internal clock signals used to decode the signal sent out by the encoder.
Like with the transmitter circuit, a voltage regulator (VR1)limits the
supply voltage to the ICs to 5 volts.S1is the power switch for the receiver.
You place a 10 microfarad capacitor (C2, C4, C6)and 0.1 microfarad
capacitor (C1, C3, C5)between the +V and ground buses at the +V input
of each of the ICs. These are used to filter electrical noise from the DC
motors; that noise can prevent the ICs from operating correctly because
they won’t consistently have the correct supply voltage. If you don’t add
these capacitors to the circuit, the motors could occasionally stop run-
ning or stop responding when you press the transmitter button.242 Part III: Let There Be Light