IC1decodes the signal sent by the transmitter. Pins 10, 11, 12, and 13 are
outputs of the decoder that are at 0 volts if the corresponding toggle
switch on the encoder is closed(connected to ground). The outputs are
at about 5 volts if the corresponding toggle switch on the encoder is
open(not connected to ground). These outputs stay at one voltage until
they receive another signal to change.
The 10 microfarad and 0.1 microfarad capacitors(C1–C4 and C7–C12)
placed between the +V and ground buses at the +V input of each of the
ICs are used to filter electrical noise from the DC motors. That noise can
prevent the ICs from getting the correct supply voltage.IC2is an LM555 timer that generates a square wave at Pin 3. This square
wave alternates between 5 volts and 0 (zero) volts. The frequency of the
square wave is determined by the values of R2, R3,and C5;we discuss
this process in Chapter 9. When you tell Sensitive Sam to slow down,
this square wave causes the voltage to the motor to switch on and off
rapidly, with 6 volts to the motor half the time and 0 volts to the motor
half the time. When you ask Sam to speed up, 6 volts are sent to the
motor constantly. This is a simple form of pulse width modulation,which
is commonly used to control the speed of DC motors.
C6 is a capacitor that reduces the occurrence of noise on Pin 5 of IC2,
which could cause false triggering of the LM555. This might occur if Pin
5 were left unconnected.Q1turns the horn on and off. Q1 is a transistor that turns on when
decoder Pin 12 is at 5 volts. Turning on Q1 allows a current to flow
through the buzzer. To turn on the buzzer, you put the toggle switch con-
nected to the encoder Pin 12 on the transmitter in the open position and
push the transmit button. To turn off the buzzer, you put the toggle
switch connected to encoder Pin 12 on the transmitter in the closed
position and push the transmit button.
Q2and Relay 1control Sam’s speed. Q2 is a transistor that turns on
when decoder Pin 10 is at 5 volts. Pin 4 and Pin 6 are normally con-
nected (NC), and Pin 4 and Pin 8 are normally open (NO). Turning on Q2
allows current to flow through the coil in Relay 1 and connects Pin 4 to
Pin 8. Pin 4 is the output of Relay 1, and Pin 8 is where you input the
square wave from IC2 into Relay 1. When Q2 is off, no current flows
through the coil in Relay 1, and Pin 4 is connected to Pin 6. This makes
the output of Relay 1 approximately 5 volts. To ask Sam to go full speed306 Part IV: Good Vibrations