The receiver module separates the coded signal sent by the transmitter from the 433.9 MHz carrier wave. The output of the receiver module at Pin 2 is the decoded signal, as shown in Figure 13-2.

VR1 is a voltage regulator that takes the 6 volts supplied by the battery pack and supplies a steady 5 volts because the transmitter module has a maximum voltage of 5.5 volts. We provided a separate battery pack for this section of the circuit to provide a stable supply for the radio receiver.

S1 is the on/off switch for the circuit.

IC1 decodes 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.

IC2 is 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.

Q1 turns 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 connected 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.

Q2 and Relay 1 control 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 connected (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 speed

Use Q3 and Relay 2 to tell Sam to start or stop. Q3 is a transistor that turns on when decoder Pin 13 is at 5 volts. Turning on Q3 by having the start/stop transmitter toggle switch closed allows current to flow through the coil in Relay 2 and connects Pin 4 (the output of Relay 2) to Pin 8. This tells Sam to run his engines. When the start/stop transmitter toggle switch is open and Q3 is off, no current flows through the coil in Relay 2, and Pin 4 is connected to Pin 6; this makes the output of Relay 2 zero (0) volts.

IC3 is an H-bridge motor controller. Although this controller is capable of controlling more functions than just going straight ahead (as you can read about in Chapter 11), all we need it to do here is supply the voltage to drive each motor forward. The battery pack attached to Pin 8 of IC3 supplies power for the motors. You connect the output of Relay 2 to the enable pins (1 and 9) of IC3. When 5 volts is provided by Relay 2 to the enable pins, IC3 supplies power to the motors. When 0 volts is connected to the enable pins, IC3 doesn’t supply power, so Sam just sits there.

The left and right sensors allow Sensitive Sam to take control of himself. When the track curves or Sam drifts so that one of the sensors is over the black electrical tape, power is cut to the motor on that side. This causes Sam to move away from the tape. When the sensor again hovers over a reflective floor, power is restored to that motor, and Sam straightens out.

On the schematic, the orange (O) and green (G) wires connect to an LED, with R4 and R6 limiting the current to protect the LED from damage. The blue (B) and white (W) wires connect to a phototransistor.

When the sensor moves over a reflective surface, such as hardwood floor, the phototransistor is on, and the base of Q5 or Q4 is connected to ground. This turns off Q5 or Q4, which leaves the output of Relay 3 or

Relay 4 connected to Pin 11, allowing the motor to run. When the sensor is over a nonreflective surface, such as black electrical tape, the phototransistor is off, and the base of Q5 or Q4 is connected to a positive voltage through R7 or R5, turning on Q5 or Q4. This disconnects the output of the relay from Pin 11 and also shuts off the motor.