Logic Circuits

As computers and solid state control modules become more prevalent on automobiles, some of the logic gate symbols that represent their internal circuits will show up more often. It is necessary to know not only what the logic symbols stand for, but to understand the basic operation of the circuits they represent when you analyze wiring diagrams during troubleshooting. Therefore, you should know a little about logic circuits and the symbols used to represent them. A logic gate symbol is simply a shorthand way of representing an electronic circuit that operates in a certain way. Understanding the logic symbols can make understanding the operation of a circuit much quicker and easier than if the circuit were represented by showing all the transistors, diodes and resistors. The logic symbols shown in diagrams in the EWD and New Car Feature book show what pin voltages must be present for an electronic controller to function properly.

Again, anything connected with a computer is based on the digital on/off language. The same holds true for logic circuits, which are made up of transistors combined in units called "gates." These gates process two or more signals logically. In essence, they are switches. Depending on the input voltage, the gate or switch will be either on or off.

The first thing to learn about the different gates is their symbols. Once you know the symbols and how each gate works, diagnosing a computer related problem will be easier.

NOR GATE

A NOR gate combines the OR gate and the NOT gate, so it functions like the OR gate except the output will be the opposite. This means that the output is only on if there is no voltage at both inputs.

TRUTH TABLE FOR "NOR" GATE

Inputs

Output

b —) y

3- C

Inputs

Output r^o

*-) J

LOGIC SYMBOL FOR "NOR" GATE

Inputs

Output

A B

C

0 0

1

0 1

0

1 0

0

1 1

The exclusive OR gate limits the output to certain combinations of inputs. An even number of Is will produce a 0 or low output. An odd number of Is will produce a 1 or high output. The XOR gate symbol is different than the OR gate in that it has an added curved line to denote an exclusive feature.

LOGIC SYMBOL FOR "XOR" GATE

Inputs

Output

A -\

\

)- C

B -J

/ y

TRUTH TABLE FOR "XOR" GATE

Input

Output

A B

C

0 0

0

0 1

1

1 0

1

1 1

By combining two NAND gates together, a circuit can be created called a Reset-Set flipflop. The R-S flip-flop will toggle as the inputs switch between Is and Os. The unique feature of the circuit is the ability to remember or hold the last output (1 or 0), if both inputs are 0. The R-S flip-flop can be found in the moon roof and light auto-off systems of Toyota vehicles.

R-S FLIP-FLOP WIRING DIAGRAM SYMBOL A -

Inputs

-C Output

R-S FLIP-FLOP CONSTRUCTION

Inputs

Output

TRUTH TABLE FOR R-S FLIP-FLOP CIRCUIT

Inputs A= Set B=Reset

Output C=Q

0 1

0

1 0

1

0 0

Hold

1 1

1

The following worksheet demonstrates how logic symbols can be used to determine how a system should operate and how malfunctions in seemingly unrelated areas can affect the system's operation.

Taken with permission from the Toyota Advanced Electrical Course#672

1. Explain both the purpose and different types of inputs used by the computer.

2. Name the type of output signal most often used by the computer.

3. Name the components that are typically used as output devices.

4. Explain the difference between Analog and Digital Signals.

5. Explain both the purpose and complete name of an A/D converter.

6. Draw both an Analog and Digital signal.

7. Explain the binary numbering system and why it is used.

8. Explain the function of the Microprocessor.

9. Describe the purpose of the RAM (Random Access Memory)

10. Describe the purpose of the ROM (Read Only Memory)

11. Describe the purpose of the PROM (Programmable Read Only Memory)

12. Explain the basic function and list the truth table of an "AND" logic gate circuit.

13. Draw the equivalent mechanical circuit of an "AND" logic gate circuit.

14. Explain the basic function and list the truth table of an "OR" logic gate circuit.

15. Draw the equivalent mechanical circuit of an "OR" logic gate circuit.

16. Describe the basic function and list the truth table of a "NOT" logic gate circuit.

17. Describe the basic function and list the truth table of a "NAND" logic gate circuit.

18. Describe the basic function and list the truth table of a "NOR" logic gate circuit.

19. Describe are the two basic components of a "FLIP-FLOP" logic gate circuit.

SENSORS AND ACTUATORS

SENSORS ACTUATORS

AIR FLOW METER

b^--

ECU

#10

EFI

Ne

NO. I AND NO. 4 INJECTORS

DISTRIBUTOR____

• Engine RPM Signal

#30

NO. 2 AND NO. 6 INJECTORS

THA

NO. 3 AND NO. 5 INJECTORS

WATER TEMP. SENSOR

STJ

COLD START INJECTOR CONTROL

INTAKE AIR TEMP. SENSOR

IGt

THROTTLE_ POSITION SENSOR_

• Idling Signal

• Throttle Position Signal

I DL

ESA

VTA

] IGNITER |

IGf ISC,, ISC,

1

IGNITION SWITCH___

• Starting Signal (ST terminal)

* Ignition Signal (IG terminal)

STA

I IGNITION COIL |

I DISTRIBUTOR I

SP,

1

NO. 1 VEHICLE SPEED SENSOR

| SPARK PLUGS J

NO. 2 VEHICLE SPEED SENSOR

OX,

ISC

MAIN OXYGEN SENSOR

ox,

ISCj.ISC, EGR

SUB-OXYGEN SENSOR"

EGR GAS TEMP. SENSOR*' |—ÏÔÎ5---

EGR CUT CONTROL

KNOCK SENSOR 1——J

FPR

NEUTRAL START SWITCH

NSW

FUEL PUMP SPEED CONTROL

[ FUEL PUMP RELAY J

AIR CONDITIONER COMPRESSOR RELAY

CRUISE CONTROL COMPUTER

A/C

OD,

SUB-OXYGEN SENSOR" _ HEATER.CONTROL_____

______ _____________________1 OD,

i SUB-OXYGEN SENSOR HEATER |

L

SHIFT POSITION SWITCH

___E_CT_CONTRQLli___

___________-1 P

SL

{ SOLENOID VALVES |

STP

STOP LIGHT SWITCH

W

CHECK ENGINE LAMP (Diagnostic Code Display)

HEADLIGHT RELAY

-

M-RFL ,-

DEFOGGER RELAY

EFI MAIN RELAY

TDCL (TOYOTA Diagnostic Communication Link) or CHECK CONNECTOR

TE,

OD,

OVERDRIVE INDICATOR LAMP*1 (Diagnostic Code Display)

te,

SENSORS AND ACTUATORS

Computer controlled systems continually monitor the operating condition of today's vehicles. Through sensors, computers receive vital information about a number of conditions, allowing minor adjustments to be made far more quickly and accurately than mechanical systems. Sensors convert temperature, pressure, speed, position and other data into either digital or analog electrical signals.

A digital signal is a voltage signal that is either on or off with nothing in between. A switch is the simplest type of digital signal sensor. The signal from the switch could be 0 volts when off and 12 volts when on. Analog signals on the other hand have continuously variable voltage. A good example is the coolant temperature sensor. The coolant temperature sensor may vary the voltage signal anywhere between 0 volts and 5 volts depending on the temperature of the engine.

The digital signal is the easiest for the computer to understand because it reads the signal as either "on" or "off." The analog signal must be conditioned or converted to digital so the computer can understand it. (This will be covered later.)

While a vehicle may have many different sensors, there are three main categories: voltage-generating, resistive and switches. A voltage-generating sensor generates its own voltage signal in relation to the mechanical condition it monitors. This signal in turn relays to the computer data about the condition of the system it controls. A resistive sensor reacts to changes in mechanical conditions through changes in its resistance. The computer supplies a regulated voltage or reference voltage to the sensor and measures the voltage drop across the sensor to determine the data.

Switch sensors toggle a voltage from the computer high or low, or supply an "on" or "off" voltage signal to the computer. This type of sensor may be as simple as a switch on the brake pedal or as complex as a phototransistor speed sensor.

The computer uses the sensor data to control different systems on a vehicle through the use of actuators. An actuator is an electromechanical device such as a relay, solenoid or motor. Actuators can adjust engine idle speed, change suspension height or regulate the fuel metered into the engine.

This chapter describes several specific sensors used in automobiles, such as potentiometers, thermistors and phototransistor / LED combinations. This chapter also addresses actuators that complete the control process by carrying out the computer's instructions.

The Sensors and Actuators section is divided into the following areas:

Do It Yourself Car Diagnosis

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