A speedometer is a device for measuring and presenting to an operator the speed of a vehicle, such as an automobile. A sample point is selected, typically at the final output (the tailshaft) of the transmission, where a flexible rotating cable is driven. The other end of this cable is connected to the speedometer, which is a specially calibrated tachometer. The responsiveness of this tachometer and its calibrated numbering must take into consideration the gear reduction from the tailshaft to the flexible cable, the final drive ratio in the differential unit, and the diameter of the driven tires. The mechanism within the speedometer head will usually also drive one or more wheeled numeric counters called odometers.

| »

Mainfold Absolute Pressure (MAP) Sensor

The manifold absolute pressure sensor is a variable resistor used to monitor the difference in pressure between the intake manifold at outside atmosphere.

This information is used by the engine computer to monitor engine load (vacuum drops when the engine is under load or at wide open throttle). When the engine is under load, the computer may alter spark timing and the fuel mixture to improve performance and emissions.

Throttle position sensor

The TPS is a potentiometer attached to the throttle shaft. A voltage signal is supplied to the sensor, and a variable voltage is returned. The voltage increases as the throttle is opened. This signal and the MAP output determines how much air goes into the engine) so the computer can respond quickly to changes, increasing or decreasing the fuel rate as necessary

Garage Parking Sensor

If you're having trouble gracefully pulling into your parking space, or want to protect against costly damage to your garage or your car, then the red, yellow, and green lights of the Garage Parking Sensor could be just what you need!

The Garage Parking Sensor is a wall mounted "parking assistant" that warns you before you're going to hit the wall.

Using a high tech "ultrasonic" sensor the unit determines the distance between the wall and your car, and displays this to you on a "traffic light" display.

Running off four AA batteries, the kit is mounted to the garage wall. The sensor unit is positioned level with the flattest surface of your car's bumper.

| »

Knock sensor

Engine knock is characteristic of an uncontrolled combustion process and can cause engine damage. That is prevented by a knock sensor, a noise sensor near the engine.

The Knock Sensor is a Piezo Electric device that when you stress it, a voltage is produced. It senses knock and transmits information to the electronic engine management control unit. This influences process control in the engine, for example timing and fuel injection until knock is eliminated.

Knock is frequently caused by fuel that does not comply with the required minimum quality.

Piezoelectric sensors

Piezoelectricityi is the ability of certain crystals to produce a voltage when subjected to mechanical stress. The word is derived from the Greek piezein, which means to squeeze or press. The effect is reversible; piezoelectric crystals, subject to an externally applied voltage, can change shape by a small amount. The effect is of the order of nanometres, but nevertheless finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, and ultrafine focusing of optical assemblies.

In a piezoelectric crystal, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. When a stress is applied, this symmetry is disturbed, and the charge asymmetry generates a voltage. A 1 cm cube of quartz with 500 lb (2 kN) of correctly applied pressure upon it, can produce 12,500 V of electricity.
| »

pH sensors

Electronic pHi meters consist of an electrolytic cell in which an electric current is created due to the hydrogen cations completing the circuit

The pH measurement originates in the electrode system. This system consists of a pH sensor, pH Half Cell, whose voltage varies proportionately to the hydrogen ion activity of the solution, and a reference electrode, Reference Half Cell, which provides a stable and constant reference voltage.

The pH electrode consists of a thin membrane of Hydrogen sensitive glass blown on the end of an inert glass tube. This tube is filled with an electrolyte, and the signal is carried through Ag/AgCl wire. This is a pH Half Cell. A similar system, but without using a Hydrogen sensitive glass, is used as a reference. A small filter (diaphragm) connects this tube to the external liquid. This system is called a Reference Half Cell.

| »

Automobile oxigen sensor

An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 voltsi.

All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen seen is 0.2 to 0.7 volts.

The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts. This is neither rich nor lean. A fully warm O2 sensor *will not spend any time at 0.45 volts*. In many cars, the computer sends out a biasi voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts. Since the computer knows this is an "illegal" value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution.

| »

Diodes (Silicon bandgap temperature sensor)

The silicon bandgap temperature sensor is an extremely common form of temperature sensor (thermometer) used in electronic equipment. Its main advantage is that it can be included in a silicon integrated circuit at very low cost. The principle of the sensor is that the forward voltage of a silicon diode is temperature-dependent, according to the following equation:

VBE = VG0(1 - T / T0) + VBE0(T / T0) + (nKT / q)ln(T0 / T) + (KT / q)ln(IC / IC0)


| »


A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistancei with changing temperature.

If we assume that the relationship between resistance and temperature is linear (i.e. we make a first-order approximation), then we can say that:

ΔR = kΔT


ΔR = change in resistance
ΔT = change in temperature
k = first-order temperature coefficient of resistance

Thermistors can be classified into two types depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have the smallest possible k, so that their resistance remains almost constant over a wide temperature range.

| »