VHDL Adavanced VHDL Arithmetic-Circuits

VHDL Concurrent-Statements








TITLE:- RPM indicator

AIM: Design, build and test sensing disc used in RPM indicator for measuring RPM

APPARATUS: LED, photodiode, power supply.



RPM measurement is important when controlling or monitoring the speed of motors, conveyors, turbines, etc.

RPM or revolution per minute for a rotating motor shaft is measured by an electronic tachometer. A light repellant tape is taped on a shaft and the tachometer sensor measures the shaft rpm. In a car, a similar principle is used but, voltage generated by a small generator is recorded as rpm in the gauge.

Sensors for RPM Measurement

A sensor is necessary to sense shaft speed. Typical devices used for this purpose are shaft encoders (rotary pulse generators), proximity sensors, and photoelectric sensors. Each of these devices sends speed data in the form of pulses.

Two factors affect the quality of this data

• Number of pulses per revolution of the shaft (referred to as PPR). Higher PPR values result in better resolution.

• Symmetry of pulses. The symmetry of one pulse to the next can play a role in how consistent the RPM readings are. Symmetrical pulses give more accurate data.

Methods of Determining RPM

There are two basic methods to determine the RPM.

1.The Frequency measurement method

2.The Period measurement method.

Frequency measurement is better for fast-moving devices such as motors and turbines that typically turn in thousands of revolutions per minute.

Period measurement is better for devices that move more slowly, such as shafts that turn in less than 10 RPM.


Rounds per minute already is frequency. Frequency means <something repeats> per <time unit>

60 RPM = 1 round per second.

So for 400 RPM,

Divide by 60 = 6.667 rounds per second.


A light-emitting diode (LED) is an electronic light source. The first LED was built in the 1920s by Oleg Vladimirovich Losev , a radio technician who noticed that diodes used in radio receivers emitted light when current was passed through them. The LED was introduced as a practical electronic component in 1962.

All early devices emitted low-intensity red light, but modern LEDs are available across the visible , ultraviolet and infra red wavelengths, with very high brightness.

LEDs are based on the semiconductor diode . When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. The LED is usually small in area (less than 1 mm 2) with integrated optical components to shape its radiation pattern and assist in reflection.

LEDs present many advantages over traditional light sources including lower energy consumption , longer lifetime , improved robustness, smaller size and faster switching. However, they are relatively expensive and require more precise current and heat management than traditional light sources.

Applications of LEDs are diverse. They are used as low-energy indicators but also for replacements for traditional light sources in general lighting and automotive lighting . The compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in communications technology.



LEDs used in a traffic signal .

The first commercial LEDs were commonly used as replacements for incandescent indicators, and in seven-segment displays , first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches (see list of signal applications ). These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level. The invention and development of the high power white light LED to use for illumination. Most LEDs were made in the very common 5 mm T1¾ and 3 mm T1 packages, but with increasing power output, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs.

Like a normal diode, the LED consists of a chip of semiconducting material impregnated, or doped , with impurities to create a p-n junction . As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers— electrons and holes —flow into the junction from electrodes with different voltages . When an electron meets a hole, it falls into a lower energy level , and releases energy in the form of a photon.

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide . Advances in materials science have made possible the production of devices with ever-shorter wavelengths , producing light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

Most materials used for LED production have very high refractive indices . This means that much light will be reflected back in to the material at the material/air surface interface. Therefore Light extraction in LEDs is an important aspect of LED production, subject to much research and development

A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation.

Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction .

Some photodiodes will look like the picture to the right, that is, similar to a light emitting diode . They will have two leads, or wires, coming from the bottom. The shorter end of the two is the cathode, while the longer end is the anode. See below for a schematic drawing of the anode and cathode side. Under forward bias, conventional current will pass from the anode to the cathode, following the arrow in the symbol. Photocurrent flows in the opposite direction.

Principle of operation :

A photodiode is a PN junction or PIN structure . When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced.

Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown , resulting in internal gain within the photodiode, which increases the effective responsivity of the device.

Phototransistors also consist of a photodiode with internal gain. A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction . The electrons that are generated by photons in the base-collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfe). Note that while phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes. Phototransistors also have slower response times. A simple model of a phototransistor, would be a forward biased LED (emitter–base) and a reverse biased photodiode (base–collector) sharing an anode (base) in a single package such that 99% of the light emitted by the led is absorbed by the photodiode. Each electron-hole recombination in the LED produces one photon and each photon absorbed by the photodiode produces one electron-hole pair.


The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents.

Materials commonly used to produce photodiodes include:


Wavelength range (nm)





Indium gallium arsenide


Lead(II) sulfide


Because of their greater band gap, silicon-based photodiodes generate less noise than germanium-based photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm.


1) Threshold voltage VT =

2) No. of times voltage across photodiode exceeds VT =

3) No. of holes =

4) Time =


RPM = No. of times voltage exceeds VT x time

No. of holes




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