Photovoltaic Cells

Photovoltaic Cells (Converting Photons to Electrons)

The solar cells that you see on calculators and satellites are also called photovoltaic
(PV) cells, which as the name implies (photo meaning "light" and voltaic meaning
"electricity"), convert sunlight directly into electricity. A module is a group of cells connected
electrically and packaged into a frame (more commonly known as a solar panel).
Photovoltaic cells are made of special materials called semiconductors such as
silicon, which is currently used most commonly. Basically, when light strikes the cell, a
certain portion of it is absorbed within the semiconductor material. This means that the
energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons
loose, allowing them to flow freely.
The solar cell works in three steps:
- Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such
as silicon.
- Electrons (negatively charged) are knocked loose from their atoms, causing an electricpotential
difference. Current starts flowing through the material to cancel the potential and
this electricity is captured. Due to the special composition of solar cells, the electrons are
only allowed to move in a single direction.
- An array of solar cells converts solar energy into a usable amount of direct current (DC)
electricity.
PV cells also all have one or more electric field that acts to force electrons freed by
light absorption to flow in a certain direction. This flow of electrons is a current, and by
placing metal contacts on the top and bottom of the PV cell, we can draw that current off for
external use, say, to power a calculator. This current, together with the cell's voltage (which is
a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell
can produce.

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MUSCLE STIMULATOR

6.    MUSCLE STIMULATOR

6.1 INTRODUCTION
The neuro-muscular stimulator stimulates the nerves (neurons) of the part of the body to which the electrodes are attached. It provides relief for headache, muscular pain, or fatigue (due to exertion of muscles) and also revives the frozen muscle which impairs movement. The main function of this electronic instrument is to provide energy to nerve of the muscle and to remove cellulites.
Current stimulator is one of the most commonly used instruments used for diagnosis and treatment of a wide variety of neurological and muscular disorders. It works by forcing electric current into a human system, to elicit information to treat the system.
The muscular stimulator circuit generates 150 µs pulses at 80 Hz frequency, which is then given to the secondary of the transformer to generate 60V positive and 150V negative peaks at very low current so that the patient in not subjected to any shock. The intensity of the current can be varied by changing the resistance which is connected to the pulse generator. Pulse generator is the heart of the circuit involves CMOS timer (7555) to generate pulse train.