# Fibre optic cables

Optic fibre cables are comprised of four layers. Data is transferred using beams of light by utilising an effect known as total internal reflection.

Light travels into the glass core, and so long as it is above the critical angle, total internal reflection will occur, causing light to reflect off of the inside of the glass core repeatedly until it reaches the other end.

# Coaxial cables

This type of cable is comprised of four layers. The innermost, is a copper wire which is the conductor, and carries the signal. This is then covered by a dielectric insulating layer, which has a copper braid wrapped around it, with a further outer protection and insulation layer on the outside of that. The copper braid acts as a screen, which greatly reduces leakage in and out of the cable.

# 555 Astable

The amount of time that the signal is at 0v (Time Low), is calculated by the following equation:

The amount of time that the signal is approximately 5v (Time High), is calculated by the following equation:

The total period is calculated either by adding together the time high with the time low, or by using this condensed equation:

Finally, the frequency can be calculated using this formula:

# Wavelength & Frequency calculations

This equation allows you to calculate wavelength from frequency, and frequency from wavelength.

In this equation:

(lambda) represents wavelength (in metres).

c         represents the velocity of light (which is approximately 3.00×108 m/s).

f         represents frequency (in hertz).

Wavelength is the distance between two peeks of the signal.

frequency is the number of oscillations in one second.

# Ohm’s Law

Ohm’s law is the basis behind electronic equations. It establishes a relationship between voltage, current, and resistance. He stated the following relationship:

In addition to this, we can deduce two more equations from this one.

Because of this relationship, it is possible to calculate any of these values, with just two of the three.

# Resistors in series and parallel

Resistors in a series alignment

Calculating resistance in series is the easier of the two. You simply add them together to find the total resistance.

Resistors in a parallel alignment

When calculating resistance in parallel, it’s a little more complicated. But so long as you follow the formula, it should be okay. In addition to this, you can double check your results, because it should always be lower than the value smallest resistor.

# Capacitors in series and parallel

Capacitors in a parallel alignment

This is the easier of the two to calculate, the total capacitance is simply the sum of both of the capacitors.

Capacitors in a series alignment

When calculating capacitance in series, it’s a little more complicated. But so long as you follow the formula, it should be okay. In addition to this, you can double check your results, because it should always be lower than the value smallest capacitor in the series.

# Difference amplifier

A difference amplifier has two inputs. It compares them both, and then cancels out any similarities between them. Once any common voltages have been cancelled out, the differences are amplified.

# Comparators

A comparator is one of the simplest op amp subsystems. It simply compares two voltages, and the output either goes high or low depending on which input is the higher voltage.

Generally, one of the inputs is fixed to a set voltage (a reference voltage), whilst the other varies depending on an analogue input. Here’s an example:

Using a potential divider calculation, the voltage at the non-inverting input can be calculated as 3v. Because of this, when the voltage at Vin is above 3v, the output will be low. When the Voltage at Vin drops below 3v, the output will be high.