Here we continue with our tutorial on process components. The next process component we will consider is the Field Effect Transistor FET.
Now there are many types of FET, for the purposes of our studies we will consider an n Channel enhancement mode MOSFET. As before, this excellent resource fully explains the construction and operation of a MOSFET
MOSFET’s make for excellent electronic switches. When switched on they have a very low resistance and thus can supply large currents to an electrical load without incurring major power losses.
The MOSFET we are using is a voltage controlled device because no current flows into the gate at all. Indeed by varying the gate voltage we can effectively control the resistance between the Drain and Source connections giving further control over the current supply. The following slide shows the connections required to operate the MOSFET and some information regarding the current it can provide.
The next slide shows possibly the simplest possible application for this type of MOSFET.
This type of component is fairly simple to use in a circuit providing you follow some basic rules. It finds uses in thousands of different types of circuit as a voltage amplifier, current amplifier and switch. Once again we are only going to study one instance of a circuit, a special type of electronic switch known as a comparator.
As before we will look at the component we are going to use in terms of its inputs and outputs before looking at it in a real circuit. The Op Amp we are going to use is the LM311.
Before we do, lets look at a physical component and its circuit symbol.
Now even a basic explanation of an Op Amp Comparator requires a little more depth than the previous components. The next slide should give you a basic understanding of the operation.
Therefore if we set one of the voltages so that it stays stable and allow the other voltage to vary e.g. the voltage across an LDR in a voltage divider, we have a situation where we can precisely set the point at which the comparator will switch on and off.
In the next slide I give you a real world comparator circuit which could be used by you as a part of this unit.
The 555 Timer
One of the most useful chips in electronics is the 555 timer. It is relatively easy to use but as before requires more in-depth study than the transistor circuits we encountered earlier.
We need to be able to use the 555 timer in monostable and astable modes. I will begin with monostable.
Monostable is the term for electronics circuits which have only one stable state. i.e its either on or off. A monostable circuit has to be triggered and is perfect for switching on an alarm for a set period of time. Lets investigate one.
In the slide above I forgot to mention that making J1 (switching it on) triggers the circuit i.e it allows current to flow to ground through the discharge pin.
This simply means that the output alternates between 2 stable states, on and off. In other words it is an oscillator (produces a waveform). The following slide will explain the operation of a 555 time in astable mode. There is also a video which goes into more depth if you require it.