Reading Schematics

Article : Andy Collinson
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Reading Schematic Diagrams
This page was created in response to all the email I receive, on how to read a schematic. Learning to read a schematic diagram, is similar to map reading. There are three main items in a schematic, wires, components and connections. I'll identify each item with the examples below. You need to know which wires connect to which component and where each wire starts and finishes. With a map book this would be equivalent to knowing your origin and destination points and which roads connect to the motorway network, etc. However schematics are a little more complicated as components need to be identified and some are polarity conscious (must be wired up the correct way round) in order to work. You do not need to understand what the circuit does, or how it works, in order to read it, but you do need to correctly interpret the schematic. Here are some basic rules that will help with reading a diagram. Look at the circuit diagram shown below: an example schematic diagram Wires
Any straight line represents a wire. Note that a wire can have one or more bends in its section. Below the blue lines represent wires, and for simplicity, I have labelled them as A,B,C. There are just three components here and it is easy to see where each wire starts and ends, and which components a wire is connected to. As long as the wire labelled A connects to the switch and negative terminal of the battery, wire B connects to the switch and lamp, and C connects to the lamp and the battery positive terminal. When the switch is operated this is a closed circuit and lamp will light. When the switch is open, no current flows in the circuit and lamp will not light. This condition is called an open circuit.

In the previous diagram, a switch, a battery and a lamp were used, and this was an electric circuit. An electronic circuit has components that can control the current electronically. The electronic components control the flow of electrons and common schematic diagrams, include active components such as diodes and transistors. Integrated circuits, field effect transistors and thermionic values all control the flow of current. A component is represented by its symbol and the following page has many electronic symbols:

Link to Electronic Symbols

wires joining and not joining Wire connections on a circuit diagram are drawn either connected, or not connected (no contact). There are two ways of drawing connections, see the left figure. Wires that are connected have a round "dot". This dot indicates that the wires form a junction and should be connected together.
Wires that are not connected are indicated by either a "jumper", indicated as a small arc, or as a connection without a dot.

Rotated Configurations
Before moving on, it is important to realise that any schematic may be drawn in a number of different ways. Fig 1 and Fig 2 below are two identical schematics. At first glance they may look quite different, but are in fact, identical. If you mentally look at each wire in both diagrams, you will see that they the same start and end points. In Figure 1 a segment of wire connects terminal b to one end of R1, this is a resistor. In Figure 2 the same wire starts at point b but has a right angle bend before connecting to component R1. The same is true for all other wires, just that the diagram is drawn with battery and transistor rotated through 90 degrees.

Component Designations
It is common practise to label components with both a letter and number. These two characters form the component designation. In diagrams 1 and 2 below, a single resistor has been designated R1. The other number 1k represents its value. Common letters, used to identify components include R for resistors, VR for variable resistors, C for capacitors, D for diodes, and S for switches. A transistor is usually designated by the letter Q, although in older schematics the letter T might be used. The transistor has three terminals labelled, E, B and C.

Typical Schematic

In Fig1 there are two wire junctions as indicated by a "dot". A wire connects from battery positive to the C (collector) terminal of the transistor, and also a wire runs from the collector terminal to one end of the potentiometer, VR1. The wires could be joined at the transistor collector, battery positive or even one end of the potentiometer, it does not matter, as long as both wires exist. Similarly, a wire runs from battery negative to the lamp, and also from lamp to the other end of VR1. The wires could be joined at the negative terminal of the battery, the lamp, or the opposite tag of VR1. In drawing Fig 1, I could have drawn the wires from the lamp and bottom terminal of VR1 back to the battery negative terminal and placed the dot there, it would still be the same.

If you now look at Fig 2, you will see that one wire junction appears at the negative battery terminal, the other junction in a similar place.

Integrated Circuits
Pinout for CMOS4050 Integrated Circuits or IC's for short have a lot of connections. When you see a schematic containing one of more IC's it is generally common practise that the power supply terminals have not been drawn. The reason is usually for clarity, but what will always be shown is the model number of the IC. Take a look at the pinout , shown left, for a CMOS hex buffer IC. Its model number is 4050 in the CMOS series. 4050B is the same part, but in this version all the outputs are buffered.
Now in this layout pin 16 is labelled as NC. NC stands for no connection, indicating that this pin has no function and not connected to the rest of the IC. Pins 1 and 8 are labelled Vdd and Gnd respectively. The Vdd terminal will go to positive power supply and ground pin to the negative or ground terminal. Note that a buffer is represented by a triangle. The flat side represents the input and point of the triangle, the output. Note that the device contains six CMOS type buffers. Pin 2 or terminal A is the output for the first buffer, and pin 3 or terminal B is the input for the buffer. Similarly the other buffers have terminals labelled, C, D, E,F, G, H, I, J and K and L. These six buffers are also called logic gates and can be used in any order. You do not have to use all of the gates on the IC. In fact, any unused gates, the input should be connected to ground. This prevents false triggering and instability of a logic circuit. The important rule to remember is that IC's are often drawn without power connections. You should check the datasheet for the IC to find the pin numbers for power supply lines and wire them accordingly.

Circuit Layout
Sometimes the way a circuit is wired up may compromise its performance. This is particularly important for high frequency and radio circuits, and some high gain audio circuits. See the diagram below:

emitter follower circuit Although this audio circuit has a voltage gain of less than one, wires to and from the transistor, should be kept as short as possible. This will prevent a long wire picking up radio interference or mains hum from a transformer. Also, in this circuit input and output terminals have been labelled and a common reference point or earth is indicated. The earth terminal would be connected to the chassis or metal framework of the enclosure in which this circuit is built. Many schematics contain a chassis or earth point. Generally its just to indicate the common reference terminal of the circuit, but in radio work, the earth symbol usually requires a physical connection to a cold water pipe or an earth spike buried in the soil.
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