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How to read schematics.
Note: This page on how to read schematics is a very simple introduction covering the basics of reading schematics.A schematic is a diagram representing a circuit in which:
- Symbols represent components,
- Lines represent wires.
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to Symbols. Jump to Components. Jump to Building blocks. Jump to Design document. Jump to Simple schematic: Example. Jump to Simple schematic: Good practice. Jump to Simple schematic Identifiers Jump to Simple schematic: Values. Jump to Simple schematic: BOM |
Click here for more symbol examples and descriptions. |
How to read schematics : Symbols
Each component in the circuit is represented by exactly one symbol in the schematic and the schematic lets you understand the operation of the circuit by showing how each component is connected.Note: To fully understand the schematic you need to understand the basic operation of each component.
A few schematic symbols are obvious and closely follow the physical form of the component e.g. a resistor or a loudspeaker
| Resistor | Loudspeaker |
 |
 |
But
most
components are represented by symbols that do not look like the
component at all:
| Capacitor | Transistor | ||
 |
 |
 |
 |
Note: There are two symbols for
the capacitor and either is acceptable for use - in general there may
be several different symbols for a component but they will look more or
less the same (there are also more for the transistor - not shown -but
they always have the arrow).
The
symbols show the component's
internal structure for more information about the circuit operation.
They give you just enough information to understand how the
circuit operates i.e. they give a broad overview of circuit operation.
For
this broad overview
its not necessary to know component
detail (in a large circuit you would be overwhelmed by the
data - even for an old BC109 transistor there are about 30 parameters).
This
is where the design document is needed - for describing detailed
circuit operation.
The
capacitor symbol just shows two separated plates - which
is all a
capacitor is made of (separated by the dielectric). In fact
the
'real' capacitor could be
chosen from any number of different capacitor types depending on what
you need the circuit to do e.g. high stability, low cost, high voltage.
Other
examples are transistors and again the same symbol will represent many
different components e.g. metal cased, plastic cased, different pinouts
etc. This is because the fundamental operation of the
transistor can
be represented by one symbol and this symbol lets the designer know how
the circuit operates.
The point is that all these different components use the same schematic symbol so that you can concentrate on what the circuit does rather than the component details.
How to read schematics : Components
Different components that fall into the same component class are all described by the same schematic symbol e.g. different type transistors or different power resistors - they may all have very different physical shapes but each class is represented by one symbol. This simplifies the schematic so that it is easy to understand (not too much detailed information).The
capacitor symbol just shows two separated plates - which
is all a
capacitor is made of (separated by the dielectric). In fact
the
'real' capacitor could be
chosen from any number of different capacitor types depending on what
you need the circuit to do e.g. high stability, low cost, high voltage.
Other
examples are transistors and again the same symbol will represent many
different components e.g. metal cased, plastic cased, different pinouts
etc. This is because the fundamental operation of the
transistor can
be represented by one symbol and this symbol lets the designer know how
the circuit operates. The point is that all these different components use the same schematic symbol so that you can concentrate on what the circuit does rather than the component details.
How to read schematics : Design document
Detailed
operation of the circuit should be written down in a separate
document that describes how each part of the circuit works.
It should detail any tricky parts of the circuit i.e.
non-obvious circuit operation.
This along with the schematic lets another designer understand how the circuit is intended to operate.
This will come naturally as you become familiar with building and using circuits and as you do so you'll notice popular circuit configurations or popular device types e.g. a relay driver circuit or an RS232 level shifter MAX232.
Once you have used them you will recognize them in future which makes reading schematics much easier.
This along with the schematic lets another designer understand how the circuit is intended to operate.
How to read schematics : Building blocks
One very important point in reading schematics is recognizing blocks of circuitry.This will come naturally as you become familiar with building and using circuits and as you do so you'll notice popular circuit configurations or popular device types e.g. a relay driver circuit or an RS232 level shifter MAX232.
Once you have used them you will recognize them in future which makes reading schematics much easier.
A
good reference book will also show you standard building blocks -
especially transistor circuit blocks e.g. switching,
amplifiers, long-tailed-pairs, current mirrors etc.
Note the absolute best bible on electronics is 'The art of electronics' by Horowitz and Hill. I bought this book at university a while ago! (1987) and it is still as useful today. In fact it is still so useful that I have started re-binding it - the front cover fell off in overuse (it was a paperback)! It covers all the basic electronics from using diodes to transistors to computers and many subjects in-between.
Note the absolute best bible on electronics is 'The art of electronics' by Horowitz and Hill. I bought this book at university a while ago! (1987) and it is still as useful today. In fact it is still so useful that I have started re-binding it - the front cover fell off in overuse (it was a paperback)! It covers all the basic electronics from using diodes to transistors to computers and many subjects in-between.
How to read schematics : A Simple example
The
following schematic shows the outline design of a push
- pull amplifier (standard transistor building block). This
is an incomplete design just used as a schematic example (not to be
built).
Wires are represented here by green lines indicating connections i.e 'real' circuit connections.
Wires are represented here by green lines indicating connections i.e 'real' circuit connections.
How to read schematics : Simple example
When wires cross over at right angles (perpendicular to each other) without a connection dot then they do not connect. With no dot both resistors would connect and separately the capacitor and transistors would connect.
Note: its not a very usable circuit so don't build it! (you need diode to bias the transistors) and component values!
How to read schematics : Good practice
In a good schematic the operation of the circuit is clear - as you can see here the logical flow of operation of the circuit is from left to right (the dotted arrow shows circuit operation from input to output).If you see schematics that are not arranged logically then they are much harder to read.
Usually inputs are placed on the left and outputs on the right. Sometimes this is not always possible but it improves readability.
Note: One way out of drawing the circuit from left to right is to draw part of the circuit within a dotted box and label the box with the circuit's function. This lets you read the circuit more easily (some of the circuits on this site use this method).
How
to read schematics : Simple example - operational flow
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Note: Using a schematic drawing tool greatly increases the readability of the circuit - you can find some just terrible diagrams that you can hardly read on the web - these are hand drawn and scrappy and virtually impossible to use and they give you no confidence that the circuit will work. If you can't read it properly then maybe the author could not!
If you would like to try out a schematic tool then this is the one I use ( I have used it to create all the schematics and graphics on this site).
Note: ProSchematic is a very easy editor to use as all the main controls are easily accessible at the top and creating a new library component is as easy as drawing the schematic in the first place.
The other great advantage in using a schematic tool is that you don't need to rub anything out and squeeze new components between symbols you have previously drawn and you can build new schematics based on your existing designs.
How to read schematics: Part Identifiers
The next part of reading the schematic is to identify components. There must be one instance of each component on the schematic so a good tool will let you check the schematic for duplicate parts (that you may have added accidentally yourself).Each schematic component corresponds to a physical component in the circuit so the identifier has to be unique.
Each part is prefixed with the base type of the part so for capacitors the prefix is usually C and for resistors it is R. Transistors are either Q or TR but you can use any prefix you like. The following diagram has all the prefixes for each component.
The prefix is followed by a number and uniquely identifies the component.
How
to read schematics : Simple example - Identifiers
How to read schematics: Component value
The next part of reading the schematic is to identify component values. These are the values that the designer used for the circuit to make it operate correctly.They are the value assigned to a particular component e.g. resistance for a resistor or they are the specify the component type name e.g. a BC109 for TR1 - BC109 is the specific transistor to use.
Forms of notation for components R, C and L.
| How to read schematics : R,C,L values | |
| Component | Notation |
| Resistor 10 Ohms | Value 10 or 10R |
| Resistor 4.7 Ohms | Value 4R7 (does not need decimal point) |
| Resistor 10,000 Ohms | Value 10k |
| Resistor 1,000,000 Ohms | Value 10M |
| Capacitor 1 micro Farad | Value 1u (u used instead of greek mu) |
| Capacitor 100 nano Farad | Value 100n |
| Capacitor 12 pico Farad | Value 12p (referred to as puff pF as small!) |
| Inductance 100 milli Henries | Value 100m |
The following schematic shows some values for each component:
Note: This is not a designed circuit so don't build it!
How
to read schematics : Simple example - Values
How to read schematics: BOM
There should also be a Bill Of Materials (or BOM) to go with the schematic if the schematic has more than about 30 components. The BOM details how many components there are and what their values are.Here is an example for the above schematic:
Item Part Value Description Qty Parts
1 1u Capacitor 1 C1
2 Signal Input pin 1 IP1
3 8R loudspeaker 1 LS1
4 10k resistor 2 R1 R2
5 BC109 NPN Transistor 1 TR1
6 BC179 PNP Transistor 1 TR2
For a Free Trial of a useful schematic drawing tool click here.
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