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:
connect symbol pins defining connections in the real circuit.
|Jump 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
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symbol examples and descriptions.
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 thebasic 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
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.
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.
How to read schematics : Simple example
Note: Connections dots connect wires together
(horizontal and vertical wires at point X are connected together. If there was
no connection dot at point X then that would indicate two separate
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!
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).
In the above
schematic the input 'Signal' goes through a capacitor past some resistors and
through two transistors and then to the loudspeaker. The operation of the
circuit is clearly from left to right.
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!
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.
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.
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|
|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!
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
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