Circuit Diagrams

In the last section, we already used a circuit diagram showing how our bulb is connected to the battery with a switch, this is what we would refer to as a schematic, showing the layout of component parts joined up by wire. To summarise, we’ve used the following parts so far:

Schematic Symbol  Description  Circuit Schematic Reference
 Symbol-Wire  Wire (Conductor)  
 Symbol-Cell  Battery (Specifically a cell)  BT, B or BATT
 Symbol-Bulb  Bulb  DS
 Symbol-Switch  Switch (Normally OPEN)  S or SW

The schematic symbols that we use comply with an international standard knows as IEC 60617.

We’ve also included the ‘Circuit schematic reference designators’ to get you used to these from the beginning. The first one listed is the standard reference, with commonly used variations also given where applicable. You may see these letters on circuit boards indicating where components go, or on more complicated circuit diagrams (schematics).

These references indicate components, which are then usually listed with more specific detail in a bill of materials. Where several of the same component type exist, they are distinguished by a number to identify them separately, for example DS1, DS2 and DS3 would cover 3 bulbs in the circuit.

Interestingly enough, bulbs don’t have their own schematic reference designator, we need to use DS, which is also used for ‘Display’, and can also cover some other types of display device as well. Here is when the additional detail in the bill of materials (a list with details of the component parts in the circuit) helps identify what we are looking at if that details is not included next to the component on the schematic.

Looking at the symbols we have so far:

  • Wire is shown as a straight line, as in a real circuit, the wire simply connects components together
  • Batteries have a long and short line, the long always being the positive end. Some components that we will cover later are polarised, and need to be connected the correct way around otherwise the circuit won't work! Polarised refers to the opposite ends, so for our battery would be Positive and Negative, or for a magnet would be North and South
  • Bulbs are shown simply as a circle with an X inside.
  • Switches are basically a junction in the circuit that can be opened (OFF) or closed (ON). Here, the symbol shows a switch that is normally open (until you press it). Other switch symbols exist, and we’ll cover those in later chapters.

So far, all the components have been in joined from one directly to the next. As circuits start to get more complex, we will encounter joins in the wire, where more than one component is connected at a particular place, or where a wire crosses over another, but does not make contact.

Schematic Symbol  Description
 Symbol-Wire-Join Wire joint – An extra wire is connected
 Symbol-2-Wire-Join Double joint – Two extra wires are connected
 Symbol-Wire-Crossover Crossover – The 2 wires are not connected together, one crosses over the next
Alternative representation of a crossover, the loop shows that the second wire is not connected, but jumps over the first wire. This is used sometimes for clarity, but is not standard on normal schematics.
 Symbol-Wire-Crossover2 Alternative representation of a crossover, the loop shows that the second wire is not connected, but jumps over the first wire. This is used sometimes for clarity, but is not standard on normal schematics.


Explaining these symbols in more detail:

  • Wire Joint – We can see that an extra wire has been joined onto the main circuit. The black dot where the lines join indicates they are connected together.

  • Double joint – Two wires have been joined onto the main circuit, not to be confused with a crossover

  • Crossover (Standard) – The wires are crossing over one another, but are not connected together

  • Crossover (Non-standard) – This symbol is often used in circuits aimed at beginners for clarity to exaggerate the fact that one wire crosses over the other, and is not connected. Not used in standard circuit schematics.

New component symbols will be introduced as we encounter new components in this Masterclass. Soon, you will be able to decipher something like this:

Fig3-PSU-Circuit-Diagram