The symbol shown above is used to represent a common connection in all parts of circuit schematic. The symbol is used to simplify schematic diagrams by eliminating lines that are all electrically connected to the same point in the circuit. In DC circuits this 'common' point is usually the minus terminal of the circuit's power supply.
It is important to remember that the common of an electronic circuit is very often referred to as GROUND but this does not mean it is connected to EARTH.
For the diagrams at this site the COMMON symbol represents a connection to the MINUS terminal of a single sided DC power supply or battery. Or the Neutral of a dual or split type of power supply.
AC portions of an otherwise DC circuit may also use this symbol to indicate a common connection. The centre taps of power supply transformers for example.
Please note that the common symbol shown above DOES indicate a connection to an EARTHED ground in many schematics that can be found on the internet and in product documentation.
In many residential and commercial AC power distribution system diagrams the following symbol, or one similar to it, is often used to indicate an 'EARTHED' ground connection.
The symbols shown above are representative of those used in North America. Other countries may use different symbols as standard but they mean the same thing.
If a circuit has a direct connection to an EARTHED ground, it is there for safety of the user and equipment. DO NOT break an EARTHED common connection for any reason.
Breaking the EARTHED connection in a household wiring system will defeat the protection provided by a Ground Fault Interupter.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
11 January, 2012