Download Electrical Symbols

Electrical Symbols
ordinary wire
lamp/bulb
battery or cell
on/off switch (SPST)
two-way switch (SPDT)
simple resistor
variable resistor
fuse
diode
motor
light emitting diode (LED)
M
(or)
light dependent resistor
buzzer
ammeter
voltmeter
reed switch
reed relay
Notes about Some of the Devices
Variable resistor: This is sometimes called a ‘dimmer’ because it could be used to
adjust the brightness of a bulb. When its resistance is increased, it reduces the size
of the electric current passing, slowing a motor, quietening a bell, or dimming a
lamp, depending on the rest of the circuit. As its resistance is decreased, the current
through it increases, allowing a motor to speed up, a bell to become louder or a bulb
to become brighter.
Fuse: This is a device which puts a maximum limit upon the current that can flow.
If the maximum is reached or exceeded, the fuse ‘blows’ cutting off the current. This
involves a thin wire inside the fuse being heated up by the current. If the current
exceeds the limit, the wire becomes so hot that it melts, breaking the circuit and
thus cutting off the current. (Some fuses are ‘re-settable’, and they may involve
breaking the circuit using a springy mechanism, which can be pushed back, once the
problem has been solved). Fuses protect the wiring from over-heating, and can help
to protect the device itself.
Diode: Basically, diodes are one-way electrical devices. If you try to pass a current
the ‘wrong way’ through a diode, it will not go, so no current flows. In the correct
direction, a current will pass (usually with little or no resistance occurring). To give
protection to some diodes (typically LEDs) a resistor is often installed in series.
Motor: These are converters of energy, from electrical to kinetic. Typically they
consist of coils of wire supported on a central spindle, forming an armature, which
is positioned in a magnetic field. When a current flows through the wire coils, it
produces a magnetic effect, which interacts with the magnetic field, causing the
armature to turn.
Light Dependent Resistor (LDR): This is a resistor which varies according to the
intensity of light falling on it. As the light is increased, the resistance decreases,
allowing the current through it to increase. In the dark, the resistance of an LDR is
very high, so the current passing is tiny.
Reed switch: This is a switch which is operated by a magnet. Most such switches
are normally ‘open’ (off) and are closed by the presence of a magnet, but it is
possible to get reed switches which are the reverse of this.
Reed relay: This is a reed switch, for which the magnetic influence is in the form
of an electromagnet. A small current through the relay is sufficient to produce an
electromagnetic effect, which then closes the reed switch. Reed relays can come in a
variety of switch-types; the type described above is the sort we have at Aldro.
Ammeter: This is a device for measuring electric current. The unit of current is the
ampere, usually abbreviated to amp. Ammeters are connected in series with any
devices for which they are to measure the current.
Voltmeter: This is a device for measuring potential difference (electrical ‘push’).
The unit of potential difference is the volt. Voltmeters should be installed in parallel
with devices in a circuit.