The letter I seems to be an odd choice for the English language, but it was chosen in the
early days of electricity to represent intensity of current which we simply call
current today. The unit of current, the ampere, is named after the French scientist
André-Marie Ampère in recognition of his work on the relationship between
electric current and magnetism. Ampère referred to electric current as
"l'intensité du courant électrique", so I was a logical choice
to represent intensité (intensity).
I am grateful to Barry Caruth for suggesting a search of the internet for
"Ampère" and "l'intensité du courant électrique"
which returns many sites as evidence (most of them French) enabling me to answer this
question with confidence.
Further information:Quantities used in Electronics |
Voltage and Current | Ohm's Law
Why do some books use zig-zag lines for resistors in circuit diagrams?
The zig-zag line is the old symbol for a resistor and you may find it in older books
and magazines. Unfortunately a few publications still use it! The correct modern
symbol for a resistor is a rectangle.
Further information:Resistors |
Circuit Symbols | Circuit Diagrams
My project has a resistor labelled 47, does that mean
No, it means 47
which is 1000 times smaller!
would be shortened to 47k (or 47K).
The ohm ()
symbol is often omitted from circuit diagrams and component layouts but the k
(meaning kilo = 1000) will always be included if it is needed.
Why do resistors have odd values like 47k and 56k, but not 50k?
There is a good reason for these odd values and it is explained on the
A project on another website lists a 10kW resistor! What does it mean?
It almost certainly means a 10k
resistor. This is a common error which occurs when the web page specifies a Greek font.
If this font is not available on your computer you see the character in your standard font
and it happens to be W which is the symbol for watt, the unit of power!
I avoid the problem on this website by using a small image for
In a few projects a low value resistor with a high power rating is required but the power will
be something smaller like 5W, never 10kW which is more powerful than an electric heater!
My soldering iron was supplied with a hook, do I really need to buy a stand as well?
For safety you must buy (or make) a stand for your soldering iron. Please don't use
the hook because it leaves exposed the very hot element and tip of the iron - it is too
easy to accidentally touch them and burn yourself. If you can't afford to buy a stand
you could try making your own with a spiral of stiff galvanised iron wire (a coat-hanger?)
screwed to a block of wood. Ideally the stand should include a damp sponge for safely
wiping the tip of the iron when it needs cleaning.
Further information:Tools for Electronics |
Where can I buy heatproof cable to replace the ordinary cable on my soldering iron?
Silicone heat resistant cable is sold in 1.5 metre lengths for exactly this purpose by
part number 85-0590 (look in the Soldering Equipment section). If you use another
supplier make sure you buy 3-core mains flex with a current rating of 3A (the proper
name for mains appliance leads is flex, not cable). Please note that to change over to
the new flex you will need to borrow a second soldering iron! This is because the
flex is soldered to the iron's element. Make sure that you connect the wires correctly
in the iron and in the mains plug which should have a 3A fuse.
Further information:Tools for Electronics |
My teacher says that Christmas tree lights are a series circuit, so when
one lamp blew on Christmas Eve why didn't they all go out?
Traditional Christmas tree lights are connected in series and you are correct in thinking
that if one lamp blows all the lamps should go out.
The problem is that Christmas tree lights are not like ordinary lamps! When they blow
they automatically short-circuit (they become like a wire link) so the circuit is still
complete and the other lamps remain lit. This makes it easy to see the blown lamp, but
do remember to switch off before changing it.
Teachers - please be careful when giving Christmas tree lights as an example
of a series circuit!
Further information:Lamps |
Series and Parallel Connections
What component has a black stripe in the centre (it looks like a diode)?
A small component about the size of a resistor or signal diode with a single black stripe
in the centre is a zero-ohm resistor, it is really just a wire link! These
components are used on commercial PCBs because they are easier for machines to handle than
small pieces of wire. The single black stripe is logical because it means zero in the
resistor colour code. Ordinary resistors have at least four stripes.
Diodes have a single stripe near one end, not in the centre.
Further information:Diodes |
What is a "short circuit"?
A "short circuit" is a connection of very low resistance such as a wire
which provides a very easy path for current. Think of it as an electrical short-cut.
It is normally used to describe a fault or accidental connection rather than a
For example: if the leads from a battery touch one another they create a
very low resistance connection across the battery, so we say they have caused a
short circuit across the battery. Current will flow through this short circuit
rather than through the proper circuit. This stops the circuit working and it may cause
a fire because the leads and battery will become hot with a large current flowing.
Further information:Voltage and Current |
What does "open circuit" mean?
"Open circuit" means no connection.
It is usually used to describe a break in some part of a circuit which could be
deliberate (such as a switch in the open or off position) or a fault (such as a
broken wire or burnt out component).
Further information:Voltage and Current |
How do I choose a relay to use with one of your projects?
The 555 timer chip used in many projects can supply current up to 200mA so it can power
most relays directly. However, you must connect a signal diode (a 1N4148 for example)
in parallel across the relay coil to protect the 555. Note that this diode is connected
'backwards' so that it will normally not conduct.
Further information:Relays |
Model Railway Signal (example project with relay)
I want to use a large number of LEDs, do I need a resistor for each one?
No, you can usually connect a few LEDs of the same type in series and just use one resistor.
The number of LEDs you can connect in series depends on the circuit's supply voltage.
This arrangement has the advantage of reducing the total current required by the circuit.
Please see the LEDs page for more details:
Further information:Connecting LEDs in series
I want to build up a stock of components, what should I buy first?
Most people build their first few projects from complete kits, but if you want to try
adapting published projects or designing and building your own circuits you will need to
have a small stock of components available. There is a page with advice on buying a
starter kit of components.
Further information:Starter kit | Links (for suppliers)
What is a Darlington pair?
A Darlington pair is two transistors connected together so that the current amplified by the first
is further amplified by the second transistor, giving a very high gain of 10000 or so.
What does 'sinking a current' mean?
It means current is flowing into the output of a chip. This happens when the output
is low (0V) if there is a device connected between the positive supply (+Vs) and
the output. It is the opposite of sourcing a current which means current is
flowing out of the output. Most chip outputs can both sink and source current.
Further information:Integrated Circuits (Chips)
Are 'time period' and 'time constant' the same thing?
No, they have different meanings although both are time.
Time period is the duration of a single pulse or the time for one cycle of a repeating electrical signal.
Time constant is a property of a changing system, such as a capacitor charging and discharging.
Further information, time period:Electrical Signals |
Astable | Monostable |
What is a PIC?
A PIC is a Programmable Integrated Circuit microcontroller, a 'computer-on-a-chip'.
They have a processor and memory to run a program responding to inputs and controlling outputs, so they can
easily achieve complex functions which would require several conventional ICs. I can strongly recommend the
PICAXE system because it is easy to program (and re-program) the PICs with a standard computer - no specialist
equipment is required other than a low-cost download lead. The programming software and extensive documentation
is available to download free of charge, making the system ideal for education and users at home.
Further information (including downloads):www.picaxe.co.uk
Why does my circuit count 3 or 4 when I press the switch once?
The bouncing output from a switch
This is likely to happen if a switch is connected directly to the clock input of a counter.
Switches contacts tend to rapidly bounce open and closed a few times when the switch is operated.
The counter sees this as several clock pulses, not the single pulse you expect.
One solution is to make the switch trigger a monostable circuit with a short time constant
(0.1s for example) and use this to drive the clock input.
Further information:Counting Circuits |
555 Monostable Circuit
What software do you use to draw the circuit diagrams?
I created the circuit diagrams (and all the other drawings on the website) using a vector drawing
application called Draw. It comes as standard with all RISC OS
computers, but is not available for Windows computers. The diagrams are converted to GIFs for the website
for maximum compatibility.
Further information:Circuit Diagrams |
'SMD' means Surface Mount Device. SMDs are components with small pads instead of leads for their contacts.
They are designed for soldering by machine onto specially designed PCBs and are not suitable for educational
or hobby circuits constructed on breadboard or stripboard. Do not buy SMD components for your projects!