Frequently Asked Questions

Circuits | Components | Soldering | Other

FAQ on Circuits

Why is the letter I used to represent current?

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: Voltage and Current

What is a "short circuit"?

A "short circuit" is an unwanted connection of very low resistance such as a wire (almost 0ohm) 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 deliberate one.
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: Resistance

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: Resistance

What is a schematic?

It is another name for a circuit diagram.
Further information: Circuit Diagrams

What does 'sinking a current' mean?

It means current is flowing into the output of an IC. 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 IC outputs can both sink and source current.
Further information: Integrated Circuits

Are 'time period' and 'time constant' the same thing?

No, they have different meanings: 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 | 555 timer
time constant: Capacitance

My teacher says Christmas tree lights are a series circuit, so when one lamp blew 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.
Further information: Series & Parallel

Why does my circuit count 3 or 4 when I press the switch once?

switch bounce

This is likely to happen if a switch is connected directly to the clock input of a counter. When a switch is closed its contacts tend to rapidly bounce open and closed a few times producing an output like the diagram. 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

Components FAQ

My project has a '47' resistor, does that mean 47kohm?

No, it means 47ohm which is a much smaller resistance. 47kohm would be shortened to 47k (or 47K). The ohm (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.
Further information: Resistors

Why do resistors have odd values like 47k and 56k, but not 50k?

There is a good reason for this and it is explained on the Resistors page.

A project on another website lists a 10kW resistor! What does it mean?

It almost certainly means a 10kohm resistor. This error occurs when the web page specifies a Greek font and it is not available on the computer so you see the character in your standard font. This 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 ohm. 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!
Further information: Resistors

What component has a black stripe in the centre (it looks like a diode)?

Zero-ohm resistor

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 | Resistors

How do I choose a relay to use with one of your projects?

The 555 timer IC 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 | Diodes

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.
Further information: 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. Please see the page with advice on buying a starter kit of components.

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.
Further information: Transistors

What does 'SMD' mean?

'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.
Further information: PCBs - SMD

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:

Soldering FAQ

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 Rapid Electronics.

Rapid Electronics: silicone cable

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 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 (with a 3A fuse in UK).
Further information: Tools | Soldering

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 | Soldering

Soldering iron stand, photograph © Rapid Electronics

Photograph © Rapid Electronics.


Rapid Electronics have kindly allowed me to use their images on this website and I am very grateful for their support. They stock a wide range of soldering irons and other tools for electronics. I am very happy to recommend them as a supplier.

Other FAQ

I'm interested in electronics, where should I start?

I suggest that you start with a few simple projects, learning how to solder and how to identify the common components. You will need some tools to construct the projects. It is best to buy kits to be sure you have the correct parts. Many people then want to start learning how the circuits work and maybe try designing their own, usually by adapting a published circuit. You can read through the study section of this website. At this stage it is worth buying a breadboard for trying out circuits without soldering so that changes can be easily made and the parts re-used. The 555 timer circuits are great for simple projects.

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 GIF images for the website.

What do you recommend for a child too young to use a soldering iron?

Circuits can be built on breadboard without any soldering and I feel that's suitable for most children from about 8 years old. Bear in mind though that electronics components are small and should be kept away from babies and very young children. It is vital to find projects with suitable diagrams (because the breadboard layout is quite different from soldered stripboard or PCB layouts) so I recommend two excellent books by Øyvind Nydal Dahl:

Note that experienced experts often develop and test circuits on breadboard so it is a valuable skill to learn and not just for children!
Further information: Breadboard

Another question?

Consider asking your question on a forum for electronics such as All About Circuits.