IAN LANG ELECTRONICS
GCSE Electronics is a very new qualification offered in England and Wales and first began to be taught in 2009, with last year being the first examination paper. I've managed to get hold o a copy of the specimen and I must say, for an entry level qualification, it does get shockingly hard in places. With this in mind, I've gone through it and given answers and underneath the answers I've given a reason for that answer.
Those of you not doing GCSE but who are trying to find your feet in Electronics will get some benefit from this too, as some of these questions are ones that beginners and hobbyists are always pondering. So without further ado, let's leap in and get the exam done. There's two question papers, we'll start with E1, Discovering Electronics. Click the link left for paper 2, Applications of Electronics.
There's lessons to be learned in terminology here. An input is when the circuit is affected by outside influences; this could be temperature, light, movement , magnetism, in fact anything that can be sensed or measured or even somebody pressing a button. An output is something that the circuit does by itself either automatically or as a result of an input.
Therefore, wherever you see the term "sensor" it's an input. It could be a gas sensor, a magnetic sensor, a humidity sensor or any of dozens. If it's a sensor it's an input.
A solenoid is activated by electro-magnetism. Your circuit provides the electricity to power the magnet. It might do it once every so many seconds without an input, or it could do it when a button is pressed. It doesn't matter how or why it does it, it's the circuit that's doing it. And so it's an output.
When you are doing this or any other kind of exam, look out for tricks. Examiners are your enemy and love to catch you out on points of hair-splitting. It's tempting to say the answer to the last bit is a time delay. In an abstract way you'd be right, but in actual terms you'd be wrong; the time delay could work the latch but the latch is the device that keeps the system on for a length of time. Don't let them trip you up.
Whoa, horsey, there is a shockingly nasty trick being played here. Look at the front of the question. It says:
"Here is a list of electronic components"
It doesn't say that all the following questions will refer to that list. That's nasty, because they give you a variable resistor in that list, but a fixed resistor as a component symbol. What a bunch of !$%&*@
The bulb goes on.
The bulb goes off.
The reason for these two answers hangs on what comes after point P. If you look, there's a 1k resistor and straight after that, a transistor. It's actually a bipolar junction type of NPN construction. You can tell by the way the arrow's pointing. Remember this:
NPN is Not Pointing iN.
PNP Points iN Proudly.
Where the arrow is is the emitter. At the top is the collector. In the middle and on the other side of the vertical line from the collector and emitter is the base. That is what's connected in the diagram above to the resistor. A PNP, when connected to 0V conducts, and the further away from 0V it gets, the less it conducts until it reaches the point where it isn't conducting at all. But this one isn't a PNP, it's an NPN. What it does is not conduct at all until it gets a voltage over 0.7V, and then the further over that it goes, the more it conducts, until it's conducting as hard as it can, at which point it's said to have achieved saturation. The conduction actually depends on the current, and the resistor before the base makes sure that too much current isn't going in there because if it does it'll blow up. Shoving 6V through a 1k resistor will bring that transistor into saturation, and so a conductive path is made between collector and emitter. This gives the bulb a path to ground, and as the other side is connected to 6V there's a complete path for it and so the bulb goes on. If you remove the 6V at point P, or indeed put it to 0V, there's nothing to bias the transistor into conducting. Therefore the bulb doesn't get a path to 0V and it goes out.
Transistors can be shown with or without a circle. With is the European standard, without is the American standard.
Another nasty one because it isn't immediately clear from how they've labelled the voltages what it is they actually want. V1 looks like between point P and 0V, which is then 0V. V2 looks like base-emitter, 0.7V and V3 collector -emitter, so that's your 6V.