Water and Electricity

Water and Electricity don't mix. We all know that, right?

DO NOT UNDER ANY CIRCUMSTANCES POUR WATER OVER YOUR HEAD WHILE YOU HAVE YOUR FINGERS IN A MAINS SOCKET.

But water flows, like electricity does. If you take some water, in a jug, then you tip the jug, the water flows, downwards, because the gravity pulls it down. Right? Come on, it does.

If you put water under pressure, from a tap, through a hose, it comes out the other end. You know it does. You can see it.

Electricity flows like that too. If you turn the tap on more, water flows faster. If you put more pressure on the electricity, it does the same.

The "pressure" on the electricity is called Potential Difference, or PD. We measure it in volts (named after a bloke called Voltaire which is a daft name but he was French). The more PD, or volts, the more "pressure".

You can alter the flow of the water in a hose by standing on a hosepipe. You know that.

You can alter the flow of electricity by using a resistor. It's like standing on the hose. It resists the flow, which is why it's called a resistor. A bigger resistor resists it more. We measure resistance in Ohms (Ohm wasn't French, he was German).

Now, here's a thing. If you turn the pressure up A LOT, by turning the tap up A LOT, you can still stop the water flowing by standing harder on the hose. You know this. Now, what happens when you turn the pressure up to the max, and stand REALLY hard on the hose? Guesses?

Eventually, the hose will burst. There will be water all over the shop.

Right?

Right.

So, we have our little LED. We have some "volts", or electrical pressure, or PD. We have our LED full on. We need to make it go dim. What do we do? Well, we can put more resistance in the way, so only a little bit of electricity leaks out, but what will happen in the end? Something will give. Electricity, when resisted, escapes in the form of heat. If you turn an electric oven on, you can see this by putting a pan of boiling water on it. It boils. An electric heater or boiler or kettle works like this.

So, we can, with a bit of electrical or electronic trickery, dim our LED by putting some reistance in the way. And it will get hot.

Do you have a dimmer switch in your house? Does it get red hot when you dim the bulb? No? Good. It works because the mains coming into your house is AC (alternating current). The way a dimmer works is by cutting the power 50 times a second (this is the 50 cycles you hear about) after a period in the cycle.

Here is 50 cycles per second (or 50Hz, named after Hertz, another German).


Image result for sine wave

Now here it is again if you chop bits out of it like a dimmer switch does:



We're going to do the same thing, but with a digital "leg".

Here is our digital leg:


Image result for square wave pwm



We're going to make it go on and off. This shows it going on and off so it's on half the time and off half the time.

If we make the "on" time much longer than the "off" time, we get a bright light. If the "off" time is much longer than the "on" time, we get a dim light.

See what we're up to yet?

I hope so.

Please let me know, as usual, if we're going to quickly or too slowly?

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