Arrival Craft – Match the Component to the Name / Symbol, Resistor Code colour by number
Introduction – Last week, we made circuits to turn LEDs on and off.
Show a model, to remind everyone.
Another way of thinking of electrical current is like something flowing through a hose.
Show a length of clear pipe with a knotted string in it.
We can pull the string through the pipe and it goes round and round just like an electrical charge.
But what happens if I cut the wire?
Cut through the pipe and string.
There’s no circle of string anymore and I can’t keep pulling it through.
In our first week we made Steady Hand Games.
Show a Steady Hand Game to remind the children, if there’s time, see if anyone wants a turn.
The whole of the game is really one big switch, since a switch is just a way of opening and closing a circuit.
In your Electronics Kit today you have push switches. We’re going to add them to our LED circuits.
Use giant model to show how to add push switches to circuits.
I made the giant model using card and velcro, so we could move giant components around a giant breadboard. The children found it really helpful.
Regroup: These switches are boxed in so it can be hard to see how they work. I’d like you all to have a go with foil and card and see if you can make a push switch of your own and use it to turn your LED on and off.
Individual Task – Make a simple push switch out of card and foil.
Regroup: Sometimes we want our circuits to be a bit cleverer. Rather than needing a person to switch them on and off by hand, we want them to respond to their surroundings.
Street lamps, for example, switch on when it’s dark and off when it’s light.
Does anyone know how they do that?
Street lamps are fitted with LDRs – light dependant resistors – which respond to the amount of light around them.
We’re going to replace the switches in our circuits with LDRs.
Individual Task: Make circuits so that LEDs come on when there is light.
Regroup: With our circuits, the LEDs came on when it was light and went off when it was dark – the opposite way round to how we would want a street light to work.
But, LDRs have a high resistance in the dark and a low resistance in the light, so we need another component to get this function.
We can use a transistor.
Has anyone heard of transistors before?
Transistors have three legs. They are able to function as a clever switch. If a small current flows across the base and emitter, the transistor allows a larger current to flow across the collector and the emitter. This means we can use the transistor as a switch that is turned on by a small current of electricity.
For example, we can use the transistor as a clever switch that will turn on the light when the LDR is in the dark.
Demonstrate on the big model, then show on the little breadboard.