29th May 2008

- Components and their uses –

Often the link is made between a flow of water and a flow of electrons – both are associated with the term “current” to describe the flow from one place to another. Similarly the drop from a point at which something has potential energy to a point ‘lower down’ is analogous to both a waterfall and the ‘potential difference between the positive and negative terminals of a battery.

In both, the potential to do work is effectively removed when the potential drops to zero. This ‘zero’ point of potential energy has to be measured against a particular point whether it be the level of the bottom pool in the case of a waterfall or of the zero-voltage line in a circuit

In both, the potential to do work is effectively removed when the potential drops to zero. This ‘zero’ point of potential energy has to be measured against a particular point whether it be the level of the bottom pool in the case of a waterfall or of the zero-

If the drop from 9 volts down to zero volts were to be compared to a waterfall with a step half-way down its height we have a similar situation to a circuit having two resistors of the same value wired in series and working as a potential divider.

So if you need to control voltage in a light sensitive circuit for example then by having a large resistance in the place of R2 there is always a ‘high’ voltage at point ‘a’ - so a transistor or thyristor will switch on. If the large resistor is at R1 then there is always a low voltage at point ‘a’.

Of course by having a ‘light-dependent-resistor’ (LDR) as one of the resistors – the voltage across the LDR will drop as more light falls on the component. By also including a variable resistor as the other component in the voltage-divider, total control can be achieved and the circuit set up with the sensitivity required.

Of course by having a ‘light-

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# 11 of 12 pages