INPUT DEVICES – SENSORS AND OUTPUT DEVICES – INDICATORS

INPUT DEVICES – SENSORS

Every electronic circuit must have some kind of input device so that we (the outside world) can "communicate" with it.

THE SWITCH

When the switch is closed it allows current to flow through it.

TYPES OF SWITCH

There are various types of switches that can do slightly different jobs in a circuit

SPST - sinqle pole, single throw

SPDT - single pole, double throw

DPST - double pole, double throw

DPDT - double pole, double throw


The diagrams show you examples of each type.

STUDENT INVESTIGATION
Which switch would you use to:
(a) change the direction of an electric motor
(b) switch on a light
(c) switch off a green light and switch on a red light
(d) switch on two motors at once

THE LIGHT DEPENDENT RESISTOR

This is a resistor which has a resistance that changes with the amount of LIGHT that falls on It. In the DARK its resistance is LARGE (millions of ohms), in the LIGHT its resistance is SMALL (tens of ohms). Very little current will flow through It In the dark.

THE THERMISTOR

This is a type resistor which has a resistance that changes with TEMPERATURE. In the COLD its resistance is LARGE (thousands of ohms), in the HEAT its resistance is SMALL (tens of ohms). Very little current will flow through it when it is cold.

OUTPUT DEVICES – INDICATORS

Every electronic circuit must have some kind of output device so that it can "communicate" with us (the outside world).

THE LIGHT EMITTING DIODE – LED

This is a diode that emits light when a current is passed through it. lust like a normal diode it will only work one way round. A protective resistor is usually placed in series with it.

THE BUZZER

This will make a noise when a current passes through it. Like the diode the version we have only works if connected the correct way round. The current needed to operate the buzzer is much larger than that needed to light the LED.

THE RELAY

A small current passing through the coil will close a relay switch and so allow a much larger current to flow in another circuit. The reed relay could also be closed by holding a magnet next to it.

SMALL TORCH BULB

This is simply a small bulb that lights up when a current is passed through it. The bulb works either way round and the bigger the current the brighter the bulb.

Remember that for all these devices:

The bigger the resistance of a device the bigger the voltage drop across it.
In other words:
when its resistance is LARGE the output across it is HIGH (1)
when its resistance is SMALL the output across it is LOW (0)