The railway barrier shown below has been designed and manufactured for a model railway system. The motor inside the post drives the barrier up and down. The motor is the output to a 555 timer circuit.

1. There are two types of 555 timer. What is the name of the 555 circuit shown below?

3. The relay is causing major problems because an important component is missing. Name the component and draw its symbol in the correct position on the circuit diagram above.

The diode protects sensitive components such as the transistors from back E.M.F. When a current to a relay is removed the relay can emit current stored in its coils. This can be sent the wrong way round the circuit damaging components. The diode operates like a one way valve preventing this from happening.

A second version of the railway barrier has been manufactured. This is mechanically operated by a rotating device.

The pivot allows the barrier to rotate at a specific point. In this case the barrier rotates upwards and downwards about the pivot.

HOW IT WORKS: As the eccentric cam rotates in a clockwise direction the barrier is pushed upwards at a constant rate. It then descends at a constant rate back to its original position.