555 Timer

In simple basic terms, the 555 timer consists of a flip-flop with set and reset inputs controlled by comparators. This timer IC has a supply voltage range of 4.5 V to 16 V DC, and if you power it at 5 V then it becomes compatible with other TTL logic devices. Its maximum operating frequency is greater than 500 kHz for oscillator applications.

For reliable time-delay circuits, up to a maximum of one hour is obtainable using a 1000 µF capacitor, and a 3.3 MΩ resistor. The 7555 CMOS version of this chip allows higher resistor values of up to 100 MΩ, and therefore larger delays of many hours are obtainable.

One very useful feature is that its output pin can sink or source up to a maximum of 200 mA, and therefore it can easily drive a small relay, bulb, or loudspeaker. To drive larger loads, you will still require a suitable bipolar junction transistor (bjt), or a Darlington.

This timer IC is one of the most versatile components ever invented, and consequently it saturates the internet with all sorts of innovative circuits. Fortunately, for GCSE Electronics, you will be glad to know that you need to understand two circuit configurations, which are for astable and monostable modes of operation.

In addition, you need to know how to manipulate their respective equations to find suitable component values. Unfortunately, in the exam, you will not be able to use an online calculator for this; therefore, it is better to learn how to manipulate the formulas by hand to calculate the value of any component. You will find all the equations and more in the following pages of this multi-page article.

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