555 timers, who needs them?
The NE555 timer first appeared in 1971. Although it is now nearly 40 years old, many exam boards still include it in their specifications. This seems rather odd in a fast developing subject such as electronics. With an ever expanding range of new technology and devices to learn about, perhaps it is now time to relegate the 555 to the museum along with germanium transistors, valves and transistor multivibrators.
The 555 provides a relatively cheap, stable and user friendly solution to timing problems. Over the years we have made extensive use of the timer in the classroom: we have circuits that we know how to fault find, a bank of ideas to meet every occasion and it features in every electronics textbook. Why contemplate using alternative devices?
Monostables and astables are of course, fundamentally important building blocks for electronic systems and are essential to any electronics course. The 555 timer can be the key component, but it is not the only device that can provide these functions. My favourite astable and monostable circuits for the classroom are based on the 40106 schmitt trigger inverter. See fig1 and fig 5. These circuits are ideal for flashing leds, producing sound with piezo sounders, debouncing push buttons, and generating time delays.
Although they are easy to explain and understand, the GCSE Electronic Products and AS Electronics courses that I teach require students to remember the circuit topology for both the 555 astable and 555 monostable. In this age of Information Technology, is it relevant to require students to memorise schematic diagrams?
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