555 timers, who needs them?

A reason for teaching the 555 timer is to study the RC time constant and the potential difference across a capacitor as it charges and discharges though a series resistor.  It is the RC network that is important, not the 555 timer.  A series RC network, which is a form of potential divider, does more than provide a time delay, it can be the basis of a filter and is an inherent part of all circuits due to the effect of stray capacitance.  A good understanding of the RC network leads to an understanding of a range of electronic situations so lets teach it as a general principle rather than the specific case where it is used with the 555 timer.

Why am I against the 555 timer?  Well I am not really, it is just that the 40106 schmitt trigger inverter is a far more suitable device for the majority of classroom situations and is much easier to explain and understand.

In my experience, the 555 timer is not particularly robust in the hands of students.  The discharge pin is easily blown as is the output.  The timers do not take kindly to reverse supply connection and require a relatively complex circuit to implement.  In contrast, the 40106 is nearly indestructible, and very simple to implement.  Students are more successful in using it and you get 6 invertors for the price of 1!

Some might argue that the benefit of 555 timers is that they can be used for precision timing as their switching levels are accurately set at 1/3 and 2/3 Vsupply.  (Whereas the hysteresis levels of a 40106 may very between devices.)  The timing of a 555 circuit is precisely given by formula which can be used in their design.  This may seem a big advantage until you remember that the timing accuracy of a 555 circuit is not just determined by the precision of the switching levels, but also by the accuracy of the timing components used.  Add the 5% tolerance of the timing resistor to the 20% tolerance of the timing capacitor and you can see why it is impossible to achieve timing accuracy without trimming the timing resistance.

When working with large time constants matters are made worse by the effect of leakage through the timing capacitor.  In practice, I find timing accuracy unimportant but when it is, I use a PIC controlled by a crystal!  In fact, for the simplest implementations of astables and monostables why not use PICs?

There are situations where the 555 can be useful:

  • When you want to enable and disable an astable with a control signal.
  • When you need a non-retriggerable mononostable, one which starts timing when the button is pressed rather than when it is released.
  • When you need to drive a loudspeaker directly from an astable.  (For this application, choose your device carefully because CMOS 555s do not have the same output characteristic as the NE555 which can sink and source 200mA)

Of course, all of these applications can be achieved without using a 555! Some examples are given in figs 2, 3, 6 and 7.

I suspect that one reason we are still using 555 timers is because some habits die hard.  If old trusty circuits serve us well why change them? Well, we need to change our ways to keep the subject up-to-date, relevant and to use improved circuits and methods to increase pupil success rates.   We also need to present students with a range of options so they can make design decisions.  I hope this article will encourage you to use alternative more robust and easier to use monostable and astable circuits.  Although I have only mentioned the use of schmitt inverter circuits, there is also a world of NAND gate and NOR gate versions to explore and delight in.


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