555 - Love it or hate it! (by Brindsley Archer)

Component: 555

Category: Timing and clocking IC's

-The CMOS version can't source nearly as much current as it can sink. Some manufacturers' versions are good for <1mA over the temperature range. Direct substitution into old circuits from old books is bound to end in tears.

-The bipolar version can source much more current but the crowbar effect of the output stage necessitates tantalum caps on the supply and good decoupling to ensure the 555 doesn't upset other electronics in the product.

-Multiple pulses have been observed on a 100MHz scope on the output during the transitions. If the output drives a LED or buzzer that's ok but if it drives logic then it could be unreliable.

-The output drive capability of the bipolar version is attractive to drive some loads directly. However it's not suitable for inductive loads. The inductance of a movement meter in a Jetboat rev counter that needs a tenth of the "rating" of the 555 was enough to made the 555 erratic. Extra parts to divorce the inductance from the 555 defeated the simplicity of the approach.

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