Maximum voltage allowed in trigger pin of 555 bistable

555maximum-ratingszero crossing

I want to measure the frequency of a bicycle hub generator, consisting of a variable AC voltage output. Nominally, the RMS voltage is 6V, but with no load attached the voltage peaks can be tens of volts.

My plan is to use a 555 in bi-stable mode, attaching its output to an Arduino interrupt to count its "rising" event. What I need to know is if I need to, and how to protect the TRIGGER pin from over-voltage. Most probably the 555 will be powered from the Arduino (Vcc = 5V).

So the question is:

What is the maximum (positive and negative) allowed voltage on the TRIGGER pin of a NE555? How should I typically protect this pin if external circuit is expected to go over the voltage limit?

I took a look at the datasheet, but I'm afraid my current knowledge is not enough to interpret it.

Best Answer

Per the 555 datasheet, absolute maximum voltage on the trigger pin is equal to VCC.

The easiest way to protect the pin is to use a series resistor and a schottky diode to VCC:


simulate this circuit – Schematic created using CircuitLab

The resistor should be sized sufficient to limit the current shunted through the diode; anything in the range of 10k to 1M is likely to be fine in your application.

In a pinch you can omit the diode and rely on the protection diodes in the 555, but it's generally considered a bad idea to do so.

Since your output is AC, too, you will need an additional diode to block negative voltages, or a resistor divider to shift the voltage appropriately.