I'm looking at using the SN74LS14 for turning a 3V peak-to-peak audio signal to a square (rectangular) wave. If my reading is correct, the positive-going threshold (V_T+) is 1.6V and the negative-going threshold (V_T-) is 0.8V. Since the input has a 1.5V peak, which is less than V_T+, would the output be a constant LOW? If so, is there a more appropriate Schmitt trigger for a 3V peak-to-peak signal?

In fact, what's SN74LS14's recommended input voltage? I can't seem to find it anywhere.


1 Answer 1


The SN74LS14 is intended to be used with TTL logic levels, not with random analog voltages. The specs you quote say that a voltage above 1.6 volts will be seen as a High, and below 0.8 volts will be seen as a Low. The actual switching threshold will be somewhere between 0.8 and 1.6 volts, but there is no guarantee exactly where.

You should use an analog voltage comparator such as an LM339 to "square up" your analog signal. With a comparator, you can set the switching thresholds and hysterisis to suit your application.


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