Electronic – Schmitt trigger not working

Do you have in place the 10 nF input coupling capacitor? Perhaps what is going on here is an input impedance effect.

If we consider the comparator inputs on the 555 to be basically like open-loop op-amps, that means that their input impedance depends on the device used (CMOS versus BJT). Negative feedback isn't there to help with impedance.

The CMOS inputs will have a high input impedance and so the input of your circuit is basically dominated by the two 100K biasing resistors, which create an impedance of 50K. In combination with the 10 nF coupler, the input passes through everything above around 300 Hz. (The -3dB corner frequency is 318 Hz). This is well below the input signal frequency.

But what if the input impedance is somehow seriously degraded with the NE555?

Just a thought.

Such considerations simply don't apply to the normal applications of Schmitt triggers. Schmitts are generally used where the input slew rate is much less than the output slew rate. With a slow input, the trigger point is not clearly defined, being affected by other factors such as noise and feedback from the switching transients as the output changes. Once the trigger point is reached, the positive feedback pulls the threshold quickly to the other level, and oscillation is prevented.

However, it is entirely true that Schmitt triggers exhibit metastability, and this was demonstrated in 1977/1979 by a series of remarks in the IEEE Transactions on Computing, by E.G.Wormald and Thomas Chaney. See http://fpga-faq.org/FAQ_Pages/0017_Tell_me_about_metastables.htm for a discussion of the subject.

So basically, Schmitt triggers are not much use for fast signals. For slow signals, they are very effective, but in these cases the low frequencies make metastability much less of a problem, since high sampling rates don't make sense, and there is relatively more time for metastable conditions to resolve themselves.