Difference between NE555P and 555CN?

I'm working on a circuit that uses two 555 timers. At the electronics store, they gave me a handful of 555 chips, I only noticed the difference when I finished my circuit: the R1 value is shorted out, my R2 value is 150 Ohms, and the electrolytic cap is 10 uF. (see the schematic with R3, D1 and D2 replaced with a single piezo speaker).

The thing is, if I put in the 555 chip marked LM555CN, it appears to be 'always on'. But, if I use the Texas Instruments chip marked NE555P, the circuit works as expected, that is, it generates an annoying screech on a piezo speaker. What is the difference between these two seemingly identical chips? Is it a quirk in the TI chip that it works when R1 is just shorted out? And lastly, is there a way around this problem other than me sitting at the electronics store picking out the TI chips? Here is the date sheet of ne555p

Also note - the piezo speaker doesn't have any oscillator in it, that is, when you apply raw power to it, it just clicks once and sits there, that's why I have the second 555 timer, and it's not an option to change out the speakers at this point.

seandogue27 days ago

There should be no difference as far as I know. As I recall, the P and N were both suffixes that designated it is a plastic pinned IC. You may simply have a bad 555 or accidentally connected it wrong. It happens to the best and worst of us. I've blown out more 555s than I care to remember ;)