HI, 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).

ImageThe 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?

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.

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