variations within the 555 timer family

IamSmooth, Mon Sept 04 2006, 09:44PM

I used two 555s for an experiment: the NE555P and the ALD555P. I set the 555 as an astable multivibrator. I have a 10k resistor going from Vcc (8) to the TRIG. I have a 1 mf cap going from TRIG to GND. The DISCH, TRIG and THRESH are all connected.

When I use the NE555 I can measure a nice sawtooth waveform at the TRIG pin. When I use the ALD555P the sawtooth waveform has a much higher frequency. I didn't measure it, but it looked 5-10x higher. Also, the NE555 ran even if I let the RESET pin float or connected it to Vcc. Connecting it to GND reset the output to zero. The ALD555 would not run if I let the RESET pin float.

Can anyone explain why there are these differences and how they would affect circuit design? I know these are different 555s; one of the is CMOS and the discharge component is a MOSFET instead of a transistor.