As part of a larger project I used a 555 timer wired for astable operation just as described in National's datasheet. I am using the 'ramp' or sawtooth voltage across the capacitor as my output. My goal was to operate at 30 kHz. I cranked thru the formula a number of times as I fit in parts that I have on hand. I breadboarded the circuit and it worked although at a significantly different frequency - not way off but enough that component tolerances didn't explain it. I convinced myself that the breadboard (an old, low cost one from Radio Shack) was at fault though it would seem that at 30 kHz there should not be a problem. I rebuilt it on a circuit board with similar results. Thru all this I'd look at the ramp voltage - paying attention to the timing more than the actual voltages. Eventually I looked closer at the waveforms.
Per the literature and numerous tutorials found on the internet I see that the comparators on the 555 are supposed to charge and discharge the capacitor at 2/3 and 1/3 of the supply voltage. There is little mention of anything else other than some advice on the min and max values for R and C.
My supply was 6 volts - the capacitor was swinging between 4 volts and about 1.75 volts (not 2 volts as expected). If I work backward, using the measured capacitor charge and discharge points, the predicted frequency is right on target.
Now, during all this I had swapped out the 555 thinking there might be a fault and results were similar- though I only measured ramp voltages on the last one. I'm not convinced entirely that there isn't some other obvious fault. C was .001 microfarads, Ra is 22k, Rb is 12k
Thoughts, comments and any insight would be appreciated.
Per the literature and numerous tutorials found on the internet I see that the comparators on the 555 are supposed to charge and discharge the capacitor at 2/3 and 1/3 of the supply voltage. There is little mention of anything else other than some advice on the min and max values for R and C.
My supply was 6 volts - the capacitor was swinging between 4 volts and about 1.75 volts (not 2 volts as expected). If I work backward, using the measured capacitor charge and discharge points, the predicted frequency is right on target.
Now, during all this I had swapped out the 555 thinking there might be a fault and results were similar- though I only measured ramp voltages on the last one. I'm not convinced entirely that there isn't some other obvious fault. C was .001 microfarads, Ra is 22k, Rb is 12k
Thoughts, comments and any insight would be appreciated.