So we agree on what is expected. Have you tried several different capacitors? This one may be mismarked.
Is the supply well regulated? Do you have a bypass capacitor across the supply pins of the chip? For *stable* supplies, the voltage should have a very minor effect on frequency. Bumpy supply voltages have a pronounced effect on the cycle.
I'd try bringing the value of Ra up to 2.2K, Rb to 15K, and C to 470pF. The equation will still give you just about the same frequency numbers, and you won't be working the poor discharge transuistor quite so hard.
See if it works. That would cause both of your "Series of Unfortunate Events".
I have a question concerning the function of the 555 oscillator in astable mode. In particular the external elements i have connected to the 555 are those depicted at the datasheets of all the 555 ics (LM555, NE555 etc). The 2 resistors are above
1K Ohm (Ra=1K & Rb=6.8K), the C capacitor's value is greater than 0,0005uF (C=0,001uF) whereas a
0,1uF capacitor is connected between the "Control Voltage" pin and the ground. The above connection with the aforementioned values for the external elements adjust the 555 so that it theoretically produces a frequency of 98KHz, more or less. The problems lies in the fact that the experimental frequency, produced by the ic, is lower than the theoretical by around 30KHz. The discrepancy of theoretical and experimental value appears also in other "valid-practical" values of Ra,Rb & C with different deviations. I have also tried the LinCMOS version of 555 (TLC555) with the same results. Another odd phenomenon, equally unwanted with previous one, is the shifting of the produced frequency whenever the value of power supply of the 555 is changed, always ofcourse within the allowed limits (4.5->16Volts). The datasheets of the various 555 mention clearly that the HIGH and LOW times, and hence the frequency, are independent of the power supply.
I am not the expert even with simple things like 555 timers, but I have used them a lot (perhaps proving I am not engineer)... Following info on my datasheets, that second cap (as opposed to the timing cap) is often 10 nF. I cannot say this is the problem. However, with regard to the timing capacitor, I have found large differences between different types of caps and also variation between different manufacturers of same types of caps with same value. Also, most of us in here (including me) are hobbyists and use surplus store components, which includes resistors. I rarely buy or use 5% or even 10% tolerance resistors. The unmarked ones are claimed to be 20% tolerance. I find they are better than this at room temperate, but maybe they are not stable in a live circuit. And then there are differences between 555 timers from different vendors and most of us are not using the stringently toleranced military grade ones that are rated for higher temperature stability. I note that the 555 can get hot in some circuits. Many, including me, are guilty of trying to get this poor little IC to drive more than the couple of hundred mW it was intended to sink or source.
I once added up all these variations that I actually encountered using my usual surplus shop components and could easily account for nearly twofold discrepancies in frequency. I also suspect that the info in the datasheet is really and truly valid for tightly toleranced components.
Another permutation was the steadily increasing frequency which disappeared once I changed the timing cap for another with exactly same value from same vendor. I think these tiny caps can get damaged during soldering and then misbehave.
I do not care to upgrade because I have no need. A discrepancy between theoretical and actual frequency and duty cycle always occurs and this can be fixed by swapping timing caps and use of one or more potentiometers at Ra and Rb. I have indeed used the high precision components and still needed to tune the 555 to get the exact outcome I want.
There are two issues developing here. One is that the poster got a large deviation. This is not likely due to the vendor of the 555, but smaller scale differences do exist between vendors. It could be due to a bad timing cap resulting from mismarking, inaccurate construction or damage during soldering. The timing cap is my usual first suspect.
Yes, we all agree.
"Junk", as you refer to it, is the stuff of life for many hobbyists. I have learned much from junk. I have fixed a lot of junk. I have built junk, and I have encountered many people who have praised my junk. Indeed, I even maintain "The Astronomy Junkyard"....
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I disagree. Whatever the application, and whatever the precision
required, having to go back and redo work because of careless errors
made is sloppy workmanship.
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Aaaarrghhh!!!
Charge time = t1 = 0.693(Ra + Rb)C
Discharge time = t2 =0.693 (Rb) C
Total period = T = t1 + t2 = 0.693 (Ra + 2Rb) C
1 1.44
Frequency = --- = --------------
T (Ra + 2Rb) C
Depends on the application. If I wanted high precision timing, I would use a crystal and would say that not using this is a sloppy design to begin with. The lack of use of a crystal by the OP suggests that the design is permitted to be a little "sloppy". If precision timing is not needed, the use of the crystal would constitute a sloppy thinking process, because it just adds extra components and cost and maybe even trouble-shooting and additional failure modes to the design. At least where I live there is a drammatic price and time difference in buying precision components (online usually) and picking the lower precision components off the shelf at the local surplus shop. It sounds like the OP will be quite content, and learn much (just like me), with common lower precision components and a little tuning with a trim pot.
I note that many multimeters have cap testers. No idea how accurate they are, but since the timing caps seem to have higher inaccuracy than the rest of the typical astable 555 circuit, I would think checking the capacitance would be informative in predicting the frequency. In worst case, one could get relative values to compare caps, which is still helpful.
BTW... when I write "theoretical", I mean specific equations for frequency and duty cycle using stated values on components Ra, Rb and C. These equations are only approximations regardless of how precise components you add to the 555. A case in point, for instance, a couple somewhat different constants for the frequency constant in the numerator for the astable calculation exist (1.49 and 1.44). These values are mentioned in books about the astable mode and are discussed independently of vendor, yet they differ by about 3.4%. Until now, I have never heard anyone claim otherwise and have expected to have easily 5% deviation from calculated frequency and/or duty cycle. Maybe you know something I do not?
Actually, I have been curious about where that constant comes from to begin with.
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