555 Astable Tolerances - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 10, 2022, 12:20:54 pm
content: I am in the middle of renovating an old arcade game and in the sound section it includes three tone generators...From the schematic the frequencies of 8R, 8S and 8T should be... 138.75Hz, 190Hz and 267Hz... mine seem to run at... 125Hz, 142Hz, 221Hz so a bit slow... not sure about that... C#3, F#3, C#4 versus B#2, C#3, A#3Now all the values look right but the caps on the board appear to be run of the mill ceramic... which got me to thinking...If the resistors are 1% tolerance... what is a good choice of capacitor type for C17, C18 and C19? In practical terms aren't capacitor tolerances of 10% pretty typical and hence an astable with fixed resistors is going to have a wide tolerance?$$f=\frac{1.44}{\left ( R_{1}+2R_{2} \right )\times C_{1}}$$Is it just they were being cheapstakes on my board and a 1980s mylar capactior would have better tolerance?

author: Gyro
author_rank: Super Contributor
timestamp: January 10, 2022, 12:39:19 pm
content: QuoteYes, probably, unless your frequencies are easier on the ear!MKS Polyester or MKP Polypropylene would be suitable replacements, you will be able to get 5% or better. At 10nF you could get closer tolerance FKP too, but that's probably overkill.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 10, 2022, 05:40:49 pm
content: I have just gone through my packet of Mylar Polyester Film Caps... 10 of them... the best is +1% and the worst is +4% with the best three being <2%... I guess that is as good as it gets. (for Mylar)(Assuming my DE-5000 LCR meter is perfect)

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2022, 06:06:29 pm
content: Mylar is not bad, just remember the 555 timer IC itself is pretty poor, you will get different results by changing them, though you could easily trim them with a 10k pot connected across the supply rails, and with the wiper connected to pin 5 of each 555 timer to do fine adjustment. Or simply add a 220uF 16V electrolytic between pins 1 and 8 of each IC, along with 100n between pin 1 and pin 5 as well.

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2022, 09:32:51 am
content: I had a play with the 7555 a few years ago, to see how close to a 50% duty cycle it gives, with the timing resistor connected to the output. The answer is: it can be off by a few percent, more if too lower value for R is used.Schematic.R = 1k, C = 470nF, V = 5V56.3% dutyR = 1k, C = 470nF, V = 15V51.7% dutyThe above isn't surprising, as 1k is far too low, which loads the output considerably. Note it's worse at 5V, than 15V, because the P-MOSFET on the output side has less gate drive and a much higher on resistance.Increasing R to 10k, or above certainly helps.R = 10k, C = 47nF, V = 15V48.4% dutyR = 100k, C = 4.7nF, V = 15V48.2% dutySee the thread linked below for more information.https://www.eevblog.com/forum/beginners/portable-low-frequency-square-wave-generator-circuit/msg2167870/#msg2167870The bipolar NE555 might be a better, or worse. I didn't test it. Note that connecting the timing resistor to the output of the bipolar 555 will be less accurate, as the output stage is more asymmetrical.