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I am trying to create a 20 minute timer for 20-20-20 Rule with some bunch of resistors and capacitors available with me.

I am using a 10M reistor here in place of R1 and some 10K/20K in place of R2.

Since i want a beep of a second or two as up signal so i used Not gate at output and then added a small active buzzer.

It seems like it is working for small resistor value and not for over 1M as some comment i found here. Is this really true? And isn't it possible the output to be low for more time and up for small time so i can avoid Not gate.

enter image description here

Any ideas are appreciated.

I can definitely change the capacitor value but want to know about Resistor limit on these cheap 555 timers.

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Threshold current parameter on the bipolar NE555 is typical 25nA, maximum 250nA at 25°C.

It appears to be recommended to have double that current available at the peak, so with a 5V supply, the maximum value for R1+R2 is 3.4M\$\Omega\$ and 10M with a 15V supply. See Note 1 at the bottom of page 5 of the datasheet.

That's assuming the capacitor has negligible leakage. If the capacitor leakage is significant you need to further reduce the maximum resistance.

A 20 minute timer is not very practical with the NE555 or even very pleasant with the CMOS versions (which don't have the same limit on resistance value). A digital circuit such as a microcontroller or a CMOS counter chip plus an oscillator is a much better way to go.

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  • \$\begingroup\$ Thank you for the answer. Really helpful. Yes i definitely have a plan to use some arduino but jut trying to learns basics about components and using some practical idea like 20 minutes timer to relax eyes as a motive to continue. thanks \$\endgroup\$
    – Moons
    Jan 8 '20 at 9:06
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The threshold pin current for the LM555 is specified as typically 0.1 uA up to a maximum of 0.25 uA. And, 0.1 to 0.25 uA through a 10 Mohm resistor will drop maybe 1 volt to 2.5 volts and might alter the actual threshold trigger points too much for reliable operation.

Also, if you are using an electrolytic capacitor for C1, it's own poor leakage current might swallow up a lot of the current needed to charge the capacitor and basically you don't get a successful ramp and trigger. If you look at figure 13 in the data sheet linked it shows that a 10 Mohm resistor can be used to give 100 seconds delay but only if a 10 uF capacitor is used and I would urge you to choose that capacitor tech carefully and stay away from electrolytics if you can: -

enter image description here

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