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