Re:Silicon die teardown: a look inside an early 555 timer chip
by altcognito on Tuesday January 25, 2022 @12:29AM
(#30068468)
How do you get such a wide range of times with such a tiny capacitor? I imagine there is some magic with multiplying this effect across multiple caps?
Re:Silicon die teardown: a look inside an early 555 timer chip
by kens on Tuesday January 25, 2022 @01:31AM
(#30068839)
For a short delay, you use a small resistor and a small capacitor. For a long delay, you use a large resistor and a large capacitor. The result is that it supports "timing from microseconds through hours." The timing resistors and capacitors are external, so you can select whatever values you want for your application.
Re:Silicon die teardown: a look inside an early 555 timer chip
by bonzini on Tuesday January 25, 2022 @03:02AM
(#30069377)
It's not necessarily tiny. You could use an electrolytic capacitor with a capacity of a few millifarads, for example. However, the tolerance of larger capacitors is larger (usually 20%, and up to 50%) so that quickly becomes expensive. It's more likely that you'll generate a 10 second or less pulse with the 555 and then make it longer using (for example) T flip-flops. A single 744040 ripple counter IC for example can divide the output by up to 4096.
How it works is that when you charge a capacitor through a resistor, the capacitor will charge relatively slowly. The rate depends on the value of the components. What the 555 chip does is monitors the voltage level on the capacitor. When it reaches an upper level, the chip discharges the capacitor. When the voltage reaches a lower level, the chip starts charging the capacitor. Thus you get oscillations with the desired timing.
Think of it kind of like filling your sink with water. When the sink gets 2/3 full, you pull the plug from the drain. When the water level drops to 1/3, you put the plug back in the drain. This will give you periodic oscillations. The timing depends on the size of the sink (capacitor) and how much you open the tap (resistor).