[Igor] made a VU meter with LEDs using 8 LEDs and 8 comparators. This is a fast way to get one of 8 bits to indicate an input voltage, but that’s only the equivalent of a 3-bit analog to digital converter (ADC). To get more bits, you have to use a smarter technique, such as successive approximation. He shows a chip that uses that technique internally and then shows how you can make one without using the chip.
The idea is simple. You essentially build a specialized counter and use it to generate a voltage that will perform a binary search on an unknown input signal. For example, assuming a 5 V reference, you will guess 2.5 V first. If the voltage is lower, your next guess will be 1.25 V. If 2.5 was the low voltage, your next guess will be 3.75 V.
The process repeats until you get all the bits. You can do this with a microcontroller or, as [Igor] shows, with a shift register quite simply. Of course, you can also buy the whole function on a chip like the one he shows at the start of the video. The downside, of course, is the converter is relatively slow, requiring some amount of time for each bit. The input voltage also needs to stay stable over the conversion period. That’s not always a problem, of course.
If that explanation didn’t make sense, watch the video. An oscilloscope trace is often worth at least 1,000 words.
There are, of course, many ways to do such a conversion. Of course, when you start trying to really figure out how many bits of resolution you have or need, it gets tricky pretty fast.
Omw of those things I feel “I know all about” but didn’t really know how it worked.
I wonder how hard it would be to add a capture and hold circuit? Presumably that could be as simple as a capacitor you sample gently… Maybe even via a buffer.
The sample-and-hold circuits was one of the first things I learned in University, when I was learning about opamps.
Never used the knowledge anymore, because there are so many dedicated chips out there. ;) Even multiplexed ones.
And many of the microcontrollers I used have multiple analog inputs. But they have only one ADC, and the inputs are actually multiple sample-and-hold circuits, which are sampled serially one after another.
I need this to help me make decisions in my life. My binary searches tend to be weighted.
I submit that navigating through life is all about successive approximation… albeit without a sample-and-hold at your input.
Not new, of course … the Radio Shack Color Computer used successive comparison to read the joystick potentiometers.
Comparators seem a little overkill for an LED VU meter.
I guess gone are the days of using a ladder of resistors to trigger a few transistors.