Hi. In my previous video, we looked at drive-in LCD displays and I'll link that one in down below and at the end. for those who haven't seen it because it might not make a huge amount of sense. although I'll try to explain it without requiring the other video.

This is a response video to some comments on the previous video. A couple of people couldn't quite figure out how we were getting the voltage doubling across here cuz what we were doing is driving an LCD with a 5 volt digital logic and XOR gate and a buffer here. Just you know, 5 volt TTL type logic are in fact it. We showed an example with an Arduino actually driving the thing and we magically got +5 volts and minus 5 volts down here for a total of 10 volts peak-to-peak Where does the 10 volts peak-to-peak come from? They couldn't quite figure it out when you only had a single 5 volt supply, and effectively just by adding some digital logic gates with magically magically created 10 volts peak-to-peak How is this possible? So they thought, eh, maybe I actually got it wrong.

or maybe the math measurement I showed it on the oscilloscope was actually wrong and it was the wrong scale or something like that. No, that's not the case. It is. we do actually get 10 volts peak-to-peak across this LCD for a 5 volt supply.

so it is actually really voltage doubling and stick around I'll actually demonstrate in a minute. now. let me actually redraw this and try and explain it a bit clearer cuz it's a lot of people rightly don't understand this alternate phase thing and actually getting a difference or a subtraction function, a difference function of a waveform and how it can actually double the voltage. So let's say that we have a 7 for HC o for inverter, for example.

you're familiar with those I'll show you this on the bench in a minute that this actually does work and is powered from your typical +5 volt. So you got +5 volts and you've got your circuit ground down here and you're feeding a 5 volt peak-to-peak square wave. And of course, if you measure just the output here, we'll call that B You get your five volt peak-to-peak square wave, right? It goes from zero to five. Zero, Five Zero five.

But if we probe across a and B the input and the output here because they're got alternate phases because it's inverted and this is exactly what we're doing with the LCD we're effectively connecting the LCD across the input and the output of an inverter. Like that, you magically get 10 volts peak-to-peak And this is not just some theoretical, you know, magical pie-in-the-sky thing. You actually get 10 volts peak-to-peak because you're subtracting a minus B You're getting a difference function and that gives you double your voltage. but it's still not clear.

I'll try and explain further. Now let's say you've got an oscilloscope. Here's my crude oscilloscope. This is the screen and we got our positive and negative input ie.

like that's our BNC input, the probe positive and your ground clip lead is your negative. Okay, so let's assume that we connect single-ended Lee to the circuit common here with your ground clip LED on your oscilloscope and then you probe either signal A or B like this. What will you see? Obviously, you'll see exactly what you expect a five Volt if that's the ground reference on your scope that you'll see a positive zero to five volt peak-to-peak signal regardless of anywhere in your digital circuit. you probe because you're measuring everything relative to the circuit common.

so that's what everyone's familiar with doing exactly this. But when you're talking about a different signal which we're looking at here, you've got to not think in terms of the circuit common. That is incorrect when you're actually measuring a difference. one signal minus another signal a difference.

A A differential probe, for example, just measures the difference between the two input lines. It's not a relative to anything and we'll demonstrate this on the bench in a minute. So you've got to wrap your head around not trying to think in terms of relative to the circuit voltage. So let's demonstrate this by taking out the negative ground lead of our source code.

Ground is not the same as Circa ground here unless you actually connect it to the circuit ground. So let's actually connect that up to point A up there and then let's probe point B there. What you end up seeing here is this signal going negative I Use that term in quote marks because it's relative to the negative input here, not to this circuit ground. So we'll try another way to think of it.

If you use the ground as your reference, then what do you get? You get a waveform that switches between plus five relative to this and this reference. ie. +50 right? So that's your regular TTL signal that you're familiar with. But if you take your reference from up here, what happens? You've changed your entire reference.

It's no longer relative to this ground. So what if you move your reference? not from here going like this, but up to here like this. If you've got a zero on the end, just assume you've got a zit. Imagine you've got a zero on the input there.

Well, the outputs are one right? It's positive. So it goes from zero to +5 volts. But then what happens if you've got a 1 here. What happens to the output here? Well, it's got to go to a zero.

So it actually flips negative down like this. It flips to another level. but - 5 volts like this, so it actually toggles. If you use this as a reference, it tolls between plus minus five.

Like that, effectively, you basically get a voltage doubling peak to peak. so you're effectively going from like this to this. Your reference is now up here and you're flipping like that. So you're flipping the polarity so it's got to be double relative to one of those pins.

I Hope that's clear, let's just go to the bench and verify this. Okay, so what I've got here is a seven 4hc, one four which is a Hex Schmidt inverter two Schmitt trigger. It's just the same as the Oaf Orbit. It's got Schmidt inputs anyway.

I powered that from our five volts and I'm probing the input and the output. So I'm just feeding in a one kilohertz square wave and we're getting the inverted out. So Channel 1 Channel 2. So let's have a look up on the scope here and exactly what we get.

This is exactly the same as the last video. Basically, we've got 5 volts per division here. Okay, so 5 volts and 5 volts with both channels and you'll see that there. 1 division and so there.

5 volts peak-to-peak signals Channel A and Channel B and you'll note that they're out of phase when one's 0, the others high, and vice-versa So our LCD has hooked across those two pins and this actually gives you a good indication of what's going to happen. your voltage reference. Actually, instead of being ground here, we shifted up to here and you'll note that it goes up by one division and down by one division. And if we look at our math function here, we've got a minus B which is our basically a diff differential function and the scale is 5 volts and the comment has said or maybe had the scale wrong.

it's doing it wrong but no look, it's actually 10 volts peak-to-peak But is this something happy funny happening in this Scylla scope? Is this math operator incorrect? Well we have a way to verify this. So if you don't trust pesky math functions like this I'm sure you will trust an instrument available on the Eevblog store by the way at a discount price. If you put in the coupon code bargain probe it's anyway. it's linked in down below you can and on Amazon.com as well.

it's on a hundred bucks off on Amazon Anyway, this is the Evo Eevblog Hvp 70 differential probe. A very nice differential probe. If I may so say myself, it's a 10 to 1 division ratio and this measures the true differential voltage between these two probes. So we can put these probes across the input and the output here.

and we'll do that. We'll hook input and I'll put it for convenience sake. There we go. We've hooked it across the input and the output, so our voltage reference is no longer relative to our signal ground input.

Here, these scope probes do not matter anymore. It's measuring the absolute differential voltage across the input and the output and what are we get? Well, let's switch on Channel 3 which is 5 volts per division and let's have a look. where is our signal? It's that purple one there. Tada.

It matches absolutely perfectly to the software mathematical function that we had in there, which was the you know, the A Minus B It's exactly right. 5 volts per division. It's 10 volts peak-to-peak across the input and the output. So if you put your LCD or anything else across the input and output of a digital logic gate like that, you get 10 volts a real 10 volts peak-to-peak simply changing your reference.

As I said from, you know, I base it a little ground down there. It's sort of like it's shifting up so to speak. I Know this isn't a hundred percent accurate thing, but it kind of gives you an idea of how changing your circuit reference ground can actually give you double the voltage. So anyway, I hope you found that video interesting I Hope that's an adequate explanation if I come up with something a better physical representation of of how this actually works.

or if you got a better way to explain this, then leave it in the comments Anyway, if you liked the video, please give it a big thumbs up. And as always discussed down below, catch you next time.