Let's take a look at a classic volt stick type device. I say: volt stick type device, because the very first of the units were called volt sticks all the modern ones that call themselves. Vault sticks are really just clones, although the original manufacturer is still there and they can justifiably call it full stick. I can recall buying one at full price.

It was quite a novel item at the time. That's quite expensive, i'm pretty sure the original had an aluminium body and that's they've changed to plastic now for safety. So the point of these is that if you have a live warrant here, i've got a live wire at 240, volts carefully folded over and heat shrinked. At the end, if you place your volt stick tester next, it will light up now.

You might not actually see that too. Well, so tell you what i'm just going to take the exposure off here like this and then demonstrate it. So when you put it up, actually it pulses it's 50 or in the case of american pulse at 60 hertz. But it's quite good in the sense that you don't need to break the wire to actually test.

If there's a presence of voltage on that wire. One thing to note: you can't use it through screen cables and you can't hold the wire like this if you're actually testing, because if you do it creates an equipotential field across it and it will stop it testing reliably. So you have to keep your fingers well away from where it's going and also in some instances, wet cables can also affect right. Tell you what i'm just going to bring the lighting back watch your eyes and we're back so the way this device works.

Is it detects the capacitive coupling of current from a live cable? All cables have that slight uh capacitive coupling it's very low, but it's enough to be picked up by a very high level amplifier in here and that provides the tip couples on capacitively and then there's another capacitive connection through your body to ground. And some of these sticks, they just won't light like this one when you're not holding it. You have to actually hold it to provide that reference to actually make it like. This stick also has the really annoying habit of making a bad connection, because the battery holder inside is too tight and it means i'm just gon na thump this.

It means that uh. No, it's make a bad connection. I don't use this one, it's just yeah. It's not a good design, but i'll show you what's inside uh, but i was going to say there that uh, sometimes the vault sticks will actually uh they're sensitive enough, that they can just act like an antenna and actually light up, but others you actually have to Be holding it to actually do that, so let me open this one and we'll explore it, and then i shall reverse engineer it.

I can remember taking my first one to bits: this is a little brass cap in the end, i wonder if i can pull no. I won't be able to pull the other end off hold on i'm just going to use techniques. So here's a little brass cat makes connection to the negative end of the batteries with a little wire going up there to the side. Let's try and get the batteries out see.

This is why this isn't a great design, because it snugs the batteries and uh. It means that when they move inside, if you bump it in your toolbox, it can actually pull them away from the connection to the end. So there's that wire there - and this comes out here and the circuit board comes out to reveal three little transistors. It's all through home, a resistor, an led and a little antenna based on a spring they've got a wire out the middle of them 10 and that almost defeats it to degree of actually having a little long spiral antenna but um.

Let's uh, let's explore this. I shall take it apart. I shall take some pictures of it and then we can explore the circuitry and reverse engineer it. One moment please and resume now that i've taken the pictures and i have reverse engineered it.

So here's the three transistors they're c3199 they're just standard npn transistors we've got the led at the end there and we've got a three mega: ohm resistor here orange black and green orange black and then five zeros. I should say three: zero and five zeros to give that full volume. The circuitry is not what i was expecting. I thought this was going to be what's called a darlington, just a three cascaded: three transistor darlington array um.

So i've done some experiments. Let me show you the actual circuitry first, so here is the actual circuit this little spring. Here, it's not an inductor. It's just the antenna feeds into the base of a transistor.

The current that's coupled onto that capacitively from the live wire that effectively acts like a capacitor and it couples onto that, and it induces a very tiny base. Current this transistor, the transistor amplifies that up and it's probably like micro, amps, nano amps, pico amps. It's very low, but this transistor boosts it up by a few hundred times and then - and this is quite interesting - because i've never really thought about this - it's a kind of one of those moments that you go. Oh that's clever! In the past, i've built similar devices, but i put my resistor there.

The resistor is designed to stop it being too sensitive and, as a result, even with a high value of resistor, it was a bit still it wasn't. It actually pulled it down too much. They've put a pull-down resistor three mega ohm at the second stage, because this is where that tiny amplification then feeds into the next transistor, which then multiplies another few hundred times, and then it feeds the next transistor that multiplies a few more hundred times and it lights. An led the placement of the led in this one is odd.

I expected this not to be connected. I expected that to be the classic darlington, a darlington is just where they cascade the transistors and the first one c multiplies it by 300. The next one, the 300 again then 300 again and it results in a really massive gain. I expect it to look like that, but it's not - and it's quite clever because uh this one as the led turns on the rail here actually pulls down towards a zero volt rail, so it kind of self-regulates and that limits the current through the led.

I think they've done that to give us a constant current thing with led, i'm not sure it operates down to 2.5 volts and the current is typically around about 10 milliamps. So, ultimately, that's it. A capacitively coupled signal comes on, gets amplified, multiple stages to light the led. I shall show you the ones i built after i've.

Uh shown you my other version. I thought what, if you cascaded the darlingtons and then you put the transistor d, the led should say down to the zero realm. I was expecting that uh as the voltage increased across, ultimately the the led. It would raise the emitter here and uh.

Then it would actually mean it would be harder to turn on so that would self-regulate in reality. It's not a great approach, because the led is already two volts and then it's you've got the base voltage with reference to the meter above that, and it just made it very insensitive. So i'll show you afterwards, it's not a good version. It takes too high voltage.

It would work fine at higher voltages, but at the three volts it's just on the border line of operation, it means it's the dimmest of the them all. I also built one more or less based on my thought of what this was going to be. In this case, the antenna goes to the first transistor, then the second then the third i've put there a 3.3 mega ohm resistor, it's a standard value here as the first uh as this sort of gain adjuster. That is very clever.

That is really clever, um and then i've just without an any current limit. I've just put the led straight there. This one operates the lowest voltage two volts and it draws about 20 milliamps. I think that's fine, but let me show you the result of my experiments.

I have my live wire here to test them, so let me just grab these devices. I cobbled them together out of discrete transistors uh bc. 5 bc547. So this is one based on the original circuitry as you'll turn it on, and when it's brought near the wire it lights up.

Let me just bring that up a little bit closer here. It works. Fine, it's got a reasonable sensitivity works quite well. The next one is the led to zero volts.

This was not a great one. There's the led's in a different position, i'll just bend that up. This is a terrible idea, i'll probably snap it off. When i hold this one nearby, you can see it's lit, but it's not very bright because of that voltage difference.

So it's not that great, but my favorite, my favorite of these designs is the plain uh darlington this one, which actually makes the led brighter and it's the simplest design. Ultimately it's the one i thought it originally was. But having said that, theirs is quite clever. This one is advantage it'll work down to the lowest voltage uh.

It worked right down to about two volts and because it's not relying on uh acting as some sort of like it's not got some sort of current regulation circuit in it, and it makes it quite bright because it does like that little bit higher intensity. It's quite good, so my favorite of the ones i built is this one, the simple darns. Let me show you that schematic again it was a it. Was this one very, very simple: if you want to make one of these experimentally uh just use any npn transistors you can get, the we've got the collector emitter and the base, and basically speaking the base is the original input.

Then you've got the emitter switching the current through to the base the next and then the next image, or switching it to the base of the next just amplifying up all those times. There's not really that much point making. One of these, though, because uh you can get them quite cheaply in the pen form and they're, usually quite reliable. You also get the audible ones.

This one is just basically glowing all by itself largely because of that i've got a live wire lying in the bench, but they are they're so cheap and you know available. I wouldn't necessarily say i would say it's useful to add to your toolbox. You don't have to buy an expensive one, it's a useful thing for quick checks and double checks before actually carefully separating mars. If you you've, no guarantee that they're isolated, but as just as part of your toolbox, it's actually pretty good um and they're cheap.

So by all means you could build one if you wanted it just for the fun. This is very similar to the ghost detecting circuit within 10 on it, i'm kind of looking at that resistor there and thinking that would be an improvement to the ghost detector. So i may revisit that now all that did was detect a dc static field. Now, oh another thing, that's worth mentioning.

If you want to test these things, uh just swipe them down the shirt and you'll, usually uh you'll get a stat. The static charge will actually make them glow and flicker uh, just while you're swiping them. That's a good uh basic test uh. If you have nothing else that you can guarantee to test on the vicinity, that would give it the proper indication.

But there we go super simple circuits, uh, quite interesting. I quite enjoyed building those ones and it's just the classic capacitively coupling voltage detector.