If you work in the electrical industry, you may have come across the classic LEDs voltage indicating test lights. Let me show you these operating and then I'll show you the subject of this video which is this ancient possibly the Grandaddy of them all. It's a Steinal Electrical tester so I'll bring in this socket, defeat all the safeties with this death adapter I shall then stuff this in and the LEDs will light. I'll shoot it so you can see.

like because I'm not going to be very bright, they're they're lit and it's displaying AC I'll demonstrated by the plus and minus both lighting and 230 volt which is the voltage that I'm measuring which is probably close to about 240 volts because that's what the voltage is here. However, I think a lot of these or the original design to probably copying this original stainer one and I bought that. This was my first fancy set of test lamps but round about the 1980s and if we take a look at the circuit board in it, it's strangely complex. Let me point out some things in this.

then we'll Zoom a bit closer because uh, I had it took me a long time to reverse engineer this I Literally Well, to give you an idea, I had literally had to print out a much bigger one just so I could see the components. But anyway, let's focus down under here and take a look at this. There is a capacitor. I Thought the capacitor was going to be part of the power supply circuitry with things like Zener diodes to actually give a stable supply for LEDs or power the probe.

It's not, it's just basically connected between the two probes. It may be for filtering and also to provide a slight load on the circuit to avoid false readings. It has four diodes here and I thought that'll be a bridge rectifier and four diodes here and I thought that'll be a separate Bridge right far. Not sure why it turns out all those diodes are just Uh series Pairs and it's one big bridge rectifier.

There were a couple of links from a to A and B to B simply because the circuitry has a sort of like the high voltage at one end going to the sort of zero volt rail the other end and they wanted to keep tracks off the circuit board with the full means potential between them because they they have to allow high clearance and putting links in is the best way to do that. There are two PTC thermistors. one with this resistor is dedicated to these two LEDs which are the polarity indicator LEDs and also the low voltage indicate realities and also there's one up here which is part of the LED circuitry in conjunction with this Uh, fixed value resistor which is going to get quite warm in use and there's another resistor here used for providing the reference voltage to a resistive divider that then goes down. let me show you the schematic.

that's the best bet. Jump straight into the the juicy bits here. the probes. The one at the tip is marked positive and the reason it's marked positive is just because if you're proving something when the positive LED lights, if it's DC that means well, it is your touch of something positive.

If a negative LED lights you've touched something negative. So on AC they'll both light. they'll flicker bats forward. There's the 36 nanofired capacitor.

Quite odd markings I Would guess is that going to be 500 volt rated much to 650 above it for not sure 500 volt. I guess might be the rating at the Ace AC rating perhaps higher for the DC but then there are two resistors 3.7 K by default, that's a PTC thermistor. It starts off with that resistance, but as it gets hot, the resistance increases and it basically self-regulates so it can't sort of overheat. Then there's a 506 Ohm resistor to help limit Imran in Rush current particularly if you use it in a very cold climate and that starts off the lower resistance.

And then there's a two inverse parallel red LEDs that basically stick that across whatever Supply AC or DC one or more of these LEDs will like to show you if it's D series in the party. Then comes a bridge rectifier with eight Em513 diodes, the rated 1.6 000 volts each. At one amp, they're rated A Cut Above the classic one and four double seven. And for extra safety, they've got two in series on each leg and they feed to the main negative reel and the main positive reel of the rest of the circuitry.

Rest of the circuitry starts getting a little bit. Wild Yeah, actually. uh, Drew this vertically. I'm going to have to zoom out a little bit.

sorry about that because uh, it's the way it fitted in. So here it is. Here's a positive reel from the bridge right far. Here's the negative reel from the bridge react for our butcher called zero Volts here.

this circuitry here and the 120k resistor and the PTC thermist to provide a current limited supply for the LEDs that dynamically reacts to the voltage. So if the voltage is very high initially, when you connect the probes on your off and see the LEDs let up really brightly and then fade away, that's the PTC thermistor heating up and regulating the current down to a sensible level. and we also have a tap off that rail for the 270k that then feeds the potential divider with a little decoupling capacitor here to provide stability at the top of this chain of transistors that are turned on by these. Taps To like the LEDs but it's really weird.

circuitry. There is a what looks like a PNP Darlington which is measuring the voltage differential I think between these two rails which means it may actually bypass current down to light the 12v LED brighter and this is a gas By the way, when the supply voltage is low before it's actually able to actually divide it down through, well, these resistors because the current will eventually flow through that led through the resistor array. but also, it seems to be tied into controlling the switching on these LEDs above a certain threshold. It's quite odd circuitry.

It's also creating a supply rail with a 39 volt Zener diode which is feeding loads of Npn transistors. Now, normally Npn transistors switch the negative rail, but in this instance they're collect. their collector is actually going to the positive rail, which means that the voltage that the emitter is at will affect the base turning on. I Wonder if that's how they I wonder if they actually they're using some clever technique to effectively sequentially step these LEDs in a large series line.

It's very hard to follow the circuitry because it is so devious and uh, and clever. I'm just going to zoom down a little tiny bit more, a little tiny bit more Again, watch me go too far. I Went too far. That happens.

but you've seen that there's just power rails up there anyway. Um, I'm not even sure what this one in for a one for eight diodes for at the bottom here for the 24 volt. Supply But uh, this starts off with a it's all strained circuitry. This must have been breadboarded in the workshop when they designed it and then they must have just shuffled components because that 4.7 volt zener Diode here with a 36k resistor in parallel.

it's very hard to just get your head around it. It's not a decisive uh transistor on transistor. off. it is very, very analog.

and certainly when you apply voltage to it, the 12 volt light starts growing above about 8 volts, the 20 volt one volt, 24 volt LED starts going above 16 volts, and the 51 volt one starts going above 36 volts. A very strange circuitry, very hard to get your head around. It took me a lot of time to even draw the schematic because one of the things that makes it easier to draw schematics is saying all right, that transistor is switching a relay or some LEDs and you just know how it's going to be connected with these ones. It is so old-fashioned and analog that it's actually really complex.

So no, no, I'm quite tempted to see if I can find a a generic Chinese one of these testers I do have one somewhere here I've found the case of it, but not the circuit board. and once I find that I'm going to compare them and see if they've copied a component for a component because it's almost certain that because Steinel were the daddy, they got in there first. with these Led Uh display units, these test probes, it's most likely that their design has been copied left, right, and center, and most modern probes, probably all a lot of their circuitry to this original design, which makes it very interesting. Very interesting indeed.