Was this the first LED test light?

This is the circuit board out of my first multi-voltage tester when I was an apprentice in the 1980's. It was quite an expensive unit at the time, but is designed for professional use.

The circuit seems simple enough with just a single sided circuit board and traditional through hole components, but it was an arduous thing to reverse engineer due to the very weird way the components are used to achieve a lot with the components of the era.

There are two distinct sections. The simple two-LED polarity indicator with PTC resistor to regulate the current thermally, and the main section that does devious things with standard transistors.

I think the PNP darlington is used to provide more current to the 12V LED, which has its own circuitry.

The 39V zener seems to be capping the LED-transistor supply to protect against overvoltage. The LED transistors themselves are being used in a non-standard way that possibly allows them to cascade progressive LEDs in series for maximum intensity at low current.

The LED supply is created by a fixed 120K resistor in parallel with a PTC thermistor that allows higher current to flow at lower voltages, but thermally regulates the current down at higher voltages.

In the future I'll reverse engineer a generic Chinese multi-voltage test light to see if they have copied the circuitry from what I believe to be an original Steinel design.

This also keeps the channel independent of YouTube's algorithm quirks, allowing it to be a bit more dangerous and naughty.

#ElectronicsCreators

[TopEndSpoonie]

Wow, only 6, 000 subs for the MILLION. Great work Clive. You deserve it.

[clems]

Big Clive, I just reverse engineered a very small USB charger board. I now have a deep appreciation for your work. There is nothing easy about doing this. Even on this tiny board, it was very difficult to do. So, A Giant Thanks for the hard work you do for our entertainment. God Bless ..

[AnimationGoneWrong]

What's this??? No microcontroller? No charge controlling chip? No USB connector? I LOVE it! Strange yet elegant in its design. Thanks for presenting this! You find some of the coolest stuff, and I love seeing what you'll bring us next.

[markpitts]

I love the 'bridges' on the schematic. So used to 'dots and crosses' in cad, but I still draw my hand drawn diagrams with hump back bridges as well. Takes me back to the Maplin catalogue when it was 2" thick.

[scottdebruyn]

One of my old nemesis... transistors in emitter follower configuration. It appeared to me to be using the LED's, the voltage divider resistors and the BE junction of each transistor in concert. Seems like that would be a mathematic nightmare to have designed it on paper first. You may have something there to suppose that they just ballparked it on paper, then breadboarded the circuit and then adjusted the base resistors (perhaps with pots). Like television circuitry, adjusting one would affect all the others. What a 'cluster'! 😏

[cortanajpn]

Love the scorch marks on the “deathdapter”!

[ehsanshahzad]

NEARLY 1million LETS GO BEEN HERE SINCE 20k

[harrischalk]

Its a nice change seeing a schematic without a chip, but it hasn't necessarily made it any easier to work through.

[michaelkaliski]

Worth noting that these voltage indicators have a work cycle of around 30 seconds measurement and 5 minutes cool down time due to the resistors heating up. They are not really a substitute for a multimeter but work well enough to give a go/no go indication in harsh high vibration environments where a multimeter movement might be damaged. Not really a problem with digital instruments. Better than a neon screwdriver or one of those battery powered voltage probes, but not by much. The main advantage is that it doesn’t require batteries and it can detect voltages from around 8 volts to 600 volts and give a clue whether to expect just a tingle or instant death.

[olmostgudinaf]

The curcuit is not mysterious at all. We used to make bar graph LED VU meters using exactly the same principle when I was a teenager (30-40 years ago).

The idea is very simple. Each transistor is an emiter follower. It starts conducting when the base voltage reaches the forward voltage of its emitter LED plus the base-emitter offset. The resistor divider ladder feeding the bases determines when that happens.

The darlington pair plus the bottom transistor form a current limiter. As the current through the LEDs rises, so does the voltage drop between the positive rail and the darlington emitter. But the voltage at its base does not, it is only determined by the input voltage. Thus, the darlington base-emitter voltage difference drops and the darlington closes, closing the bottom transistor (the one with all the LEDs in its collector) and reduces the current through the 12V LED.

Other components, like the diode and the zenner, cater for non-linearity of the indicator.

[jeffdayman]

Old school and very clever! Some great stuff was designed in the 1960's and 1970's before affordable u-processors were available. I knew a few designers of this sort of stuff early in my work life and they knew RC network theory, transistor logic, and PCB capacitance effects inside and out. Liked seeing this device! Cheers.

[KeritechElectronics]

An interesting and cool find. I love the complexity and simplicity at the same time - all discrete transistors, none of that microcontroller rubbish.

[nowster]

The LED chain on the right is 2V each step. The middle transistors will switch on when their base is 0.6V above their LED voltage (apart from the last with the diode). The bottom right transistor is a constant current sink. The chain of resistors on the left will be carefully chosen to switch the transistors on at 2V intervals (apart from the one with the diode). Very clever.

[Pulverrostmannen]

That was a very nicely drawn schematic you did there, very impressive details and straight lines. and about the travel adapter safety, let´s face it, we all been a conductor at some point when working with electricity in one or another way :P

[ManWithBeard]

The more I look at it the cleverer it gets. On the left is a potential divider with mostly resistors, on the right a series string of LEDs. For all intents and purposes we can assume the LEDs have a fixed voltage drop, but the taps on the potential divider all rise together. The surprising thing to me is that the logarithmic scale indicated by the LEDs might actually be required for the device to operate properly, because as the voltage increases you need each potential gap in the divider to be smaller than the last one (so a bigger increase in voltage at the probes is needed) so the LEDs light up in the right order... Really confusing TBH.

Anyway, in case you're not familiar with transistors being used as they are here, this is a classic example of what's called an emitter-follower configuration (if you were using MOSFETs you'd call it a source follower). The name explains very well what it does: the emitter's voltage will "follow" what's being applied to the base, minus the transistor's threshold voltage. The advantage of doing it this way as opposed to connecting it directly is that very little current is drawn from the base, so it doesn't influence the voltage from the potential divider that much. That makes it easier to calculate what those resistors need to be and also means that the LEDs come on in discrete steps, instead of turning on more gradually. They also happen to function as diodes to prevent the current from going back into the potential divider at the step below and messing stuff up that way. Elegantly done.

[RomanoPRODUCTION]

BigClive had pleasure in drawing this schematic :) I love that :) Thank you BigClive :)

[RODALCO]

Great that you backwards engineered the workings of this electrical tester. It is a lot more complex than at first thought. We used the Bennings plunger duspol in the 70's till around the 2000's, and now the led or digital versions of the latter.

[ThePoxun]

For something as analogue as this it would be interesting to see it modelled in SPICE or similar and to investigate exactly what is going on

[gavinminion]

5:49 - My take on the BC556 darlington is that it provides power 'at low voltages' to assist in lighting the 12V LED and additional bias current for the constant current circuit formed by the BC546A and 270R resistor. As the voltage rises, so the emitter of the darlington will begin to level off at 39V, whilst the base will continue to rise, switching off the darlington. At this point, bias current and 12V LED drive will come through the string of resistors on the left. It is the BC546 which limits the current to the string of LED's (to around 2mA) and the PTC thermistor is there to reduce it's dissipation at higher voltages.

[chuckthetekkie]

I'm always interested and fascinated how circuits of the 60's-80's were designed that could do so much for how primitive the components were. Really interesting. It's amazing what people were able to do. You really had to think and consider every component choice. Today people have it easy with computer programs and AI that can help them design stuff and even simulate how components might interact with each other.

I remember my dad bought me a transistor radio kit made by Radioshack. So much fun to put together and learn how everything work together. Too bad the case was made of cardboard which got soaked in the leaky trunk of an old car and completely destroyed. Really miss that project.