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This was hellish to reverse engineer (Patreon)

Published:
2023-09-21 01:36:59
Imported:
2023-09

Content

This vintage LED test light is so "analogue" that it was really hard to reverse engineer due to the odd way the transistors are used.

Instead of being able to draw it out in a logical way, I literally had to trace every component's connection to the others, and then try and draw it as a neat schematic.  Even with triple checking, it still left me with doubts about the layout.

The veterans of analogue circuitry will be much more comfortable with the design than I am.
I bet there's even a name for the cascaded NPN transistors switching to the positive rail.


https://www.youtube.com/watch?v=czSaFinAacU

Files

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. If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:- https://www.bigclive.com/coffee.htm This also keeps the channel independent of YouTube's algorithm quirks, allowing it to be a bit more dangerous and naughty. #ElectronicsCreators

Comments

Curtis Hoffmann

A circuit design so old, even BigClive can't trace it.

Mike Page

It feels as if the 12V indication has been bolted on at some later date, suggesting the design was initially mains-y with a nominal 24V indication for truck usage. And when I say mains-y I mean UK mains, although 440V is a curious choice, being the upper +6% limit rather than nominal. That said, it is common for LEDs to light at -15% of their marked value.

Jon (0ryn) Westgate

I've seen something very similar in an LED VU meter it had stacked transistors just like what you have there.

Nani Isobel

It looks like the stacked LEDs themselves are being used as voltage references. The transistors won't turn on until the base is about .6 V higher than the voltage at that point in the stack of LEDs. It looks like the voltages on the resistive divider on the left gradually increase as the probe voltage increases. It reminds me a little of a vernier scale where the marks line up one at a time, but in this case it's the voltages.