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This was hellish to reverse engineer
🕑 Added 2023-09-21 01:36:59 +0000 UTCThis 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
Comments
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.
Jon (0ryn) Westgate
I've seen something very similar in an LED VU meter it had stacked transistors just like what you have there.
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.
Curtis Hoffmann
A circuit design so old, even BigClive can't trace it.
