I'm a self-taught/hobbyist electronic engineer, and I enjoy taking apart old scrap electronics both to salvage components and to try to study design patterns (and as a challenge!). I've recently extracted a nice 1.5" 4-digit 7-segment display from an old slot machine (I'd guess from the early 1990s), but I'm having a little trouble explaining how it's wired, and I'd love any information from more knowledgeable folks!
I attach photos of the unit in question:
- the markings on the LED digits themselves suggest they are OasisTek TOS-15102BE-1 (Common Anode, 2.0Vf).
- the anode pin(s) of each digit are wired to separate pins on the edge connector
- the segment pins are each wired first through a 1N4148 diode, and then to one of eight 3W 680Ω resistors (along with the same segment from the other digits), and then to a pin on the edge connector
- I have no datasheets, manuals, etc. and a Google search for the only markings on the board came up blank, but after poking around with a multimeter, I'm fairly happy the wiring is as shown in the schematic below which looks like a fairly standard multiplexing array. But....
My question is: what's the purpose of the 1N4148 diodes placed immediately after each LED segment? They don't seem to add any value to the functionality of the circuit, and all they do is drop voltage. I can successfully drive the display from an Arduino (using a MIC5891 high-side driver connected to the digit pins, and a TPIC6B595 on the low side to sink the appropriate semgents), but with the additional voltage loss across the diodes, and the fact I'm multiplexing, means I'm having to use a 24V power supply to get anything like reasonable brightness from the LEDs. Also, I notice the chunky resistors are getting quite warm when all segments are lit (although I'm quite pleased that the brightness between segments/digits seems nicely uniform). Or can you suggest a better way to operate this display? (to be clear, if I was starting a new design, I would do away with the multiplexing completely and just mount a TPIC6B595 to sink the segment cathodes per-digit, but I'm keen to try to use the digits as mounted, and also to try to understand the design decicion!)