old two digit LED with no 7seg driver IC... How did they do it? - Page 1
Forum section: electronics / beginners

author: dentaku
author_rank: Frequent Contributor
timestamp: July 10, 2014, 01:38:33 pm
content: I got this part from an old Cornell-Dubilier AM Transceiver Mark 20I can connect a 9V battery to it, turn the knob and the 7 segment LED switches from 1 to 40.The only IC on this thing is an MSM5807 - PLL Frequency Synthesizer.No BCD to 7 Segment driver IC or anything. The big rotary switch has 20 pins and seems to make a connection before it clicks.The switch is connected directly to the LED display so I'm guessing the switch is quite specially designed.How does this LED work?don't you hate it when you post something and realise you forgot the attachments after 15 people have already read the post

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 10, 2014, 02:10:00 pm
content: It's the switch, it controls the PLL and it controls the display.As you can see in the datasheet the divide-by-N in the PLL has only 8 inputs, so there are eonough pins left on the switch.And since these switches were used in almost all 40 channel CB tranceivers at that time they probably produced a million

author: dentaku
author_rank: Frequent Contributor
timestamp: July 10, 2014, 02:45:18 pm
content: Quote from: PA0PBZ on July 10, 2014, 02:10:00 pmNow that's a fancy switch

author: grumpydoc
author_rank: Super Contributor
timestamp: July 10, 2014, 03:02:31 pm
content: QuoteI'd have thought several concentric tracks which are arrranged to connect to the segments - eg the a segment (top horizontal) of the "ones" LED needs to be on in positions x2,x3,x5,x7,x8,x9 where x = 0,1,2,3 and so on for all of the segments. I wonder if it can be done with fewer than 14 outputs from the switch.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 10, 2014, 03:17:41 pm
content: Yep -the switch outputs a binary code for the synth, and the segment patterns for the display.This is from the days when mechanics were cheaper than silicon

author: London Lad
author_rank: Regular Contributor
timestamp: July 10, 2014, 03:29:35 pm
content: Quote from: dentaku on July 10, 2014, 01:38:33 pmYou seem to have answered your own question :-)

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 10, 2014, 03:54:40 pm
content: Quote from: grumpydoc on July 10, 2014, 03:02:31 pmI think they use 14 outputs, because for 40 channels you only need 6 outputs to the PLL.However, segment b on the ten's display can be hardwired on.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 10, 2014, 04:09:48 pm
content: Quote from: grumpydoc on July 10, 2014, 03:02:31 pmThe second digit is connected as you would expect with 7 pins connected to 7 resistors, another pin is ground.The first digit has four diodes connected to it and is a bit too complicated to bother figuring out because this is just a curiosity of course, it's not terribly useful in modern times other than to salvage that amazing turned metal knob. I wish I had two of these knobs instead of just one because they're expensive to buy new.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 10, 2014, 04:17:38 pm
content: By the way, the switch is capable of having 26 pins but this one only has 20 installed.

author: SL4P
author_rank: Super Contributor
timestamp: July 10, 2014, 11:47:40 pm
content: Quote from: dentaku on July 10, 2014, 04:17:38 pmAhh yes, I remember the days when I swapped the switches to get extra channels on my old 27MHz CB !Thanks for reminding me!!

author: dentaku
author_rank: Frequent Contributor
timestamp: July 11, 2014, 01:14:36 am
content: Quote from: SL4P on July 10, 2014, 11:47:40 pmAnd now this equipment gets put out on the sidewalk during the bi-annual large garbage pickup so I can salvage, switches, jacks, pots and nice old knobs from themSolid aluminium knobs can cost $5 each and up.