Awesome thanks for checking that out.

I got my chips today. I also fired up my PIC compiler. It won’t take much to add a needle sweep. Just a few lines of code with your data above.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Well I assembled my circuit and it didn’t work. I don’t get enough signal to trigger the NCV1124 chip. Very frustrating. It’s supposed to have the current source built in. I notice Techguy used a transistor to source current just like the original circuit.

I’m going to tinker around with it over the next few days. I think I got a little ahead of myself. I need to rebuild a breadboard of the circuit before going to a PCB so I can find out why it’s flat lining. I’m getting only 10mV of signal and that isn’t enough to trip the comparator on the ic.

Tomorrow is another day.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

So I've had to fire the guy that resoldered my amp. Turns out he was over confident in his abilities and couldn't be bothered to check for solder bridges.

The output was shorted to pin 14 which caused it to sit at ~2.5 volts instead letting the pull up bring it to VCC. Made the output cycle between 2.5 and 0 which the tach correctly ignored...

With that corrected, the tach now works. I'm going to hold off on potting it for now and will look into compounds that aren't such a pain to remove if it needs repair in the future.

I did measure both the voltage across the pickup and the current. Voltage was ~2.3 and current was about 27mA. The amp drew about 50mA at rest without the tach connected as displayed on my bench supply.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

Hey that is great that it was something fixable.

I’m still working on my amp design. I’ve actually been working on it everyday full time for the last three weeks. I’ve gone through three prototypes. They all performed poorly in a number of ways. Noise, choking out after a mile, losing signal at low rpm, quitting at higher rpms. I’m starting to get a lot of respect for Techguy512. I’ve restrained myself from posting because of my design’s ups and downs and daily changes. I had to actually go backwards a few times and you’d see what an effing idiot I am if I gave a blow by blow account.

Everyday I knock down one problem and improve my design but it changes significantly. Just saying I haven’t given up yet.

Looking ahead I’m not sure what I’m going to do with this. Selling a unit like tech-guy did can become a can of worms. Especially when the pickup, cabling and tach have to all function perfectly in advance. This thing has to be bullet proof because I don’t want a tech support or RMA job in retirement. But something in this thread made me feel I should stand up to this. Not to mention we are living in Covid hell and I needed a hobby.

More to come.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

I'm more of a fixer of broken things than a maker of new so my hat's off to you for your continued efforts. I'd have given up after the first few failures.

The boy's response when we went out on the mail run yesterday was "all the things move!" He's just learning to drive and should probably pay more attention to the road than the dash but I couldn't handle it not working.

Michael

__________________
Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

My prototype is finally working satisfactorily. I didn't want to say that up to now because I thought I'd jinx it. Also it literally is changing by the day. I'm pretty happy with the hardware and software. I basically took the description of operation of the LM1815 and condensed it down to a program running on a PIC Microcontroller (not shown in the image but goes in the socket). At the end of the day it will probably look a lot like Techguy's. I think he came to roughly the same solution.

Had a nice drive out to test this morning. Good operation in terms of response, minimal overshoot and noise immunity. Reliability is good.

I think the next step is to see if I can condense it all down on a poker chip sized board with SMT components. The pin locations are a little off but I think I can nudge them around by a few mil here and there on my pcb editor.




Installed in the car for testing.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

I have finished my Tachometer Amplifier design. I was able to condense it all down to a poker chip sized circuit board.

I used @oldpokey’s circuit diagram and @Techguy512’s idea to use a RISC microcontroller to do DSP on the signal rather than analog signal processing using a LM1815 like VDO. Hats off to those gentlemen. I really spent a lot of time researching everything they posted here. They really put in a lot of work and I want to recognize their hard work here.

Even with their posts I still took a little over a month to get to this point. The design was challenging in a number of ways. I also haven’t done any electronic design after retiring a decade ago. I was very burned out as an engineer and didn’t want to look at another line of code or circuit trace. This design reminded me of why I used to love engineering.

I looked at Techguy512’s design over the years. I didn’t think it was a big deal till I tried to make my own version of it. He was a very talented engineer and I enjoyed following his design process. I wouldn’t have taken this on if he hadn’t left us. But I realized from this thread there isn’t a new OEM unit available anymore and there is a gaping hole left to be filled. I hope my version stands up to his.

Refurbished units are a great solution. I personally picked all the silicone off my old VDO unit in 2010 and resoldered every joint with fresh solder and flux. It has lasted till today. My remix of Techguy512’s design is a stopgap effort for those who cannot find a working JY unit or don’t want to refurb their old solder. Both solutions are a big jump over a cigarrette butt. I tried that in 2010 and didn’t have lasting results.

Parts are still coming in. Tentatively I’d like to offer the unit up for the same price Techguy512 did to our members. Postage has gone up a bit but parts prices have dropped since 2015 so I think it’ll all balance out.

Enjoy the video.

https://youtu.be/8Coar9x9Sjw

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

nice one

Wonderful! Thanks for stepping in and providing this product. I'm sure you'll get lots of buyers as these cars become sought after classics. As more and more parts become NLA, it's becoming harder to keep our beloved machines going or in some cases, to even restore them. Your work is one small but substantial bit in the long journey to keep classic MB diesels going.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

Very nice, put my down for one when you are ready. Mine works but yours is pretty cool and having a spare wouldn't hurt.

Michael

__________________
Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

Thanks Michael. I have a bunch partially assembled on my bench. I’m waiting for some last components for the production models. I ran out of some of the passives while making the prototypes. They should be here in a day or so and I can complete the first run. I’ll PM you when I have something.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Nice job

I'd be interested in one as well. Will it be open source? Or will you provide source+schematics with an order? Or will none be provided?

Thanks. I’ll PM you when have shippable product.

The package will be a complete amplifier board and installation and troubleshooting instructions. No schematics or source.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Well, that was quick! I ordered on Saturday and they delivered it today (Wednesday)!

Looks good. Now I just need to get off my arse and begin the wiring. It is going into a 300CD which didn't come with a tach. I'm transferring a tach and sending unit from a parts car

Thank you for the speedy delivery!

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now