DM7441A BCD-to-Decimal Decoder - Schematic Problem - SOLVED + Extras - Page 1
Forum section: electronics / projects

author: GreggTech
author_rank: Contributor
timestamp: October 20, 2024, 07:10:19 pm
content: Hello all,I'm working on some discrete designs of quite a few 7400-series logic chips. One of them is the DM7441A BCD-to-decimal decoder, which is apparently designed to drive Nixie tubes. The datasheet (attached) shows the logic diagrams for the 41A and 141 decoders. I simulated both diagrams in Falstad (also attached) and the one for the 141 works great. Very clever with minimal gates. But the one for the 41A is giving me problems. Only about half of the decimal numbers are being decoded properly. I noticed in the diagram in the datasheet (also attached) that there's a wire connecting a few inputs with no connection to an output. Perhaps I messed up building the circuit, but it seems like there might be a mistake or two in the datasheet drawing. Feel free to take a look - thanks for your time!

author: Analog Kid
author_rank: Super Contributor
timestamp: October 20, 2024, 07:39:20 pm
content: Quote from: GreggTech on October 20, 2024, 07:10:19 pmI looked but couldn't find such a wire in that diagram; maybe I missed something? All inputs seem to flow properly to outputs.Could you maybe highlight the connection for us?

author: GreggTech
author_rank: Contributor
timestamp: October 20, 2024, 07:47:36 pm
content: Yes, here is the connection...

author: Benta
author_rank: Super Contributor
timestamp: October 20, 2024, 08:39:25 pm
content: Yeah.A wire is missing between the output of the second top inverter and your highlighted trace.

author: GreggTech
author_rank: Contributor
timestamp: October 20, 2024, 10:20:00 pm
content: Thanks Benta, I added the wire into the Falstad simulation and unfortunately the decoder still doesn't work for most numbers. Multiple outputs turning on in some cases. If you have the time to check out the simulation, I attached the file to my original post.

author: Benta
author_rank: Super Contributor
timestamp: October 20, 2024, 10:27:05 pm
content: Sorry, don't play with Falstad.

author: GreggTech
author_rank: Contributor
timestamp: October 20, 2024, 10:49:26 pm
content: It's fun to play around with

author: floobydust
author_rank: Super Contributor
timestamp: October 20, 2024, 11:24:21 pm
content: I'd guess it's a missing dot on the schematic. Try connect that line to (buffered) C.Or do the hard work and figure it out in truth tables, K-map.

author: GreggTech
author_rank: Contributor
timestamp: October 21, 2024, 12:26:01 am
content: Thanks floobydust! That fixed one of the problems. I was playing around with it and fixed a few others, and now the schematic is correct and working perfectly. I attached it here for anyone interested.

author: GreggTech
author_rank: Contributor
timestamp: October 23, 2024, 03:43:31 pm
content: For anyone interested in the discrete schematic, here it is. I may end up increasing the value of the pullup resistors to bring the current down a bit.

author: floobydust
author_rank: Super Contributor
timestamp: October 23, 2024, 08:26:57 pm
content: A diode-matrix is another way to do it. I'm not sure your Darlington is needed in each stage.https://hackaday.io/project/165757-discrete-bcd-to-7segment-decoder-using-transistorscheck out that breadboardhttps://www.nutsvolts.com/magazine/article/transistor_clock

author: GreggTech
author_rank: Contributor
timestamp: October 24, 2024, 02:22:17 am
content: Yep, certainly it can be done like that and the gates can be made simpler with less parts. That breadboard is a familiar sight. My focus though has been to use some approximation of the designs found in the actual 7400-series chips. The main difference is I replaced the multi-emitter input transistors with diodes and pullup resistors. I'm actually thinking of putting each type of gate and all the dual/triple/quad gate chips on their own PCBs with LED indicators for input/output and wiring them together like puzzle pieces that can be used to make lots of different projects.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 24, 2024, 03:21:03 am
content: Quote from: GreggTech on October 23, 2024, 03:43:31 pmThanks for that. I loves discrete projects like this!Any chance you could post a more legible schematic? Maybe not light on dark, and a bit larger so one can actually read it?I've built a (working) discrete 555. Someday--a Z80! Well, just kidding ... maybe ...

author: GreggTech
author_rank: Contributor
timestamp: October 24, 2024, 03:31:33 pm
content: Me too! Yes I'll post a higher quality screenshot when I get home later. Flooby mentioned not bothering with the Darlingtons - I may end up taking those out and simplifying it. This is really just a copy and paste of some gates I've done for other 7400 ICs, wired together to make the decoder.That's cool about the 555! Do you have a link to it? How accurate is the timing compared to the IC? A Z80 would be sweet! I know the 6502 has been done before. I've attempted a digital clock using a standard 32.768 Khz crystal and got a lot of the design done, but got stuck on the oscillator, which would only oscillate when I touched the crystal or around it.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 24, 2024, 08:28:03 pm
content: Unfortunately it was long enough ago that my source web page is now off the air (domain is up for sale).Most of what I find in a search are kits, likethis one from Evil Mad Scientist(love that name! they're also the ones who did the 6502, though some of us think they cheated a little by using SMD components).Here's the schematic I used:

author: GreggTech
author_rank: Contributor
timestamp: October 25, 2024, 11:09:38 am
content: Nice, that's right, Evil Mad Scientist. I remember seeing Dave assemble the 555 kit, or something like it. They did cheat, go big or go home.I attached two versions of the schematic - unfortunately this is the best I can do at the moment because it's so big and I only have them in Falstad. One version has the Darlingtons, one does not.

author: GreggTech
author_rank: Contributor
timestamp: October 25, 2024, 11:18:30 am
content: Actually I just realized the output is inverted on the simplified one, so add a transistor and also one of the resistors in each inverter pair is unnecessary.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 25, 2024, 07:56:49 pm
content: Thanks. Captured for possible later use.