Geoff Bunza SMA20 Low Cost 17 Channel DCC Decoders Ver 6.01

I had difficulty determining how this would work and have attempted to Fritz the decoder for a circuit for my dc to dcc Model train conversions and so others may see how it is made.

Having said that I may have this wrong and I suggest you read Geoff’s Blog here:

Geoff Bunza’s 17 Channel Multifunction Decoder

Hopefully the Fritzing is attached below.

Geoff Bunza 17 Channel Multifuntion Decoder.fzz (44.8 KB)

I am open for correction as I am only learning the basics of electronics and I learn from advice and practice.

Hi Daz. What do you need to understand about the operation of Geoff’s decoder? I just happened to see this today because I am working on something else for the DCC-EX documentation. We’ve put this circuit into KiKad and Geoff has the original Eagle files available. The schematic and breadboard look fine after a quick glance.


Hi Daz & Fred,

Daz look very closely at your schematic and board layout.
ProMini D10 is not connected to SN754410 pin 15.
Pin 15 connects to DTR ain the Progamming header (last pin).
And the Pro Mini schematic template does list D10 next to D2 it shows DTR where D10 would normally be. Also D11, D12, and D13 are omitted.
'Hope this helps.
Have fun! :slight_smile: Best regards,
Geoff Bunza

Hi Geoff

Thank you for the reply, I was struggling to get my head around the flow and I wanted to get a grip on the parts needed.

I have read through all the SMA’s from SMA10 to SMA20 and it has been an interesting journey.

I would like to use your decoder to convert my DC loco’s to DCC and I really only want to run forward / reverse and lights front and back.

I appreciate your great knowledge and sharing this with us and I will make the changes that you have suggested.



Hi Fred

Thank you for the reply and looking at my circuit, Geoff was kind enough to look at my circuit and provide the advice needed.

I am in the throws of getting things prepared for when I build my layout and the conversion to DCC which is a costly one for all my locos. I found that I can get all the parts for the circuit about $20aud cheaper than buying the commercial ones and being on a pension is a lot of savings.

Once I have this sorted my next step will be working with DCC-EX for my plans hopefully starting before or after Christmas.

Thank you again and we appreciate all the work you guys are doing with DCC-EX as it helps us non technical one keep up with the times.