NuTone LD49N Door Chime – Part 1 – A first look

I am very easily side-tracked and for some time I have been interested in adding chimes to my various radio controlled clocks.  Westminster chimes  or variants thereof are often found in vintage mantel clocks and door chimes and also find enthusiasts in the hobby crowd for whom a micro-controller and some tuned pipes or chimes can develop into an interesting project.  I’ve mused around doing a project for some time and then stumbled across an EBay listing for a broken door chime (a 1950s? V & E Friedland Limited 8 note electric chime), quickly acquired for a low price and now stored safely for some point in the future to investigate further.

In the meantime I decided to do some research into door chimes generally and stumbled across a remarkable YouTube channel by Northside Sevices.  Northside clearly love all things NuTone including door chimes through the generations and the combination of loving owners keeping their vintage stuff alive and perhaps a new demand for retro chic will hopefully keep them very busy.  On their channel I came across a video showing the workings of several chimes across the ages, with early electro-mechanical types to more modern micro controlled solutions but one I found particularly interesting is the LD49N which is a simple design from discrete logic.  Working from the video I have been trying to work out the schematic… well I said I am easily distracted!

Essentially the circuit revolves around a 4017 decade counter. The decade counter has ten states 0,1,2,3,4,5,6,7,8 and 9.  0 is an idle state, 5 in a quiet state and then the solenoids which drive the chimes themselves are mapped to the other numbers.  Despite the name 8-note chime the actual device can only produce 4 different notes, but with each note played twice during the sequence… that gives eight notes.. (someone in marketing must have come up with that one!).

I haven’t completely worked it out but this is what I have so far…


Each solenoid is connected to a Pin on the board, each solenoid pin is driven by a power transistor (with a Diode to handle reverse EMF when the transistor turns off), the transistor is driven by two states of the decade counter.  I think there is additionally a “Solenoid Inhibit” which turns off the solenoid, presumably to ensure notes don’t run into each other.

So for example, Pin 1 is driven by states 1 and 8.

Pins 2 and 4 have an additional input which is driven by logic for the Side and Rear door inputs.

I’m still trying to work out the front end logic but it is all based on a hex Schmidt invert-er chip.  The Front door input enables an oscillator made up of the 2A/2Y and 3A/3Y inverters, the clock for the 4017 being derived from 2Y.  The oscillator will run until state 0 is reached (after state 9 or by a Reset at State 5 if 4-note only is enabled).  The 3Y output seems to drive the solenoid pulse duration.

The Side and Rear door inputs seem to drive chimes attached to Pin 2 and Pin 4.  It looks like the Rear input will cause two chimes to sound, whereas the Side door just drives the chime attached to Pin 2.FrontEndLogic

It’s early days yet and I will probably need some more input from Northside but it would be great if I can fully document this interesting circuit… and after all it keeps me from doing anything useful which is always a good thing!


About nivagswerdna

Professional Geek
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