Schematics Semi Lunacy PCB | Electronic Circuits

Schematics Semi Lunacy PCB

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Well – the “virtual organ” project continues...

You might remember that I’m building some aspects of a physical interface for a “Hammond-style” tonewheel organ emulation (currently using the “Organzied Trio” VST plug-in). The first and most important part of the interface is undoubtedly the drawbars, but I’ve cracked that. Next on my list of “must-have” features is another iconic aspect of old Hammonds: the Leslie switch. Let me explain...

Hammond organs traditionally have been paired with “Leslie” loudspeakers within which sound is passed through rotating horns (or “baffles” for the lower frequency components). This system exploits Doppler shifts to generate interesting comb-filtering effects, the rate-of-change of which (corresponding to the speed of the rotating elements of the Leslie speaker) is controlled by a two (or three) position switch. In later organs, this switch was integrated within the console. However, perhaps as result of alleged antipathy between the Hammond and Leslie companies, early organs had an external switch fixed to the lower left side of the console, where the player conveniently could control it with her/his left hand.

That switch was mounted in a “half-moon” shaped case; the “semi lunacy” of our title is part of Hammond iconography (so much so there’s a crowd in the US trading as Halfmoon Electronics) and not another cheesy astronomical link to mark yesterday's lunar perigee.

Here’s one, mounted on a B3. Fortunately (for visibility) this one is white – more usually they were a sombre brown (the originals were made of Bakelite) or black.


I decided – in my lunacy – to make a half-moon enclosure for the switch I got from the good people at Warman Guitars (good prices, great service and a sense of humour).

I hacked a piece off some PVC 200mm soil pipe, glued on a “back” made of Perspex and roughed out a “top” from fibreglass PCB material (in which I cut the slot using a dental burr in my old horizontal milling machine).


Some more epoxy and fettling had the case finished...


The white “brackets” on the back are the "nylon" corner blocks that have replaced joinery skills in cheap modern furniture.

Here’s the whole shooting match installed on the temporary stand on which I'm assembling the "organ"...


All we need now is a lick of paint.

The switch grounds one of two PIC I/O lines which have pull-up resistors. The code reads these two I/O lines and sends MIDI commands to set the speed of the "Leslie" simulation accordingly. The switch currently selects between "Chorale" (slow; switch left) or "Tremolo" (fast; switch right). I've left the central position inactive (it should stop the Leslie rotors completely, but the Organized Trio software makes an irritating glitch when you de-select the Leslie simulation, so I've left it out).

Next step is the pedals – I’ve just won an old wrecker Hammond on eBay, from which I’ll rob out the pedal assembly and, perhaps, a few other bits. These will be converted to MIDI operation after even more coding for the PIC16F873.

Watch this space!

...-.- de m0xpd

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