How does Megadrum detect switches on cymbals?

[metalguy1981]

Hi All,

This is my first post as I cannot figure this out. I'm about to start building my Megadrum and understand just about all of it, apart from one detail: how exactly do Megadrum and other modules detect switch (edge/bell/choke) hits? I have observed that the ring of the hat input on my Roland TD20 is outputting 3 volts to the switch, but I don't quite understand what happens next.

On the schematic I notice that the ring of the jack coming from the hat goes to the 4051 multiplexer, but I fail to see how this provides power to the switch on my VH12 hat? As there is only one piezo on the VH11, how does the megadrum detect an edge hit? I understand that there is a separate threshold for switch hits, but what is this threshold detecting? Is it a difference in piezo output caused by the switch breaking circuit, or is it the output from the ring measuring a difference in voltage when the switch is activated? This has befuddled me!

if somebody would be kind enough to explain this simple concept to me, I'd be appreciative!

Thanks,


Dave

[dmitri]

In theory it is very simple.

1. A module outputs some voltage to the switch input.
2. A module reads voltage level from the switch input.
3. If the voltage level is above a threshold then the switch is open.
4. If the voltage level is below a threshold then the switch is closed.
5. If the switch is closed and there is no piezo signal from the head/bow input then it is just a switch press.
6. If the switch is closed and there is a piezo signal from the head/bow input at the same time the it is an rim/edge hit and a velocity of the hit is calculated from the signal of the head/bow piezo.

In practice, it is much more complicated.

[metalguy1981]

Thanks Dimitri! Just one last thing: how is the voltage getting to the switch from the Megadrum? I see no connection to a voltage source on the schematic

[dmitri]

In Atmega based MegaDrum it is Atmega itself which sources the voltage.

[metalguy1981]

Thanks for the info. Very enlightening!

[metalguy1981]

Last question, Dmitri: how come the switch input to the 4051 goes through the same diode and resistor circuitry that the piezos go through. Is there an advantage to this? Sorry to ask so many questions, but I really want to understand this.

Thanks for your time and patience,


Dave

[dmitri]

4051 is a bi-derectional multiplexer with 8 channels to 1. To understand what it does you really should start with http://en.wikipedia.org/wiki/Multiplexer