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Modwheel/Joystick dead zone
 
 
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Author Modwheel/Joystick dead zone
whoop_john
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Joined: 17 Mar 2013
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PostPosted: Fri Nov 28, 2014 11:42 am    Post subject: Modwheel/Joystick dead zone Reply with quote Add User to Ignore List

Forgive me if this has been done to death, but I want to design a modwheel and joystick assembly with a dead zone, so there is no pitch variance on return. An op-amp buffered voltage is what I am after. I could do it in software, but it seems overkill.

I think this is possible with back to back diodes, but could someone run me by the circuit explanation and if possible some example circuitry and how to set it up. I think it must have to do with the 0.6v typical voltage drop across a diode.

Thank you in anticipation.
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BananaPlug
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PostPosted: Fri Nov 28, 2014 12:00 pm    Post subject: Reply with quote Add User to Ignore List

You might want to look at the discussion of diodes in this post...
http://www.muffwiggler.com/forum/viewtopic.php?t=126272

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whoop_john
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PostPosted: Fri Nov 28, 2014 12:32 pm    Post subject: Reply with quote Add User to Ignore List

BananaPlug wrote:
You might want to look at the discussion of diodes in this post...
http://www.muffwiggler.com/forum/viewtopic.php?t=126272

OK thanks. I have seen that yellow Minimoog circuit before, but I am still not sure how it does what it does. I'll breadboard it and have a look.
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daverj
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PostPosted: Fri Nov 28, 2014 4:29 pm    Post subject: Reply with quote Add User to Ignore List

whoop_john wrote:
BananaPlug wrote:
You might want to look at the discussion of diodes in this post...
http://www.muffwiggler.com/forum/viewtopic.php?t=126272

OK thanks. I have seen that yellow Minimoog circuit before, but I am still not sure how it does what it does. I'll breadboard it and have a look.


The trimpot defines a voltage. When the modwheel is within one diode voltage drop of that trimpot voltage the modwheel won't change the output voltage. It has to go above or below that voltage enough to turn on one of the diodes in order for the output to change. So that gives it about a 1.2v ~ 1.3v dead band centered at the voltage set by the trimpot.

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whoop_john
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PostPosted: Fri Nov 28, 2014 6:37 pm    Post subject: Reply with quote Add User to Ignore List

Many thanks for the explanation. It seems to be what I need.
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Dimitree
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PostPosted: Fri Nov 28, 2014 8:06 pm    Post subject: Reply with quote Add User to Ignore List

The value to look at when choosing the diode is forward voltage?
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daverj
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PostPosted: Sat Nov 29, 2014 4:52 pm    Post subject: Reply with quote Add User to Ignore List

Dimitree wrote:
The value to look at when choosing the diode is forward voltage?


Yes, but the Vf changes with the amount of current flowing through it, so it will start conducting at a smaller voltage than the specified forward voltage.

In general you use silicon diodes, like 1N4148 to get about a 0.6v forward voltage (1.2v dead zone), or Schottky diodes to get about a 0.2v forward voltage (0.4v dead zone).

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daverj
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PostPosted: Sat Nov 29, 2014 5:22 pm    Post subject: Reply with quote Add User to Ignore List

Also remember that a pitch wheel typically only rotates about 30%-40% of the full rotation of the pot. And that minimoog circuit is assuming that, plus assuming the value of the pitch wheel pot is several times lower value than the trimpot. (minimoog used a 25K pitch wheel)

The trimpot will cause the pitch pot to give a somewhat curved rather than linear voltage, sort of like the curves shown in "method #3" on that linked discussion. Somewhere between the curve I posted and the curve "guest" posted. But if a pitch wheel pot is used it will only be the center 1/3 or so of that curve. As the trimpot is adjusted off center, the curve gets stronger on one end and weaker on the other.

If looking for a dead spot at zero volts, and the main pot goes plus/minus, then just use a single resistor to ground rather than a trim pot.

You can eliminate the curve by putting a non-inverting amp between the pitch pot and the diodes.

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