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 Forum index » DIY Hardware and Software » Circuit Bending
Circuit Bending Radioshack Rapmaster Keyboard
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johnwoo4545



Joined: Mar 15, 2010
Posts: 1
Location: Pennsylvania

PostPosted: Mon Mar 15, 2010 9:00 pm    Post subject: Circuit Bending Radioshack Rapmaster Keyboard
Subject description: How do I limit input voltage with a Potentiometer
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Hi all, This will be my very first post.

I have an old school Radio Shack Rapmaster Keyboard. When I'm running it off the batteries and they're almost out of juice, the sounds get real lo-fi / distorted, and I think it sounds awesome, esp. the beats. I want to add a knob into the DC power input circuit which will allow me to dial down the power voltage and achieve the same effect.

The DC power adapter input is 7.5V ( or 5 AA's )

Can I just add a certain value potentiometer into the power circuit? Is it that straight forward, or are there other components to consider.

Soldering is not a problem for me. I've worked as an assembly technician for 5 years soldering aerospace electronics, yet I don't know much about designing circuits.

Any help is much appreciated!
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Blue Hell
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Joined: Apr 03, 2004
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Location: The Netherlands, Enschede
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PostPosted: Tue Mar 16, 2010 10:13 am    Post subject: Reply with quote  Mark this post and the followings unread

welcome johnwoo4545


You could try a pot wired as a variable resistor (rheostat) in series with the batteries but it might melt down a bit depending on how much current the device uses ... a wire wound pot could be an option ... I guess something like 1 kOhm maybe.

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alienmeatsack



Joined: Mar 04, 2010
Posts: 137
Location: Oklahoma

PostPosted: Tue Mar 16, 2010 11:27 am    Post subject: Reply with quote  Mark this post and the followings unread

I've used potentiometers as voltage chokes/starves before on DC based projects with no problems.

Like Blue suggested, simply wire up a 1k or 5k pot inline from the battery to the device then tweak the knob and see what you get.

There are some issues with this, as starving the voltage with a pot is not the same as a dying battery's starve. The trick is to try smaller/larger pots to see what works best for your needs.

If you want to do a little more work and get close to the dying battery's effect on the unit, build a simple voltage regulator circuit instead to starve the unit of power.

Another option is to build a Pulse Width Modulator. This is a little different than simply choking off the voltage, as it just pulses it instead with full voltage. The speed of the pulses tricks the unit into thinking the voltage has dropped or risen and affects the unit. This kind of circuit could be used for example to change the speed of a DC motor. I don't know if the effect it has will be the same as starving the voltage, but it's worth trying if you are adventurous.

You'll find that the area of actual effective "starvation" is variable based on the amount of juice the batteries actually have, the size and specs of the pot, and the device itself's tolerance or lack there of for voltage less than what it thinks it needs.

One final method you can try involves literally removing a battery or two from the power side and adding a switch to do the bypass. If the device has a particular sweet spot for when it gets real messed up by the lack of voltage, it's possible that simply dropping the amount of power it is getting by this method will do the trick.

I hope this helps, and be sure to post pics and vids of your adventure!

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electri-fire



Joined: Jul 26, 2006
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Location: breda nl
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PostPosted: Tue Mar 16, 2010 11:54 am    Post subject: Voltage starve Reply with quote  Mark this post and the followings unread

In general, the exact amount of voltage starve you want can be VERY specific. Switch to another voltage regulator or shut down part of the batteries is probably too crude .You need a pot. Maybe even two pots, for crude and fine adjustment.

Grogberries uses even three pots:

http://www.experimentalistsanonymous.com/board/forums.html?topic=1688

Quote ( Grogberries ) "Normally I use a 10k pot and then mabye a 5k and a 25 ohm pot in series to tweak it when nessisary."

For lack of smaller value potmeters, the smallest value I've tried is 1K, and indeed had the experience of the usefull range being within a few degrees of pot turning.
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electri-fire



Joined: Jul 26, 2006
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PostPosted: Tue Mar 16, 2010 12:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
You could try a pot wired as a variable resistor (rheostat) in series with the batteries but it might melt down a bit depending on how much current the device uses ...


I've actually had a perfectly good potmeter ruined, it was crackling and smoking. While I could have added a small resistor in parallel with a potmeter to reduce the value I havn't persued the technique more often for lack of succes. Maybe I could give it another try some time.
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alienmeatsack



Joined: Mar 04, 2010
Posts: 137
Location: Oklahoma

PostPosted: Tue Mar 16, 2010 12:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

I like the idea of using several pots in series to fine tune the sweet spot. I hadn't thought of that! (Learning every day.)

One of my smaller bends I just found 1 place it worked best and used a regular resistor setup and a switch to get to that point. Unfortunately, as soon as the batteries dropped a bit in their power, that spot vanished.

It's tricky stuff.

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