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Old 7th July 2004, 04:36 AM   #1
Prune is offline Prune  Canada
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Default How do I drive volume motor?

I bought an 8V motor with gear box used to drive battlebots off eBay. The thing has enough torque to turn the stepped attenuator I made (barely). The problem is how to control it. I'm using both a remote control reciever and a manual knob. The reciever chip can definitely not provide enough current to drive the motor. Can anyone suggest a circuit to control the motor, with signal at input for one direction, and at the other input for the other direction? I have some spare IRF610 that I can use.

Also, since I want the knob for manual control, I need suggestion for a mechanical linkage to do that. I was thinking something like in the attached picture (the knob must control the motor as it's too hard to turn the gear box from the output side). The two pushbuttons in the linkage in the picture sense when the user is trying to turn the knob. I know it looks complicated, but I couldn't figure out any other way to do it. BTW where can I get gears? I don't have the tools to make my own.
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Old 7th July 2004, 12:43 PM   #2
Johnix is offline Johnix  France
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A nice way to drive a DC motor is 'H bridge'.

Look at the L293B or L293D (D coz including diodes) from STMicroelectronics. This chip accepts voltage levels up to 30Vdc and can deliver 600mA of current.

One pin for the direction and another one to activate or not the motor. Really easy to use.


It costs approx 2Ä
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Old 7th July 2004, 12:45 PM   #3
Johnix is offline Johnix  France
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An example using L293 :

http://www.me.umn.edu/courses/me2011...L293/L293.html

Datasheet :

http://www.st.com/stonline/books/pdf/docs/1328.pdf
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Old 7th July 2004, 01:15 PM   #4
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Cool Re: How do I drive volume motor?

Quote:
Originally posted by Prune
I bought an 8V motor with gear box used to drive battlebots off eBay. The thing has enough torque to turn the stepped attenuator I made (barely). The problem is how to control it. I'm using both a remote control reciever and a manual knob. The reciever chip can definitely not provide enough current to drive the motor. Can anyone suggest a circuit to control the motor, with signal at input for one direction, and at the other input for the other direction? I have some spare IRF610 that I can use.
I aggree that a "H-bridge" is the best way to drive the motor. You will have to find out how to interface this with your control(s).


Quote:

Also, since I want the knob for manual control, I need suggestion for a mechanical linkage to do that. I was thinking something like in the attached picture (the knob must control the motor as it's too hard to turn the gear box from the output side). The two pushbuttons in the linkage in the picture sense when the user is trying to turn the knob. I know it looks complicated, but I couldn't figure out any other way to do it. BTW where can I get gears? I don't have the tools to make my own.

Why not just a pair of pushbuttons (or a two way switch) on the faceplate? Remember, most times, K.I.S.S. works best.

With the control you have the above picture, I can see it starting to rotate in one direction then overshooting where you are holding the knob, reversing, and then repeating this as it "hunts" for the null point.

As far as gears, or other small parts, I'd look at small parts Inc. or any number of the surplus parts places on line.

Keep us posted and Good luck!

Tall Shadow
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Old 7th July 2004, 01:35 PM   #5
Magura is offline Magura  Denmark
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To get around the gear usually is done by implementing a limited slip clutch. Such can be obtained from Farnell or RS.

Magura
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Old 7th July 2004, 09:30 PM   #6
Prune is offline Prune  Canada
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I hope I can find an H-bridge IC locally. I'd build a discrete one, but I have only N channel FETs and I'd need a voltage doubler to drive the upper ones.

Tall Shadow, there's no way to know the position of the volume control if I use pushbuttons. There's no problem with overshoot; you misunderstood my diagram. The motor turns both the attenuator and the knob. The user turning the knob can just turn it enough to engage one of the switches, and then the motor turns it until the user releases it.

Due to the way the attenuator is used (in-between the balanced lines, this way I can use only two decks for balanced stereo), during a transition from one position to the other it will momentarily be at maximum volume. So I have to put a wheel and IR sensor to drive a muting relay. Talk about complicated volume control, LOL.
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Old 8th July 2004, 08:53 AM   #7
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surprise surpirise...a H bridge can also be implemented with standard bipolar transistors...would it be cheaper?? i dunno...but most of us have more bipolars in our part boxes...
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Old 9th July 2004, 02:34 PM   #8
Prune is offline Prune  Canada
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Well, the local store was sold out of L293 so I built a discrete MOSFET bridge, with small transistors to drive the FETs. Almost fried the semis, but it's working fine now. I'm gonna use an IR LED and sensor around a cardboard masking wheel to monitor for stops so the motor doesn't try to force the switch beyond its range, and another IR pair to mute between volume positions (during transition for a moment the volume will go to maximum due to the configuration).
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