Modifying a Cheap RC Boat/Boosting Engines

Discussion in 'The Projects Forum' started by IowanChef, Jun 9, 2010.

  1. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
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    So I've got my cousin in town and I'm trying to suck his brain of all his juicy robotics engineering knowledge. We settled for buying a cheap-ish ($30) "Donzi" boat with a dual motor hydro-propulsion type system. The board reads "New Bright 741A" if anyone's had one of these.. Anyway, we dropped it in the water and boy howdy is this thing slow slow slow. Clearly unacceptable, we tried boosting the voltage (transmitter runs on 2 AAs, boat on 3 AAs but we got better response with only two for some reason) but that didn't really give much back. So we started thinking, what about using the signals that controlled the motors (forward, reverse, left/right) as control voltage to a couple 9v batteries? The motors responded quite well after replacing the top caps (not capacitors, the actual plastic top on the motors. Turns out the connections were very bad/cheap/etc) with those from spare junked motors lying around, and we're thinking either using a bipolar op amp on either side or possibly transistors?

    Any opinions, advice, etc is welcome. Seems like the bipolar op amp is the way to go, but this is fairly new territory for me and he "sorta" remembers these days in college..

    Thanks, sages.
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Your 9V batteries won't do squat.

    Suggest you try 4, NiCd in series (approx 4.8V). You need low internal resistance in the battery. The 9V have high resistance. NiCd's are quite low.

    John
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    what about Li-on , john?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    The motors and batteries are the place to go, but don't boost the control electronics, lest something smoke. You'll probably need some transistors to boost the current to the motors.
     
  5. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
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    Thanks Bill! Yes, we certainly don't plan on running any more juice through the actual circuit, just using the roughly 3 volts that were initially getting to each of the two motors as a control voltage to swing the power from larger batteries to the motors. I'd considered using the same NPN circuit that you see in most color organ circuits, but that would only give us one direction of motion and I don't quite see how you could achieve the same effect with a PNP transistor.. Hex inverters came to mind but don't seem quite right.

    Relays would be adding more weight than needed, but the way these motors have reacted to taking 9 volts could mean additional battery weight might not be such a bad idea, lest we endo flip this thing..

    It puttered at a turtle's pace when we first dropped it in the pool, but after replacing the contacts to the motors we're already getting a decent boost, but maybe this bipolar op amp could work in swinging them to +/- 9v when the original +/- 3v signal comes out of the last stage of the circuit?

    Thanks again guys.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Look up H bridges, they are for dual directional control. They can be bought as RC components too.
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I considered several of the Li (e.g., Li-Po, Li-ion) chemistries and left them out because I wanted something that would be a "drop in" replacement, might increase performance a little, and not require motor controllers or complicated charging. LiPo batteries can be particularly finicky and dangerous. LiFePO (A123) are expensive and hard to find. Li-ion are not bad, but again hard to find. Also, the lithium batteries are nominally 3.2V. A single battery cell is less than 3 alkaline AA and would probably not improve performance; whereas,two lithium batteries might require some sort of regulator to keep from smoking the electronics.

    I have a little experience with a propulsion system that I think might be similar to the one used here. It was for a model airplane and/or blimp. The stick controlled each motor independently for steering. The motor controllers were integrated into the receiver. As Bill Marsden suggests, one could could take the motor supply from the receiver and amplify it, but that violated the keep it simple principle.

    So, I recommended NiCds as a quick, simple, drop-in solution that might improve performance a little. NiMH would be my second choice. They are readily available -- more so than NiCd's nowadays -- and might have sufficient discharge capability for this application.

    John
     
  8. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
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    The last stage of the output appears to be an H-bridge already, which is giving us the initial forward/reverse, left/right action. We just want to amplify the voltage..

    Are we on the same page here Bill? I've got the info for H-bridges here, but is there much point to adding one if it's already got full functionality and directional controls?

    Thanks again..
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    If you posted a schematic or high quality pictures (component side and solder side -- two seperate pics) and the motor model numbers. That might help those offering you advice on improving the performance.

    Do you have test equipment?
     
  10. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
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    I only have a multimeter for testing purposes. I've got a couple pics to show, hopefully they'll help. I'm thinking four of the six transistor-looking things are mosfets (reading 3P:J, and 5P :., respectively) and make up the H-bridge outputs, which are the two sets of terminals for the motors, the blue and brown cables coming out of the top side of the picture..

    Does this help?
     
  11. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    If it supplies two motors and runs forward and reverse, you need 8 transistors for dual H-bridges. There appear to be 8 on the board; two are separated by the hole. The PCB traces look about right for that too.

    The unit I mentioned from more than 10 years ago looks similar, but uses regular NPN and PNP transistors. They are driven by a Samsung KA2311 single chip receiver/driver. I suspect your board has been updated to use mosfets. Is the receiver a single chip device? Can you decipher its part number?

    The schematic was provided with the unit I got (rare). To increase motor voltage, you would only need to tap into the line at one spot. In your case, I would start with where the positive terminal of the battery connects and trace that line on the board. Red magic marker works well. If you are lucky, the supply to the mosfets will separate from the supply to the chip and other components.

    If going to a higher voltage, I would do it cautiously. Those little motors can draw a lot more current at higher voltage. My unit was designed to operate from 3V to 6V. A single additional AA will up you from 4.5 to 6.0V and would require no change to the board. If you do that, you should hear a noticeable increase in prop speed. First, though, are you sure the problem with speed is not just the propellers?

    John
     
  12. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
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    The chip driving the whole operation is some sort of proprietary chip (the markings beneath show holes for a chip 2 pins larger than the one they're using, as well as a couple empty component slots) and Googling the crap out of it produces no usable results. "New Bright R288-2" and "1127" with a copyright symbol and "2007." It's the only chip on the entire board.

    Is it really not as simple as using 3v to control 9v? It seems like all the hard work is done, we just want to amplify the signal coming out or use it to swing 9v +/-.
     
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