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I understand that the Hex buffer will take a TTL input and sink or source a larger current and voltage on the output side, is there a common IC that does the reverse i.e. takes a 6,7,8 or more volt input and converts it to TTL on the output side, with some protection in between ? If switching speed is a priority is an opto isolator my best option ?

Not voltage typically, just current - for buffers and opto-isolators; sometimes you may find parts that will allow for switching slightly higher voltages (it depends on the transistor or mosfet outputs). Also note that many buffers are inverted - that is, you put a logic HIGH on the input, and get a logic LOW on the output. Something else to keep in mind (read the datasheets of the parts in question - always try to read the datasheet for any "active" part if you can - and passives in the cases of diodes or similar) is that a part may require that pins are pulled either high or low, and not left floating; leaving pins floating on certain parts can damage them (or cause the outputs to be wonky).

There are voltage level conversion chips that can take (most commonly) a TTL level (5 volts) and drop it down to TTL 3.3 volts; if you are just needed to drop the voltage to supply power to a part (say you have a 12 volt battery, and need to run a 5 volt motor), then using a voltage regulator (and if needing the current - a bypass transistor; or an adjustable switching regulator) to drop the voltage to run the motor (or circuit - or whatever) can be used. But these aren't typically used to switch voltage to the motor or circuit - you would typically do that -after- the regulator or whatever.

Opto-isolators are most useful for, well, isolating a circuit; say on one side of the circuit you have a beefy motor, with transistors switching a lot of current, and with possibly voltage spikes being introduced by switching. You don't want this getting to the logic side of things, because doing so might cause the logic to behave wonky, reset your microcontroller, or burn something out. So you use an opto-isolator.

Basically, all an opto-isolator is, is an LED sitting next to a photo-transistor. When the LED is lit, the photo-transistor is turned on (note - check the datasheet for the opto-isolator; some have current limit resistors already for the LED, some don't - if it doesn't then you need to check the current and voltage needed for the input/LED in the opto-isolator, then calculate using ohm's law the size of resistor needed to pass that current, so you don't burn out the LED); this photo-transistor then controls the load, or more generally switches a beefier transistor controlling the rest of the circuit.

The key, though, is that with the photo-transistor, you can have -completely- separate grounds and power inputs for the isolated circuit - so that no noise or voltage spikes will travel back into the logic control. Your logic remains safe from the other side (unless you don't pay attention to how you route wires or traces on the board, in which case induced voltages/currents can be set up and cause problems - typically very difficult to diagnose problems, as well).

Now - for this R/C car, you may or may not want the isolation; it seems like (at least in the datasheet on page 7) they already use a transistor for some amount of buffering on the circuit (parts Q4 and Q9 for the forward/reverse, and parts Q10 and Q15 for left/right); if something goes wonky, these would probably burn out first before anything happened to the Arduino. But - a buffer or similar might be a good part to add if you are paranoid. You should also check and make sure that the manufacturer of your car actually put in those transistors (do this by following the traces - a good way to do this is to make a nice scan of the bottom - which you have - then using a paint program to flood fill the traces from part to part, while looking at the top-side and comparing things; make a rough schematic while you're at it if you want - it could be helpful!).

Something else to notice on that example circuit in the datasheet (check out your car to see if it is used): Remember the turbo function (pin 12)? Notice how it is wired in the example circuit: it goes to a transistor (Q3), that then turns on the "forward" function (Q4 - which turns on the h-bridge transistors Q6 and Q7 - making the motor spin in the "forward" direction); this flow also turns on transistor Q2, which switches the full 12 volts (or whatever the upper voltage is) into the motor circuit (instead of the normal 9 volts which is tapped from the battery at the right) - giving you the "turbo boost" function; pretty neat, huh?

Also something else; check out how your car steers - based on what I can see on the circuit board, page 7 is your likely example circuit - but notice that the datasheet lists a very different circuit on page 8 (I am not sure how this is supposed to work, unless "left" and "right" go to pins 6 and 7 or something.

Another thing (always something with me - I should write a book!) - ultimately - once you get this thing working with "probing", and you start to hook in the Arduino, you are going to want to desolder the chip from the board (another reason not to solder to the chip) - this is so the chip doesn't use up any current (you want that to power the robot and the Arduino!), and also so no power is supplied to the radio receiver portion of the circuit (consuming more current, as well as possibly getting interference into the circuit - making your robot do strange things).

Finally - I want to point out that the page 7 example circuit shows the view of the IC from the top down (exactly as I described the pin numbering in my last post) - just look at that to verify things, ok?
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I have a board I ripped out of a very old phone to practice on smiley

Here is the new diagram




Also on the solder points are spots B and C acceptable or do I need to be going over the previous solder point like A?



You said "You want to solder to that resistor anyway to limit current to the transistor's base - whatever you do, don't solder to the base of the transistor, as you may end up drawing too much current and burn out either the resistor (or the Arduino's port)."

Where am i meant to put resistors and what value? or do you mean I should trace it out to the resister and solder to the resistor rather than one of the points off the chip?
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I have a board I ripped out of a very old phone to practice on smiley

That's good - practice on it; learn to desolder parts, solder them back in, and solder wires to points (hint: tin the end of the wire first, then solder the tinned wire to the point on the board - if the wire is stranded, after stripping the insulation, twist the strands up tight, then tin the wire end - it will soak it up fairly easily. Once it is cooled, snip off the very tip if there is any excess or extra strands sticking out).

You might also practice making solder bridges between close pads and cleaning them up - this is just something you need to learn, because you -will- make them, and understanding how to clean up a bridge (or make one!) is important. Bridges are made best when the pads are close together, but if the pads are too far apart, you may need to use a piece of thin wire (solid core works best). Surface tension is the key to all soldering; you heat the solder on the pad up (until it is shiny), and if the pad is close to another, the idea is to heat both together and "shove" them together so they form a joint bridge. Then let them cool (don't leave the heat on too long, or blow on the hot solder to cool it too quick, just let it naturally cool - otherwise you run a risk of a "cold joint", which is weak and may be electrically unsound. If you have to use a piece of wire, tin it first, then heat up the pads (you have to be quick - though sometimes you might just need to do one, then do the other, and let the heat flow thru the wire to keep the other end fluid), and shove the wire around until it fits and the solder flows over it, then let it cool. It will take some practice, but you'll get it.

Your first thing though is to learn to desolder parts and solder wires - because that is what you'll need to do on this PCB. You'll solder the wires first, probe around, then once you have everything situated (and label the wires!), you can desolder the IC; desoldering DIP ICs is tricky, though. You'll quickly run out of hands (if you have a vice or can improvise something, it will help). The best way is to use a combination of a solder sucker and desoldering braid to remove most of the solder off the pads (you won't be able to get it all). Then with a small screwdriver under one end, you carefully pry on the IC (not too much force!) while reheating the pins, and "pop" the pins off. Don't spend too much time on any one pad; if it ain't working, move on to the next, and let it cool down; you don't want to lift the pads if you can help it (but it will happen occasionally). If it really isn't working, the move of last resort is to use wire dikes to cut the pins from the IC body (this renders the IC useless, btw), then desolder the pins one-by-one from the board.

Here is the new diagram



That looks perfect!

Also on the solder points are spots B and C acceptable or do I need to be going over the previous solder point like A?



You want to solder on pads away from the main pads holding the IC on - because later you will desolder the IC, and you don't want your wires falling off, right? That means the one underneath "B" is most acceptable, and the one on the other end of "C" would be better. Only solder on to the IC pad if you have no other choice (like for instance, you are using a function that the pad represents that -wasn't- brought out on the board), or if you aren't desoldering the IC. You'll know when you need to do what.

You said "You want to solder to that resistor anyway to limit current to the transistor's base - whatever you do, don't solder to the base of the transistor, as you may end up drawing too much current and burn out either the resistor (or the Arduino's port)."

Where am i meant to put resistors and what value? or do you mean I should trace it out to the resister and solder to the resistor rather than one of the points off the chip?

Well - first try to locate the resistors; if you follow the traces from the pads for pins 6 & 7 (right/left) and 10 & 11 (backward/forward), you should end up at a resistor pin (if the manufacturer followed the reference diagram in the datasheet - or something like it). If you end up at a transistor pin, then the manufacturer was flying fast and loose, and you should add a resistor; use the values as given on the datasheet, which is 690 ohms (or something close if you don't have this value). Either way, you want to solder your wire to that pin, and not the IC pin (and remember, the further you can get away from the IC and closer to the h-bridge driver circuit, the better).

Also - once you have your wires in place, and you know their function (and you know they are soldered on good), put a dab of hot glue on and around the joint to provide some "strain relief" so they don't pull off as you experiment. Hot glue is non-conductive, so it won't be a problem.

Finally - may I use your photographs and such that you have posted here for use in an article I want to write up on my website? They are fairly clear, and would save me some time and effort. I think all of this that I have written could be useful to others in the future...
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Go for it on the photos and if you need any others for your story let me know. I am very appreciative of the help and if that can help you back in some small way I am happy to help!


So now I have the wires in the next step is take the IC out? Why am I taking the IC out? Because the arduino is going to replace it?

Once I have done that what next?
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Go for it on the photos and if you need any others for your story let me know. I am very appreciative of the help and if that can help you back in some small way I am happy to help!

I'll be sure to attribute the photos (and anything else necessary) to you, of course - thank you for your permission!

So now I have the wires in the next step is take the IC out? Why am I taking the IC out? Because the arduino is going to replace it?

Once I have done that what next?

No, the next step is the probing - but first we need to do some sanity checks - read thru all of the following, so you know what is coming up, then start:

1: Verify your soldering connections - make sure you didn't make any bridges, and that the soldering job looks good; if not - fix it first (if you are worried about your soldering job, post a clear sample picture of your work here first, and I'll let you know what I think).
2: You did label your wires? If not, label them!
3: Make sure all the wires are separated from each other (none touching!) and that none are touching anything else on the board.
4: Take your meter (you do have a meter? if not - GET ONE) - set it to voltage measurement (you want a range of 0-10 volts at least) - if your meter auto-ranges, that's ok.
5: Hook your ground probe to the GND wire (pin 2) - use alligator clips or something secure.
6: Hook your positive probe (make sure it is in the voltage measurement plug on the meter!) to the VDD wire (pin 13), again with alligator clips.
7: Turn on the car carefully - put on a stand so all four wheels are off your work surface, but can still turn freely (and steering still works).
8: What does your voltage read? Make note of the voltage!
9: While on the stand, try using the remote to verify everything is still working OK - and that your "stand" isn't interfering with anything.
10: Turn off the remote.
11: Remove the positive probe of your meter from the VDD wire; hook it up to the RIGHT wire (pin 6).
12: Turn on the remote. While watching the meter, send the signal to turn the wheels right - read the voltage, and note it (also note: sometimes manufacturers reverse the order of the wires - so left is right and right is left - if you don't get a reading, try turning the wheels with the remote the opposite way, and see if you get a reading then).
13: Move the positive probe of your meter to the LEFT wire (pin 7) and repeat your testing and notes.
14: Do the same for BACKWARD (pin 10), FORWARD (pin 11), and TURBO (pin 12 - if your car has this option) wires - note the voltages you read.
15: Hopefully - the voltages you read on each of these tests match the read you got from the VDD wire.

If it doesn't - STOP HERE - and let me know what readings you got. Do not proceed. Proceed only if your readings are within 5-10 percent of VDD (there will always be some internal losses in an IC). If the voltages are the same - it means you can use the VDD wire (pin 13) as a probe wire. When probing, only touch it to the wires I have indicated, and not to any others (or any parts on the PCB).

16: Disconnect all the wires from your meter, and from the alligator clips (or whatever you used).
17: Again, make sure all the wires are separated from each other (none touching!) and that none are touching anything else on the board.
18: A good sanity check is in order - try your remote again, then turn it off if everything looks normal.
19: Now - take the VDD wire (from pin 13) - this is your probe wire.
20: Briefly touch it to the RIGHT wire (from pin  6) - did the front wheels angle right? Did they move at all? If not - do they still move with the remote? Do they return to center when you disconnect it?
21: Briefly touch it to the LEFT wire (from pin 7) - how about that?
22: Briefly touch it to the BACKWARD wire (from pin 10) - do the rear wheels spin in reverse?
23: Briefly touch it to the FORWARD wire (from pin 11) - do the rear wheels spin forward?
24: Briefly touch it to the TURBO wire (from pin 12) - do the rear wheels spin forward faster (note: if your car doesn't have a turbo mode, this may do nothing, or do the same as step 23)?

That should be it - if at steps 20-23 (and possibly 24) you don't get an action, try the remote (remember to switch it off afterward). If at any of the steps, you get the "opposite" action of what the wire is marked, it just means the manufacturer has reversed the pins (they also did the same on the transmitter - so no big deal) - just swap your labels around. If you don't get an action from your probe testing - but still do from the remote - that will be odd, and I am not sure what to tell you. If you don't get any action from the remote or the probe, that will be double odd (I don't make any guarantees on any of this - welcome to the joys of hacking).

Once you have performed the above steps, and know this is working OK - you can then proceed to the next step. But let's take this a bit at a time, ok? In these kinds of hack jobs, it is best to take it slow and easy - no rush - otherwise you make a mistake, and POOF - the magic smoke is let out of something.

Good luck!

smiley
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Hi Cr0sh, Destined,

I just wanted to point out to avoid confusion that while I have been asking questions in this thread, I am not the original poster. I trust this is not a problem for anyone as the questions are relevant and also relate to a radio controlled car.

Just to clarify why I am looking for an input buffer, my receiver operates on around 6v, at present I have this connected to an Arduino interrupt through a 1K resistor. It works very well, noise from the FETs and Motor is not a problem however I am concerned about putting 6V into an Arduino Pin. I did try a quick and dirty work around with a series diode to drop some of the voltage but would like a cleaner long term solution.

In looking through the 7400 Series ICs, I see many buffers, some that have protection in the form of built in voltage clamping diodes, but they all seem to be designed to buffer demanding outputs rather than buffer/protect inputs, is this because the opto-isolator is the preferred solution in this application ?

Thanks

Duane.




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Hi Cr0sh, Destined,

I just wanted to point out to avoid confusion that while I have been asking questions in this thread, I am not the original poster. I trust this is not a problem for anyone as the questions are relevant and also relate to a radio controlled car.

Actually - a separate thread might be better for all...


Just to clarify why I am looking for an input buffer, my receiver operates on around 6v, at present I have this connected to an Arduino interrupt through a 1K resistor. It works very well, noise from the FETs and Motor is not a problem however I am concerned about putting 6V into an Arduino Pin. I did try a quick and dirty work around with a series diode to drop some of the voltage but would like a cleaner long term solution.

Hmm - did you measure this output from the receiver, and actually see that it was 6 volts? If so, no - you don't want to do that, even if it looks OK (indeed, if this has been this way for long, you may have already damaged your Arduino...). Instead, what you might try is to set up a voltage divider using a trimmer potentiometer, adjust that for 5 volts maximum on the final output (wiper is final output to Arduino; other two legs go to ground and your output from the receiver).

In looking through the 7400 Series ICs, I see many buffers, some that have protection in the form of built in voltage clamping diodes, but they all seem to be designed to buffer demanding outputs rather than buffer/protect inputs, is this because the opto-isolator is the preferred solution in this application ?

Your concern here is voltage - not current, because the Arduino will take what it takes on an input, which isn't much BTW. So - you just need something that will take at least 6 volts, but its output remain in the 5V TTL level while supplying say 10-20 mA for the Arduino's input. If there is a buffer IC that will do this, use it. If you want to use an opto-isolator, use it too (but normally, and opto-isolator is meant to isolate completely one circuit from another - especially where there is the chance of high-voltage kick-back spikes occurring; say when you are switching large inductive loads using a microcontroller or similar).
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Hey DuaneB I don't mind at all if you post in this, but you might get better answers in your own (however you seem to of got pretty good answers in this one smiley ).


I have moved through all the steps. My soldering was better than i expected smiley. I managed to find a black and red wire and using blue for the controls.


The car doesn't appear to have a turbo. Looking at the bottom of the board, the point doesn't appear to go anywhere so this would make sense.

Forward/back/left/right all work and the left and right flick back into position when I remove the probe.

I love the idea of labelling the wires, i haven't done it but I am going to buy some stickers to fold around them because it will really suit my purposes in the long run!


So what should I (we) do next? (by the way I almost have my arduino running with a wifi shield, so looking forward to controlling it from my phone!).



kind of an aside cause we aren't there yet.
Will it be possible to run the motors slower? Will it be possible to turn the wheels in increments rather than just left or right? (I am guessing the answer to the second is definitly no). Will I be able to power the arduino from the cars battery?

What sort of things should I be looking for to be able to do those things? (i bought the cheapest truck i could find in order to test, but if this works out I plan to spend more to get one which can do those things).

Again thank you for your time and patience. You are giving me way more help than I ever expected to get.

« Last Edit: January 16, 2012, 03:02:59 am by Destined » Logged

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Hi,
   To get back on topic, you should be able to control the speed of the motor with the Arduino. Here is a nice set of experiments that Oomlout.co.uk provide with their Arduino Starter Kit, it also has a servo which would give you the proportional steering.

http://oomlout.com/products/ARDX/ARDX-experimenters-guide-WEB.pdf

See page 12 for controlling a toy motor of the type in your car and page 14 for controlling a servo which would give you the proportional steering. I am assuming your car doesn't have proportional steering so you would need to buy a small servo from somewhere.

Duane B

http://rcarduino.blogspot.com/



 
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Thanks for the link. I am hoping to find an RC car which does this I can mod so I use the base of car (and build a fibreglass covering to make it look like what i want).


Is the next step to take the move/turn wires and put them into the digital outs of the arduino and set them high and low to switch them on and off? Or will that blow up my arduino?
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Hey DuaneB I don't mind at all if you post in this, but you might get better answers in your own (however you seem to of got pretty good answers in this one smiley ).


I have moved through all the steps. My soldering was better than i expected smiley. I managed to find a black and red wire and using blue for the controls.


The car doesn't appear to have a turbo. Looking at the bottom of the board, the point doesn't appear to go anywhere so this would make sense.

Forward/back/left/right all work and the left and right flick back into position when I remove the probe.

I love the idea of labelling the wires, i haven't done it but I am going to buy some stickers to fold around them because it will really suit my purposes in the long run!

Great! I'm glad everything is working! Regarding labeling of wires - they do make wire labeling kits with labels with numbers, letters, etc; you can find ones meant for home-wiring at Home Depot or other big-box DIY stores, but they aren't as useful for small electronics. Fry's Electronics (if you are in the USA and have one in you area) carries labels meant more for general electronics. You can also find such labels online. Then again, you can probably get by with a roll of masking tape, or other tape you can write on.

So what should I (we) do next? (by the way I almost have my arduino running with a wifi shield, so looking forward to controlling it from my phone!).

Well - the next thing is to remove the receiver IC from the board (so you don't get spurious signals from other transmitters, and so the chip doesn't consume power you need for your robot and/or Arduino). Ideally, you would also want to sever any power connections to the RF circuit that feeds the chip (doing this, though, will require some review of the PCB along with a schematic, which you would have to create - to know what parts or traces to cut to remove power from the RF circuit - the circuit, though, will probably follow the general idea of the reference design in the datasheet, so that can help you understand the traces).

After you remove the IC, you should still have the wires in place (btw - did you note what voltage the outputs from the chip were - you never mentioned it?)...

Assuming the voltages from the outputs are TTL-equivalent - you should then be able to hook your ground wire to the Arduino's ground, and the control wires up to various digital pins on the Arduino (be sure to hook the drive motor's wires up to PWM pins - so you can control the speed!). Then in your code, just perform digitalWrite() for the steering controls and analogWrite() for the PWM to the drive motor.

One thing to note: DO NOT BRING BOTH PINS/WIRES ON EITHER H-BRIDGE HIGH AT THE SAME TIME

In other words - don't activate the LEFT and the RIGHT wires (or FORWARD/BACKWARD wires) at the same time (logic HIGH); these h-bridges have -no- protection for this (IIRC) - it will short out the h-bridge (and the magic smoke will be let out). Make sure in setup() to set the pins as outputs, and set their states to LOW.

kind of an aside cause we aren't there yet.
Will it be possible to run the motors slower? Will it be possible to turn the wheels in increments rather than just left or right? (I am guessing the answer to the second is definitly no). Will I be able to power the arduino from the cars battery?

As noted before, you can use PWM to control the drive motor; for steering, left and right is likely all you have - replacement with a servo might be possible as other's have noted. As far as powering the Arduino from the car's battery - that depends on the battery's specs - care to share 'em?

What sort of things should I be looking for to be able to do those things? (i bought the cheapest truck i could find in order to test, but if this works out I plan to spend more to get one which can do those things).

Well, for proportional control (which is what it's called), you're unlikely to find it in a cheap vehicle; you'll want to look into a true R/C vehicle - be prepared to spend a bit of money, though. The nice thing it, though, with such a vehicle you can easily control it with the software Servo library.

Again thank you for your time and patience. You are giving me way more help than I ever expected to get.

You're welcome; this kind of question comes up often enough, though, that an article on my website I could link to, with all the detail, would be a better solution...

smiley



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I am prepared to spend if i can get it going. Actually i might go down the hobby store and look and ask some questions. I am from sunny Australia so most of the stuff I have to get online or improvise (actually improvise a lot because I want to get this working so I can justify the purchase of more. )

The battery on this car is a 9.6 volt, 750mA NiCd rechargeable. Honestly I am amazed the car came with it at the price. I did measure the voltage, but I can't see it in my notes which probably means I wrote it on a post it note(yeah smart I know)(edit: about 3.5, but it was really hard to measure cause I just had to stick the multimeter in while holding it on which is hard to do without a third hand. I figured holding control in palm still allowed me to hold the probe off multimeter), so I will look it up. Could you explain what TTL equivlent is? (I would rather understand than just do). Currently I am using a 9 volt clip battery for arduino, but I have read the wifi shield I have needs a fair amount of juice and I plan to hook an array of sensors up to it.

I currently don't have a solder sucker, although i might be able to find one to borrow. Is it possible to do it without the sucker?

How do I sever the antenna? It is clearly marked on the board so easy to find.

I already figured I can't turn back and forward on and will write my if statements so it always switches the other one off before starting as a failsafe.
« Last Edit: January 16, 2012, 08:29:23 pm by Destined » Logged

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Ok success.

I can now control forward/back/left/right  from the arduino, which is what i set out to do.

However when running the PWM pin i run into problems.

running fast like
analogWrite(forwardPin,250); //I am using pin 9
works fine.

Running say 50% like
analogWrite(forwardPin,125);
Works for a while, but then a high pitch tone comes and the motor starts to slow to a stop. I just turn it back to full speed cause I don't want to break it. Does this mean PWM isn't suitable for this motor?


Do I need the probe wire from VDD for anything now?

Note: I haven't taken the IC or antenna out yet, I still wanted the car to be able to run that way for now. So I don't know if the IC is part of the problem.
« Last Edit: January 16, 2012, 09:06:38 pm by Destined » Logged

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Ok success.

I can now control forward/back/left/right  from the arduino, which is what i set out to do.

Congrats! smiley-grin

However when running the PWM pin i run into problems.

running fast like
analogWrite(forwardPin,250); //I am using pin 9
works fine.

Running say 50% like
analogWrite(forwardPin,125);
Works for a while, but then a high pitch tone comes and the motor starts to slow to a stop. I just turn it back to full speed cause I don't want to break it. Does this mean PWM isn't suitable for this motor?

Strange; what happens at say 75% (192 or so)? I would say it is likely due to the h-bridge driver, though I find this very odd (btw - is this after you have removed the chip?).

Do I need the probe wire from VDD for anything now?

No - desolder it...

Note: I haven't taken the IC or antenna out yet, I still wanted the car to be able to run that way for now. So I don't know if the IC is part of the problem.

It very well could be! If you don't want to go to the trouble of desoldering the IC completely, just cut the traces (or jumpers) between the IC and your control line points (alternatively, cut the trace going to VDD - this will "turn off" the IC only); but it is best to remove the IC or cut the traces/jumpers between your control wires and the IC.

The reason why could be any number of things, but you have to remember, you are applying a voltage to an output pin on the IC, and it may not like that! There could be some weird short being set up, or the IC might be doing something weird (activating the other output is a possibility - in other words, you PWM forward, and the IC gets confused and starts triggering the reverse pin at the same time).

Try getting that IC out of the loop, so to speak - then retry the PWM stuff.

smiley
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I actually should of said running it at about 30%. Running at 50% seems okay, and 75% is definitly ok. I will report back once i get the ic out.

I will take the IC out since I really have no use for the remote anymore since I can control it from the ipad now.

How do I "cut the trace going to VDD"? Do you just score the board with a blade or something?
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