RC Car Motor Problem

[Chet T16]

I bought a cheap rc car for messing about with, i opened it up and inside it had an RX-2-G chip with the outputs connected to 2-H bridges controlling 2 motors, one for steering and the other for moving. I wasn't going to use the original controller so i removed the RX-2-G IC and connected an arduino+bluetooth module to where the outputs were - setting the relevant arduino pin high turns the  motor on in whichever direction.

Firstly, i know its not ideal but the arduino is powered from the existing three AA's in the car. When i tried to turn one of the motors on the voltage dropped enough for the bluetooth module to reset. I put a 1000uF cap across the batteries and that seemed to solve it. The motor for turning the wheels works fine but the motor for the driven wheels will not work, they will 'click' a little like they are trying to turn but it's like the arduino is resetting and it repeatedly clicks. I tried adding more caps in parallel (three in total) but it made no difference. If i remove the batteries and power the car from my bench supply set to 4.5V is works fine and also if i power it solely from the usb on the arduino it works fine.

Do i just need a bigger cap across the batteries? I would've thought the batteries would be able to recover quicker than the bench power supply but i guess not?

Also i'm not sure why the wiring for the motor is setup as in the picture, can someone explain it? (both motor connections are coiled around, hard to tell from the pic)

[SeanB]

You might try adding a shottky diode in series with the power feed to the micro, to decouple the transient from it. Add another 1000uF capacitor at the micro input along with the one on the battery.

The motor wiring is done for RF noise suppression, the choke reduces the noise fed back to the board from the motor brushes.

[Simon]

yes sounds like the power rail is highly unstable. the above suggestion is a good one. will the arduino work on 3.3V providing you account for that in your program and circuit ? I mean how much can the arduino tolerate ?

[Chet T16]

It's a 5v nano board with 16MHz crystal and i believe you can't run them at 3.3v and 16MHz.

I don't have any shottkys but i'll pick some up, in the mean time i'll add the other cap and see if it helps. I'll also so if i can get the scope to trigger and compare the differences with the battery v's the power supply.

Thanks for the replies

[markus_b]

As first attempt I'd feed the Arduino and Bluetooth module from a separate source. If it then starts to work reliable you can attempt to get them to work together. But I doubt it will work, the main motor is probably too powerful for the 3 AAs already and draws too much power to allow the Arduino to survive on the same power source. I don't think some capacitors will be enough to help, maybe if you use one of those 10 farad caps sold to car stereo freaks. But those are way bigger and heavier than three additional AAs or AAAs.

[Chet T16]

Hopefully you're not right! Its close to working and the arduino isn't completely resetting itself its having a mini hiccup so i'm hoping its just on the verge of working!

[KTP]

How low a drop does a schottky have?  0.2V or so?  This puts you at best case 4.3V to the arduino capacitor.

You need some way to steal 4.5V from the batteries for the arduino capacitor without the motor being able to grab it back...a "can't touch this" circuit.  The schottky kind of does this, but with the small drop.  I guess building a small DC DC is out of the question? 

[markus_b]

Another simple test you can try: Feed the motor from your lab supply and measure the current it draws when you hold the wheels (stall current). The current drawn when accelerating from standstill will be similar.

I bet it is several amps, more than the AAs can supply. This is your main problem and why I suggest to add a second supply for the Arduino.

[Zero999]

You should expect the battery voltage to drop to below 3V when the motor starts and not rise much above 3V when running continuously.

You need to add a boost converter to provide a properly regulated 5V supply.

[Chet T16]

Yep, well having had another look at it there isn't a hope of it running as is so i'm going getting one of those small smd boost convertors from ebay to stick in and see how it goes

[markus_b]

Did you try if works at all with a separate power supply for the Arduino ?

Just two wires from you lab supply are sufficient for a test.

[Chet T16]

No, i didn't try power it seperately. Given that it works with the whole thing powered from the bench supply and the AA's did power the motors originally i couldn't see the point?

[markus_b]

I reread your original post and saw that you have evidence. What was the peak current draw you have seen, when running on the lab supply ?

What kind of switching regulator on ebay were you thinking of ?

I seem unable to find a 5V boost regulator quickly, but there are plenty of 7805 replacement regulators, but they will not work (buck, not boost).

[Chet T16]

This one, it was the cheapest at £2 delivered

http://www.ebay.co.uk/itm/251076896649?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

[Zero999]

This one, it was the cheapest at £2 delivered

http://www.ebay.co.uk/itm/251076896649?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I wonder what controller IC it uses?

Maybe it's just a blocking oscillator? I think you should measure the output voltage before connecting it to the rest of the circuit. It may not be regulated and give a much higher output voltage, when lightly loaded. The cheapest solution is often not the best. Good luck.

[markus_b]

This one, it was the cheapest at £2 delivered

http://www.ebay.co.uk/itm/251076896649?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I see, interesting item !

[Simon]

This one, it was the cheapest at £2 delivered

http://www.ebay.co.uk/itm/251076896649?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I see, interesting item !

the spec lower down in the listing says it is 5.5V output - be careful

[amyk]

This one, it was the cheapest at £2 delivered

http://www.ebay.co.uk/itm/251076896649?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I wonder what controller IC it uses?

Maybe it's just a blocking oscillator? I think you should measure the output voltage before connecting it to the rest of the circuit. It may not be regulated and give a much higher output voltage, when lightly loaded. The cheapest solution is often not the best. Good luck.

From the pictures it's a part labeled BA?19/8A?19. A quick search of "700mA 150kHz boost converter" shows tons of other sites are selling the exact same thing using the exact same pictures, and with the same incorrect spec ("non-isolated Buck" when it's clearly a boost converter.)

Edit: I think I found the schematic for something very similar.  http://akitora.com/pdf/LM2577_1.pdf (attached in case it disappears from the site, which is strange to me.) The labels, inductor, and diode match. The IC doesn't, however; its pinout matches e.g. iD8602, ML63, etc. I don't believe the 700mA rating though.

[markus_b]

I don't think your schematic matches. My main indication is that the LM2577 in your schematics has five pins, but the device on the PCB in the Ebay auction has only four pins.

[Chet T16]

Lets not forget that this is ebay so the item that arrives might be completely different anyway!

If it is regulated to 5.5V thats just within what the arduino will take directly, there is also an onboard regulator that will work for 6V

[Simon]

you need to check the arduino out. feeding it 5.5 volts knowingly from what may be an unstable supply is not a good idea. you can't put 5.5-6 volts through the onboard regulator and expect it to regulate, most likely you will get the same 5.5-6+ volts out and still do damage. maybe get a booster to 9 volts and feed that into the reg

[KTP]

you need to check the arduino out. feeding it 5.5 volts knowingly from what may be an unstable supply is not a good idea. you can't put 5.5-6 volts through the onboard regulator and expect it to regulate, most likely you will get the same 5.5-6+ volts out and still do damage. maybe get a booster to 9 volts and feed that into the reg

Just run the 5.5V through a diode.  The drop will be enough to get it safely in the range for the arduino...cheap solution.

[Simon]

possibly, try connecting a diode to a power supply and then measuring the voltage. I bet it won't be the supply voltage minus the diode drop. how much does the arduino draw ? will it be consistent ?

[Fallingwater]

What AAs are you using? You mention "the original" AAs and that you tried running the car on a bench supply set to 4.5V. This suggests to me that you're running the car on alkalines, or possibly even carbon-zinc. 
High-current devices like RC motors draw too much current for primary cells. You need three NiMH rechargeable AAs - those will deliver far more current before sagging. If the voltage of three NiMH AAs is insufficient to reliably run the arduino you'll need to use four (but try three first). Note that four fully-charged NiMH AAs are likely to provide more than 5V - possibly as much as 6V. This will go down to nominal voltage quickly under load, but all your circuitry must be able to withstand 6V if using four NiMH AAs.

If you're already running rechargeables feel free to ignore this.

[Chet T16]

The AA's are cheapo ones i bought, 14 for €1.50. It's a cheap RC with a small motor and was working fine originally. I tried adding another AA in series and it nearly worked, on a smooth surface it worked but on carpet the initial load was too high.

I'm still waiting on the boost to come.