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[on:]

Hi all. Compliments of the New Year to all.

Position : sons 7th birthday 24 Dec. He wanted a specific RC car, which he got. Not overly expensive, but Made in China all the same.

Problem was the radio. First the trigger broke ( like only a 7 yr old can do ). Fixed that.
Next the antenna got snapped in half - still worked.
Then trigger broke again in a different place. Fixed that.
Antenna got snapped again - apparently you only need a 5mm antenna anyway ( ask my son )
Then it gave up the ghost completely, and trigger broke again.
Checked the transmitter board, and the pot (?) that the trigger connects to, is not registering anything, and with the broken trigger, is virtually unrepairable.

To cut a long story short :

We have another cheap RC car that got stood on ( same 7 yr old ) and busted wheels. So I got the transmitter and receiver board from that - also runs on 6v DC, same as the battery pack in the newer car.  Found the contacts on the receiver board and connected the 2 motors ( steering and drive ). They have only 2 wires to each motor, and the power over the wires was either +6V or -6V, depending on either left / right, or forwards / backwards.

It worked great, and his face lit up and we each had a turn. Unfortunately, after about 4 minutes, it stopped, and I could smell the magic dust escaping from the receiver board. To make matters worse, guess who was driving when it escaped - me.

So he doesn't want another birthday present, or another car ( and no more of this model available ), he wants THIS one.

My thinking was that the cheap receiver board could not handle the current drawn by the bigger motor, and it fried / died after a few minutes of continuous use.

I do have another cheap board, and was thinking that I should use that, but instead of connecting the board outputs to the motors, connect them via relays or transistors that can manage the motors load from the battery to the motor, instead of pulling the current thru the board.

Can anyone please help me with a simple wiring diagram that would take a +6v / -6v feed, and trigger the motor ?  In stock I have some 5v relays, transistors, diodes and resistors.

All help greatly appreciated.
Regards

[Reply #1 on:]

You can do a search on "relay h-bridge", or take a look at the following ckt. It uses
one relay to switch motor direction and a MOSFET to turn the motr on/off, but that
could be replaced with a relay too. Pretty simple. You will probably need to use
transistor inverters to provide enough current drive the relay coils, and be sure to
wire reverse diodes across the relay coils to quench inductive spikes.

http://circuitbuilder.blogspot.com/2012/05/making-h-bridge-motor-driver.html

[Reply #2 on:]

So he doesn't want another birthday present, or another car ( and no more of this model available ), he wants THIS one.

I suggest you let him play with the toy like it is. This could be a learning moment that he should take more care with his gifts.

[Reply #3 on:]

Normally I would, but I am with him on this one - the trigger was a krappy design, and the arial wasn't completely his fault.

[Reply #4 on:]

Normally I would, but I am with him on this one - the trigger was a krappy design, and the arial wasn't completely his fault.

Problem was the radio. First the trigger broke ( like only a 7 yr old can do ). Fixed that.
Next the antenna got snapped in half - still worked.
Then trigger broke again in a different place. Fixed that.
Antenna got snapped again - apparently you only need a 5mm antenna anyway ( ask my son )
Then it gave up the ghost completely, and trigger broke again.

We have another cheap RC car that got stood on ( same 7 yr old ) and busted wheels.

Reality check!

[Reply #5 on:]

OK. So I have decided, mainly to satisfy my own curiosity, to try and get this RC car working again.

Step 1 was to take the transmitter and receiver board from an old cheap unused toy and put it in the car.
Wired it up and all worked great -- for about 5 minutes.
There was a tell-tale smell of something 'cooked' on the board.

Looking back, I think my error was the power for the car.
The old toy used 4 x AA batteries ( so max of around 5.5V ) and the 'new' car as a rechargable battery pack which, when charged, measures 6.5V.
I looked at the spec sheet for the transistors used on the receiver board ( C8050 and C8550 ) and from what I can understand, they 'expire' at 6V.

Next step was to get a second transmitter / receiver pair from another toy ( local toy shop had a sale, so got a cheapie spongebob car for less than $5 ).
Stripped out the receiver, and it has the same components ( C8050 and C8550 ).

So my thinking was to reduce the power to the board to around 5V ( so it almost matches the disposable batteries in the cheap toy ) and did this by adding a 1N4007 diode to the battery positive wire - thought it would also be a good thing to protect the battery pack if I got the wiring wrong.
Then have the 'output' from the board connected to some 5V relays, so the relays could channel the full battery voltage to the motors, without frying the components on the board.

Connected 2 relays for the left / right control, and 2 for the forward / reverse control.
Rigged up a series of 1N4007 diodes to prevent any damage to the relays / board, and tested.
Worked a bit erratic, and I found that the voltage drop on the diodes was reducing the power being fed to activate the relay coils. This proved to be an unusable solution.

Next, I figured that the 1N4007 diode on the positive wire from the battery was reducing the power sufficiently that it would be below the 6V self-destruct voltage that killed the previous transistors. So I removed the relays, kept the diode on the battery line, and powered the motors directly from the transistors on the receiver board ( just like it was in the cheap toy ).

This works and everything working smooth and as expected, except for the fact that the power from the battery is now reduced to around 5V and the motor is not running on full speed as it used to on the 6.5V

Any suggestions on how to get the 6.5V from the battery to the motors, without killing the transistors ?

[Reply #6 on:]

I think it would be easier to just buy some transistors which can handle the voltage and solder them in instead

[Reply #7 on:]

Expire ? That's a new expression for me regarding transistors.

Also Magic dust ? It's usually Magic Smoke that powers electronics, maybe yours runs on dust... interesting.

Your transistors can handle 25V, it's the Emitter-to-Base voltage that must not be over 6V.

// Per.

[Reply #8 on:]

Expire ? That's a new expression for me regarding transistors.

Also Magic dust ? It's usually Magic Smoke that powers electronics, maybe yours runs on dust... interesting.

Your transistors can handle 25V, it's the Emitter-to-Base voltage that must not be over 6V.

// Per.

I think that's your main problem right there.  You're running an old cheap Magic Dust™ receiver board on a Magic Smoke™ power source.  The two aren't compatible, and the Magic Smoke™ will force the Magic Dust™ out of the board.

[Reply #9 on:]

I think it would be easier to just buy some transistors which can handle the voltage and solder them in instead

OK. I thought about that, but the transistors are on the receiver board and supplying any 'more suitable' transistors with the 6.5V would surely mean supplying the board with the same 6.5V - and I don't really know enough to understand what that would do to the other components and the IC on the board, but my gut feeling tells me it is probably not a good idea.

Expire ? That's a new expression for me regarding transistors.
Also Magic dust ? It's usually Magic Smoke that powers electronics, maybe yours runs on dust... interesting.
Your transistors can handle 25V, it's the Emitter-to-Base voltage that must not be over 6V.

Expire / self-destruct / terminate / perish / give up the ghost / die / fail / pack up / break / melt / implode. Call it what you want, it just don't work

Magic Dust : well, you see, here in Africa things are always slightly different and unusual. I could consult the local witch doctor, but am not certain that I would get a definitive answer, and it would likely cost me a goat, or at least 3 chickens in fees. And since the 'dust' vanished almost as quickly as it appeared, I suppose that one could imagine that it had been smoke.

Again, I think that the problem is that the transistors are on the receiver board and therefore limited to the restrictions and limits of the boards components.