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I'm got a RC car, which can go forward and backward and it's supplied by ~5V battery. At this moment the circuit looks like this:

enter image description here

and the board can swap + and GRN so the motor can rotate in both ways.

My goal:

How will look the circuit if I wanna accomplish something like this:

enter image description here

And replace the 5V battery with 9V battery ( and add a resistor before the electronics? )

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I see what you are trying to do... The schematics turn out not to be quite as simple as you have drawn though.

  1. Resistor between the battery and the PCB wont work since the current draw will likely vary a lot. Use a Linear Voltage Regulator, like the jelly-beans 7805.

  2. Simply adding two transistors in front of the motor will not do the job either. I am sorry to tell you, but you need a circuit called H-Bridge. Please read through it.


The PCB most likely already implements such structures. Perhaps you are better-off plugging the stronger battery directly in and seeing if the thing explodes. I'd say you have a 50% chance of success.

Note that regular 9V batteries have quite significant internal resistance, so it may not run better at all. Try a two-cell Li-Po battery. These are much better, but require some attention when charging.

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    \$\begingroup\$ Please use caution when adding Li-Po batteries to a circuit at above the rated voltage. Li-Pos typically specify a maximum discharge rate, and if that rate is exceeded in an unexpected dump of current due to a higher-than-specified voltage, "very bad things" can happen. \$\endgroup\$ – Scott Winder Oct 24 '13 at 21:42
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There is a couple of option you could use. You could power everything from a 9V battery. Connect your board via à voltage regulator like à LM7805. Connect your motor using pn2222a transistors, but if you want forward and reverse you will need an H-Bridge. Sorry I can't draw right now, but here is a nice tutorial for connecting a set-up similar to yours.

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If you want something simple to try, use a couple of relays powered from where the motor connects to at the moment: -

enter image description here

EDIT - My knowledge of RC cars is a bit limited to toys that are either full speed ahead or full speed reverse and it has been pointed out that if the car is a little more sophisticated it will control the voltage to the motor so that a range of speeds can be produced. This means that the relay idea will only work on simpler cars of the type that I'm more used to.

A and B are the connections to where your motor is at the moment. Depending on the polarity of A and B, either the left relay closes or the right relay closes. This is set by the diodes in series with the relay coils. The diodes across the relay coils are there to protect the circuitry that feeds A and B.

Without any relays being activated i.e. A and B both inactive, the motor doesn't get fed the 9V. If either relay operates the motor gets fed 9V left to right or 9V right to left. The action of swapping active relay reverses the motor as per a semiconductor H bridge.

Your relay coils need to be rated accordingly to what voltage your current system uses so try measuring this with a meter with a 100 ohm load to simulate the relay in place. Then choose a relay who's coil will work at this voltage.

BTW the relay contacts need to be rated to switch the motor current and they also need to be rated at at least 24V dc switching. To prolong relay contact like fit a 100nF capacitor across the motor.

This will give you an insight into how the H bridge works and if you want to go straight for the semiconductor version then good luck but take advise on ones you think may be suitable.

As for powering the receiver from 9V you ought to use a regulator like a Low Drop Out (LDO) version of the LM7805. This regulator circuit needs to go between the car on-off switch and the circuitry - don't put it before the switch (on the battery side) because after a day your battery will be discharged.

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    \$\begingroup\$ This will work only if the existing controller is a bang-bang controller -- full forward or full reverse, with no proportional control via PWM. \$\endgroup\$ – Dave Tweed Oct 24 '13 at 22:47
  • \$\begingroup\$ @DaveTweed Very true I'll point that out in my answer. \$\endgroup\$ – Andy aka Oct 24 '13 at 22:57
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What you need is an h-bridge, you can buy a full solution from ebay for about $5

http://www.ebay.com/itm/Dual-H-Bridge-DC-Stepper-Motor-Drive-Controller-Board-Module-L298N-for-arduino-/251080674810

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