Let's Make Robots!

Omitting 1 battery

I had an inexpensive RC airplane.

I removed the motors, and replaced them by the attached circuit.  I replaced the motors with some 12V motors and This circuit is presently driving them.  I can control them so both run at once or the left or right independently.

I am however trying to figure out a way to get by with using only one battery.  I tried to tie them both together on one battery, but both of the motors would run and could not get them to run independently.

Is there a way to use a bridge or something to still power both by using the same battery?


schematic.gif2.57 MB

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meassure now with one wire of your meter on the +...

Now with my red lead of my meter on the + battery terminal, here are the results:

When the receiver is off and the black lead of my meter attached to either the blue, yellow, or 2 reds, I get 0V

When the receiver is on and the black lead of my meter attached to either the blue, yellow, or 2 reds, I also get 0V.

With the transmitter turned on and in neutral, and the receiver is on, I get 0V on each of the wires as well.

With the transmitter turned on and the stick that controls both motors is on, I get 0V on both reds, and I get 4.98V on both the blue and yellow wires.

With the transmitter turned on the the stick that controls each motor independently is on, I get 0V on both reds, whether I turn the stick to the right or left.  When I move the stick to the right I get 4.98V on the yellow wire and when I move the stick to the left I get 0V on the yellow wire.  Conversly, when I move the stick to the left I get 4.98V on the blue wire and when I move the stick to th right I get 0V on the blue wire.

OK. So it sounds to me like your receiver provides an active LOW signal on the blue and yellow wires. You cannot trigger an NPN transistor with an active LOW signal.

You do have some choices.

  1. You can replace the current NPN power transistors with PNP power transistors
  2. You can keep the current NPN power transistors, but get a pair of small general purpose PNP transistors to drive them.

Option 2 looks like this:

Just about any general purpose PNP transitors should be OK. You are just using them to switch the power NPN transistors.

The 10k pull-down resistors keep the NPN transistors off, unless the general purpose PNP transistors are turned on by the active LOW signals from the yellow or red wires.

Many thanks to ignoblegnome and Dannyv.  It uses 1 battery just like the schematic.  I used a pair of MPS 4126s. PNP general purpose transistors I had laying around, a diode across the motors and it works great.

Thanks for sticking with me.  I hope I can help sometime.  You guys are tops.

Glad to help. So, ah... what are you going to do with it now that it works?

Since my grandson crashed the cheap harbor freight airplane one too many times, we are going to work together since we salvaged the receiver, and put some wheels on some kind of 3 wheel device and keep the controlling on the ground instead of the air.

I have looked through the LMR website, and it is truely amazing the things that you guys come up with.

Again, thanks for sharing.

Nice it's working now. glad to help..

It's cool to share knowledge i think.. to help other people around the world.

@ Ignoblegnome: his next project will probably be to add some smoke to the airplane, and write LMR in the air :D THAT would be a cool video for youtube :)

I think there are no limits to you guys and your imagination.

I will look forward to see what you come up with next.


I totally agree with Ignoblegnome..

A diode parrallel over the motor is recommended.. placed with anode to -. When a DC-motor is turned off it could give some current back to the transistor (it's a coil) wich can kill the transistor. the diode should be fast and strong enough depending on your motor...

Thanks  again for the assistance.  This will be awsome to eliminate one battery.  I will post the results when I get it done.