Upgrading from small motor

Let's say I have a motor control circuit that's too small. It could be the motor output of a Picaxe, the adafruit motorshield for arduino or a the innards of an old RC toy. The main thing is that it has two output wires, both capable of delivering positive and negative to make a motor turn both ways.

What if you want to use a bigger motor that needs more power? Below are three circuit diagrams

The first is just a representation of the existing circuit highlighting the old contraption in a red frame

The second diagram show a simple circuit connected to the motor output. This will allow a higher voltage to drive a bigger motor. The Darlington Power Transistor will act as a switch controlled by the old/small motor power, allowing the big battery power to connect to the big motor.. New circuitry framed in blue. I know this circuit works in theory, but it will only allow the motor to be controlled in one direction. So your robot will only move forward, not in reverse...

The third diagram has an even larger circuit framed in green. Will this make the big motor turn both ways? Thus mirroring the behaviour of the small motor?

THAT'S what I really wanna know. Any ideas anyone?

Sorry mate, the 2nd circuit

Sorry mate, the 2nd circuit works, but the 3rd won’t do you much good. The motor in the 3rd circuit will only turn on when both transistors are active, and it’ll still only go in one direction.
You’ll need an H-bridge using at least 4 transistors, or a similar circuit to the 2nd one, but with a relay to switch the current direction.

Another thing to note is that the 2N6282 has quite a high Vbe voltage, so 500Ω may be a bit high for the base resistor (depending on the voltage your small motor controller can supply).

So a full H-bridge like this

So a full H-bridge like this one is the only solution?


reverse logic

Seems to me that you are reverse engineering some H-bridge logic. Here’s the simplest situation using an H-bridge in the classic way (no Enable, no Coasting or Braking):


mcu thinks
mcu says
H-bridge does


Now check the column under pinA. That would be the output of your toy on one of the motor leads. It takes three different states: 0, + or -. And each state represents an action: stand, forward or reverse (respectively).

All we need now is a (simple?) circuit that interprets signals Zero, Plus or Negative and produces new output signals to feed into your big H-bridge. Seems to me that must exist already, since some programmers use the tri-state abilities of MCU pins to send signals out.

It’s not the only solution,
It’s not the only solution, but it’s the simplest all-transistor solution. Although you’ll definitely need to reduce the base resistor values, and the 1st and 4th transistor need to be flipped over (reverse the collector and emitter connections).