I want to control a 4V motor to rotate both directions which draws 100mA and I was looking at the H-bridge schematics. The tutorial says that you should place NPN transistors at the bottom if you need >5V Power to the motor, which i dont need. This H-bridge also requires at least two inputs to switch direction. I also do not need to turn the motor off. It will run either or one direction, or the other.

I was wondering if I could put the NPN transistor at the top, so that I can control the motor with only one I/O pin (simplified schematic):


simulate this circuit – Schematic created using CircuitLab

Since I don't need >5V power for the motor, I don't need to put my NPN transistors at the bottom. Using this schematic I can also control the direction of the motor with only one I/O, without more external components.

Of course, the complete schematic will also include a resistor to limit the Base current and the fly back diodes (fly back diodes and Base resistor added):


simulate this circuit

When I/O is 5V, both NPN transistors turn on.

When I/O is 0V, both PNP transistors turn on.

Do you think this will work? I have not seen any tutorial/schematic with this H-bridge configuration.

  • 1
    \$\begingroup\$ This can easily be simulated in LTSpice. Or any of the additional free simulators. I do wonder if your MCU will be able to drive two transistors into saturation though. Simulation and datasheet inspection can answer that though. \$\endgroup\$
    – Aaron
    Commented Mar 3, 2021 at 16:08
  • \$\begingroup\$ Hm you are right! i should start using that LTSpice. \$\endgroup\$ Commented Mar 3, 2021 at 16:09
  • 3
    \$\begingroup\$ no good. the common emitter pair on the right side turn each other on. \$\endgroup\$
    – Pete W
    Commented Mar 3, 2021 at 16:10
  • 1
    \$\begingroup\$ @PeteW Right, the right side pair is just 2 diode junctions right across the supply, and the left side pair are emitter followers which can only pull down to a diode drop above ground or up to the base drive voltage minus a diode drop. The left side of the bridge could kind of work if the power dissipation in the transistors is OK. \$\endgroup\$
    – John D
    Commented Mar 3, 2021 at 16:16

1 Answer 1


As @Aaron stated, I should simulate this in LTspice.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.