A full H bridge built from four discrete transistors and no driver chip. Two NPN transistors form the low side of the bridge and two PNP transistors form the high side, with a DC motor wired across the two outputs on a screw terminal.
Build sheet and parts:
The trick is the cross coupling. Each low side transistor, when its input goes high, pulls its own output toward ground, and that falling output switches on the high side transistor on the diagonally opposite leg. So input one drives the motor one way and input two drives it the other way, with only two control signals.
Four flyback diodes, one across each transistor, catch the inductive kick from the motor winding, and a single series resistor on the supply keeps stall current in check.
This layout omits the per transistor base resistors to keep the board readable, so treat it as a routing study, not a finished driver. A real build needs base current limiting resistors on every transistor. The transistor pin order for a 2N3904 and a 2N3906 is emitter, base, collector in a TO-92, but package variants differ, so verify every pinout against the exact datasheet before you solder.
BOM: two NPN TO-92 transistors, two PNP TO-92 transistors, four axial flyback diodes, one axial resistor, a 2 pin power input, and two 2 pin screw terminals for the motor and the two inputs.
Built solo. Questions and feature ideas welcome in the comments.
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