do stepper motors need buffers??

I am investigating controlling stepper motors with the A-board.

I know that you can buy a stepper motor controller IC, but I want to do things the hard way ;)

So, now I am trying to control the stepper motor using a TTL 74194 shift counter, a hex inverter (for direction control) and a darlington transistor array. (don't be impressed! - I am just following the instructions in a book!)

Amazingly this seems to work, however, the book recommends using a hex buffer before the darlington transistor array to (and i quote....)

'..protect the translator circuit from the motor's supply voltage in the event of a transistor-to-base breakdown.'

Anyway I tried connecting up a hex buffer (sn7407n) and now things don't work. :(

I'm sure that this is a mistake in my circuit (i'm a electronics beginner) but before I try again, i wanted to ask.... is the buffer needed?? I notice on the 'Unipolar Stepper Motor' tutorial, there is no buffer being used. What do you guys recommend??

Kind Regards


A buffer is a good idea for just that reason: collector-base breakdown.

It also gives you the chance to use separate power supplies: 5V logic for the arduino and buffer, and 12V for the transistors and motor. With isolated supplies, the logic supply stays nice and cool and happy, and you get less noise back form the motors through the power supply.

Re the 7407, it's an open-collector driver. This means it won't source current through its output, it will only sink current (it will only pull the output pin to ground when told to do so).

You can either use a different buffer, or put a pull-up resistor of 1K or 3.3K or so on the output pin(s) of the 7407. If you do that you can also easily isolate the power supplies!

thanks Daniel! I wired everything as you suggested, e voila! she lives :D