Dual-battery setup

[Cyntrox]

Hello, everyone! I am new to this forum, but am a big fan of the site.

I'd like to start off with a question. I am building my first robot, and have decided to use two power sources:

• A 9-volt battery, regulated down to 5 volts for the control electronics
• 5 1.2v rechargeable batteries (for a total of 6v) for use with the engines

My trouble comes with the motor driver (L293D). While it has two voltage inputs (logic and supply), the ground is, as far as I can tell, common.

I have tried to connect the ground of the 6-volt pack to the regulated 5-volt power, this caused three of the batteries in the pack to discharge.

How can I use different voltages like this? Thanks in advance for any help.

[rbtying]

Connect all grounds (- terminals) together.

Connect the +9v rail from the 9v battery to the 5v regulator

Connect the +5v output from the regulator to the logic power pin on the L293D

Connect the +6v rail from the 5 1.2v batteries in series to the supply power pin on the L293D

Common grounds should not discharge your battery, you may have hooked something up wrong.

[Cyntrox]

It turns out that your last theory was (almost) right. My faulty soldering had short-circuited the 6v pack. Now everything works, and I have a regulated 5v and unregulated 6v rails!

[rbtying]

It's great to hear that you've solved your problem.  Good luck on your robot!