Let's Make Robots!

Robot gutless and slow....

Ok, so I finally got him put together, and it turns out that he doesnt move worth a damn.... I cant figure out what I did wrong. The tamiya twin motor is set for low range, but it still shouldnt be THIS slow... turning is almost impossible for him... Even on smooth flooring.... I shot a quick video for you all to see and hopefully help me figure out what is going on...


This is the H bridge I am using... SN754410   from sparkfun. The data sheet said i could save pins by hooking the enable pins up to a 5v source, so I did that..... VCC1 is connected to the 5v on the arduino, Vcc2 is hooked up to the same power source as the motors/IC, with common ground. My arduino has its own power source. Ignore the wire coloring, I only had 2 colors and the ones i used here were for my reference, not to indicate hot and ground.

 Power for the board is 4xAA alkaline batteries. Power for arduino is 1 9v battery. 



I have no idea what to do here....
Can anyone help? Thanks in advance.

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just looking from the physical side, are the tracks to tight? are they hard to turn by hand? and I've always found them to run better on floor boards (hard ground) rather than carpet, have you tried? Good luck.



Err.... can you dumb that down for me? Do I need to put diodes in line between the the output pins and the motors? Ive never done anything this intricate before... Sorry for the ignorance....

Take me to diode school please? 

Basically each output pin needs a diode "pointing" to V+ and a diode from GND "pointing" to the pin. 4 outputs with 2 diodes each means you need 8 total for that chip. That picture in my other post is probably the easiest way to understand how they should be wired.

It`s a little tricky to get your head around the way it works and can be hard to squeeze 8 diodes on the board. Try drawing on graph or dotted paper how you intend to fit them in before starting up the soldering iron and you should be right.

So last night after my last post, i rebuilt his gearbox for higher speed. He doesnt move at all now with the treads on. I dont know what the heck is going on....


@ aniss, i did the re-wiring of my enable pins and it seems like it did nothing.


@ignoblegnome, yes i checked the batteries


@calculon, I think both motors are turning in opposite directions when hanging a left already.... check my code above.


I just woke up, and after some sleep, maybe i will be able to figure out what is going on... In the meantime, keep coming up with ideas for me to try. you guys are being very helpful. Thank you. 

That chip needs flyback diodes. It is not a direct replacement for the L293D as Sparkfun say because the L293D has the diodes internal. The output pin circuit diagram shows ESD protection diodes but flyback diodes should be added as shown in the schematic on page 6 of the datasheet.

This pic ishows the diode arrangement better and is easier to understand than the datasheet diagram. Any diodes at least 1A should be ok.


based on your video, that looks about as fast as those motors are meant to go. the left turn is so slow because one motor is doing all the work. To turn, have one motor go forward, one backward.

Did you double-check the 4xAA batteries are new and up to full voltage? It looks like a low battery supply.

Sorry if this is obvious and you've already checked. Some times even the most experienced people forget to check the basics. 

ooh, and also....


On the arduino, there are 2 ground connections and only 1 5v connection. are the grounds common? can i use either one? 

My enable pins are wired in with the +5 on Vcc2, not the arduino. Could they be pulling current away from the motors? my grounds are all common and wired to the ground on the arduino already.... Just to make sure before i do this (its going to take some doing to get at the driver board) I should wire both enables and the lead from VCC1 to the arduino's 5v?


I really want to make sure of this before i tear into the board. Am I clear on your instructions? 

And the grounds need to be all tied together. The 9V negative and the 4xAA negative both need to go to the same ground, along with both the l293d and arduino. -- if that's not done already. can't see the picture well on the phone ;)