Let's Make Robots!

Major problem with Picaxe 120 starter kit. Please help

Hi Guys,

 

I am absolutely new to this stuff, I know a bit about the basics in electronics, a lot about logic circuits, and even more about programming.... but as you can read, there is a shortcoming when it comes to the more advanced stuff in electronics, that's why I do need your help.

 I recently purchased the Picaxe Microbot starter kit (as I thought that this would be an ideal platform to learn the basics, and to grow further from there). In combination with an SRF005 a servo a brandnew Picaxe 18x, and two Sharp GP2D12 infrared range sensors. Nice little toys and they all do work perfectly. But I have a problem since the very beginning that I have received the board. One of the two DC motors didn't do anything at all. I first blamed myself, by claiming it would have been a result of sloppy coding. But that didn't really seemed to be the case. 

As I went through all possible options (broken DC motor, etc), I started by taking my trusty multimeter into my hands, and messured through all the ports that are going out to the L293D Motor driver. And I see something really strange:

 

picboard.jpg

To give you a raw idea about the deltas between the original board and my modified setup:

 

*Position R-2 carries a 330Ohm resistor.
*The V+ and GND of Out 2 are connected to a small servo
*Out 2 is directly connected to the control of this servo (so there is a 330Ohm resistor between them) and the servo works fine
*The SRF005 is directly hooked up to the SRF004 terminal, and works fine.
*the V+ and GND of Out 1 and 0 are hooked to the Sharp sensors in order to feed them.
*the In 1 and 2 terminals are hooked up to the output of both sharp sensors for ADC, and seem to work without any problems.

That's about it. But now to my problem:

One of the two DC motors (as a drive) is not working. If I messure Out 1 and Out 2 of the LD293D I get a steady current of approx 4.2 volts and that seems to do the job driving the little motor when active. 

The other end of the LD293D doesn't seem to do that much, if active, it only gets to approx. 0.11 volt..


If I go further down the chain the PWM Out6 offers me the desired 4.2volt the Out 4, again only something between 0.11-0.5 volts.

On the Picaxe 18x itself out 5 is showing the same symptoms!!!.... Then my new 18x became my suspect, and I replaced it with the included Picaxe 18... but still the same issue. (which makes me beleive that my PIC is ok).  Please also do remember the followin: The problem did occur way before I started to solder my own material onto the board. 

 Can this be caused by a broken capacitor (not charging correctly), or do you guys think that either the PWM, or the LD293D are busted... and if, how can I test / troubleshoot this?

 

Many thanks!! All help will be really appreciated!

 

With kind regards,

 

Gereon

-Amsterdam

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Found the problem... the PWM is busted! The good news is: Once I removed it my robot is capable to go forward, turn left and right... the bad news is: the robot isn't capable to make sharp turns, nor does it go backwards or am I capable to set the speeds on this baby :( I hope Rev-Ed is so kind to send me a replacement, as the PWM installed on this kit is a custom build by them. Thanks for the help Oddbot, very much appreciated your kind support. -G
I don't know because I don't have any of that stuff. Sorry. I make my own boards and use a L298N because it handles more current than a L293D. What is the PWM chip? what number? Check the RevEd site in case it's a common problem.

I also mailed rev-ed about this, perhaps they will be capable to give us a quick answer on the topic (you might be right that this is a common flaw, but I haven't found anything "yet" on the rev-ed.co.uk site.)

 

I'll give you an update on the PWM later this day, can't remember what type they used. 

 

My actions tonigh will include the following, not specificly in the sequence mentioned below:

*Open fridge
*Grab beer
*Open beer
*Drink beer
*Stare at PCB for any flaws, or soldering issues.
*Grow gray hairs...

 
Many thanks man. :)

Do until problem solved or drunk and staggering to bed :)
Try reseating your L293D and the PWM chip. check the legs of the ICs as sometimes one can get bent when the IC is inserted. Look for bad solder joints or two connections that have been shorted together by excess solder.

... I will do that tonight (busy day at the office).

 Could this also explain the low voltage on Out5 of the Picaxe itself?

Thanks for your help, I really do appreciate this.

 

BTW... how do I reset the PWM? And will the PWM get rid of any coding done to it?

 

PS: I have had written a little test code which was somewhat similar to the following:

which was cycling through all possible motor control combinatios, and slowly incrementing the speed settings for both motors. This didn't had any effect at all :(