Am I being thick?

Mr S

New Member
I have been trying my new 18x high power project board and am currently trying to get it to just output high and low levels to output ports 1-3 to drive LEDs.

The code and programming seems sucessful, if i put a meter on the output pins on the PICAXE chip then all is good and correct, i see the low/high switching that i am expecting. The problem comes when I try to drive the LED via the FET's on the board.

The FET doesn't seem to ground the pins (for example pin0 for outpin0) as the diagram shows. I have also tried to see whether the high/low levels as measured on the PIC output pins are reaching the legs of the FET and they don't appear to be. I understand that the output ports 1-3 consist of a fixed +V power pin and a FET switched ground, it's just that the ground doest seem to be switching. I currently have 6 of these boards and none are working.

Any ideas what I am doing wrong? This seems like it should be really simple and yet I cannot get it to work. Incidentally I currently do not have the L293D motor driver IC in place as the data sheet says it is optional (and I dont need to drive motors right now) also the circuit diagram doesnt show it being needed to just switch output pins 1-3 so this shouldnt affect them right?

Any advice greatly received

Darryl
 

BeanieBots

Moderator
You need to wire the LED (with series resistor) between +Ve and the FET.
The FET outputs (and darlington outputs on other boards) can only SINK current. They cannot SOURCE it.

What are you using for power?
The FETs require about 4v from the PICAXE to turn on. Low power supply voltage might not be enough to make them work.
 

Mr S

New Member
Yep, that is what i am doing, LED and resistor in series between +V and the FET in order to sink the current. The output voltage on the PICAXE pin is ~6v too so should be enough to turn on.

I have checked silly things like bias on LED and integrity of resistor and they are fine. What does bother me though is that the switching output levels (+V and 0) are not reaching any of the legs of any of the FETs as the electrical diagram suggests they should.

I am increasingly convinced that there must be a problem with the board(s) that I have purchased.

Cheers for the suggestions.

Darryl
 

hippy

Senior Member
What does bother me though is that the switching output levels (+V and 0) are not reaching any of the legs of any of the FETs as the electrical diagram suggests they should.
That is odd if you are driving the correct pins. If those pins are not being switched then are any of the others ? It may be an idea to post your test code.

Could you also give a link or reference to the manual / circuit diagram for your board as I'm not familiar with it.

I am increasingly convinced that there must be a problem with the board(s) that I have purchased.
That seems unlikely as all tracks from the PICAXE to the drivers would have to go open-circuit. It is possible to blow up the PICAXE output pins and the entire output port, but not a common thing to happen. Reversing power can destroy an entire chip but that's usually noticeable as download will stop working.

Most "My chip/board is faulty" problems turn out to be something else, so here's hoping that's the case here.
 

BeanieBots

Moderator
I'm assuming you have the AXE035. Correct?
Where was it purchased?
Please describe your power connections and supply.
~6v at the PICAXE output is a little worrying.
 

Mr S

New Member
I can now confirm that the answer to my thread question "am i being thick?" is most definitely a yes!

Got my pinouts and code pin refs confused between the diferent data sheets and the simulator.

Thanks for your input guys, sorry to waste your time - its now working fine!

Cheers,
Darryl
 
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