AXE020 Project Board and PICAXE - 28X2 Output Pin Voltages

Flint10

New Member
Hello everyone,

I am a total newcomer to the PICAXE Microcontroller world and to this forum.

I have an AXE020 Project Board fitted with a PICAXE -28X2.
With the board powered up using the common 5v link what output pin voltages should I expect to see when set by the program to 0 using the command letpinsB = %00000000.

I seem to have a higher voltage on pin B.0 than the others. Is this normal?

The voltage in question (0.6v) is enough to turn a relay on even though the output pin is supposed to be low.

Any help will be much appreciated.

Alan
 

hippy

Senior Member
Welcome to the PICAXE forum.

The outputs are probably going through the ULN2803A Darlington driver; 0.6V would be expected at the ULN2803A output when sinking current through your relay.
 

Flint10

New Member
Thanks guys,

Yes the voltage was measured at the output of the Darlington driver, but with no load on the output.

As I want to latch the relay I am using a latching thyristor (C106D) bewteen the output pin and +12 volts. Do I need to remove the 5volt link from the board in this case?
 

hippy

Senior Member
I think it would help to post your circuit then we can better understand what you are trying to do.
 

Flint10

New Member
AXE020 Project Board with PICAXE 28-X2 - Output Pin Voltage Query CCT Diagram

Herewith CCT diagram as requested.

On start-up all inputs need to be high and all outputs need to be low so that it can interface with existing external equipment.

Thanks in anticipation.
 

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inglewoodpete

Senior Member
The ULN2803A is not a good way the switch an SCR (or 'latching SCR'). The ULN2803 is an array of open-collector darlington transistors and needs a pull-up (resistor or relay) on each output that is used.

Tell us more about the relay eg coil resistance. Why do you need a thyristor to turn it on? The darlington would normally do the job on its own.
 

MPep

Senior Member
I would remove the SCR, and connect the relay directly to the ULN2803, and the 12V rail. If you need to interrupt the supply to the relay, use an input into the PICAXE using ..... wait for it.... an interrupt! :)
 

Flint10

New Member
Thanks for your quick response.

Yes, I must admit I chose the the SCR idea before reading about the interupts command. I will look into that command further.

Cheers,

Alan
 

hippy

Senior Member
Depending on exactly what you are requiring it might not be necessary to use interrupts, simply polling the input pins and responding to what's read may work.
 

MartinM57

Moderator
Might be best to provide an idea about what the project is actually for - we're all staring into our crystal balls discussing SCRs, ULN2803, interrupts, start up values of ports and relays etc with no real context as to what we are dealing with.

With the bigger picture available I expect, from what's seen so far, the solution is quite simple (unlike the initial design ;))
 

Dippy

Moderator
Can you explain how your circuit as posted (post#8) would actually work?

As said, please explain your requirements as I'm sure someone can provide a neat solution.
 

Flint10

New Member
Thanks for all your replies.

Yes, on reflection I think I did over complicated the design. I have since tried another approach using just the darlington driver to operate the relays with some success.

My project has 7 inputs and 8 outputs. Due to the extenal equipment the PICAXE is controlling, on startup each input need to be high, and each of the outputs need to be low.
The required program logic is thus:

Typically when each input (1-6) is low the corresponding output is high (e.g. if PinC.0 =0 then pin B.0 =1). But once initiated each output must remain high until any other input pin goes low. i.e. they need to stay latched untill any another input pin (1-6) goes low.

The above senario applies to any individual input (1-6) or when 2 or more are initiated simulataneously, i.e. if inputs 1,2,3 are initiated together (go low) then outputs 1,2,3 go high and stay latched until (say) input 1 is initiated, in which case output 1 goes high and all the others go low.

However, in one special case if 2 of these inputs (1-6) are initiated together with input 7 then output pin 8 needs to go high and stay latched until another input is initiated.

Phew......

I have got the programming sorted (I hope) it was just the problem of latching the output relays. Now I have got rid of the SCR's and got the relays working I will get the program to control the output relay 'latching period' instead.

On a seperate point, instead of using the project board I have decided to redo the PICAXE -28X2 on a breadboard. I have not got an external resonator fitted so do I need to tie the 2 resonator pins (9 and 10) together?

Thanks for all of your support. I will probably be back.......

Cheers,
Alan
 

John West

Senior Member
No, it will use the internal oscillator without shorting the pins together. While the manual doesn't normally instruct here an what "not" to do, as that could turn it into a text the size of Wikipedia, it's an interesting point that perhaps should be mentioned in the manual for those who wonder about it.
 
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inglewoodpete

Senior Member
On a seperate point, instead of using the project board I have decided to redo the PICAXE -28X2 on a breadboard. I have not got an external resonator fitted so do I need to tie the 2 resonator pins (9 and 10) together?
Refer to the minimum operating circuit for the 28x1 in Manual 1 - Getting Started. It shows everything needed to get a 28x1 running.
 
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