08M2 input pin as an output (was sorry i need help...)

[mezzrow]

Excuse me, for my poor english, and for my lack of understanding...

I need to use, as a output, the input only pin of an 08m2 the way some guys have already done.
as an example you can see this:

http://www.picaxeforum.co.uk/showthread.php?21006-Driving-JY-LKM1638-from-08M2&referrerid=46065

They use a n-mosfet and the pullup comand.

But for me it does not work...

On a 4,0 v supply the pin give out, tested with an old, but still good, multimetre only 70 millivolt.
Is just the 08m2 that does not work ?
the schematic is thisView attachment LKM1638 Temperature 1.pdf
from the link above.
the mosfet i use is an BS170.

Hope someone can give me the right advice,
because i have open the box many days ago now...
and messy as i am i will soon start to loose pieces...;-)

Thanks in advance for your time.

Andrea

 

[manuka]

Andrea: Where are you? Please confirm you ARE using a PICAXE friendly DS18B20 for temperature monitoring.

 

[AllyCat]

Hi,

Your diagram shows an 08M, are you definitely using a 08M2 ?

Do you still measure only 70 mV when no other components are connected onto Leg 4 ?

Cheers, Alan.

 

[nick12ab]

According to the datasheet, the gate-source voltage of the MOSFET is 15V and when a voltage of 4V is used the drain current will not be very high. A logic-level MOSFET should be used.

Does the pin continue to measure a low voltage when the MOSFET and external resistor is removed?

Also you do not have any decoupling capacitors fitted.

 

[westaust55]

Here is a link directly to the original thread about using the 08M2 pin as an output.
http://www.picaxeforum.co.uk/showthread.php?19111-08M2-and-pin-C-3-as-output

 

[hippy]

Rather than use a circuit to overcome a limitation of a specific PICAXE, which may be sensitive to noise and particularly when relay switching and switching 2kW of mains load, it would be better to go up in size and use a PICAXE which has the required number of output pins.

That will also help if the Serial Out is used for other purposes as appears may be the case here.

The circuit should also include decoupling capacitors.

 

[srnet]

Using a pin that is input only, might be a useful dodge in a battery operated toy, but hardly suitable for the control pin for a 2kW mains heater, just dont do it.

 

[mezzrow]

Sorry i have not been very clear, as i fear because of my english.

Only the idea comes from that schematic.
I don't have a 2 kw heater at all.
i have only the 08m2 fitted with the 1 megahom to ground on pin 3, and the necessary tie to ground for the serial in.
The code only give the pullup to the pin
I use 3, not very fresh, 1.5 v AA to power the circuit.
And i do measure 70 mv with only the multimetre attacched.
How much i have to read in your opinion?

Nick12ab: don't know if it make any difference, but i use the BS170.

Thank you westaust55, but i have already look at that tread, it was your suggestion (you are the best finder in the world) in a previus tread regarding the same argoument.

Manuka : right i will use the DS18B20.

Hope this help.
Andrea

 

[AllyCat]

Hi Andrea,

What PULLUP command are you actually using? Can you make it work with any of the other pins?

The pullup resistance is about 30 kohms, so a pin should rise almost the supply rail, with only a 1 Mohm load on the pin.

Cheers, Alan.

 

[hippy]

The pullup resistance is about 30 kohms, so a pin should rise almost the supply rail, with only a 1 Mohm load on the pin.

For an 08M2 here, with a 4.5V supply and no load on input pin 3 the voltage at pin 3 switches between 0V and 4.47V as would be expected -

#Picaxe 08M2
Do
  Pause 5000 : PullUp %001000
  Pause 5000 : PullUp %000000
Loop

Adding a 2K load drops that considerably; 0.23V so perhaps the 1M resistor is not 1M ?

 

[mezzrow]

YES... indeed.

I was born in 1960... i really need a new pair of glasses...

It is not a 1M ...

In this forum I always find a great support...

Tank you all... i have to pay a drink to Hippy and Allycat... and after the arrive of the "glasses" i will close the box.

Andrea

P.S. how the tread change the title? Is it better this way, but i do not make the change.