Not Outputting the Correct Voltage Problem

[drewt1983]

Hey guys I'm having major problems here, any advice would be really handy. I have 2 18 pin project boards and the 28 pin project board. I'm having universal problems across them all.

I tell either of the 18 pin boards to send pin 3 high (with code below). Its sends pin 3 high but only outputs 600mV not the fill 4.3 volts from the 3AA cell bat pack. Tried new bats and a power pack and still does the same (both the 18 pin boars and same on the 28 pin board now as well).

I have checked the volts out with a multimeter and a scope. its always the same 600mV-610mV. My end aim it to PWM on pin three, had it all working well earlier, checked it with the scope all was working well. But now I'm having this problem, any ideas ?

Thanks for the help,

Tom

main:

high 3

goto main

end

 

[BeanieBots]

Welcome to the forum.

Are you looking directly on the PICAXE pins or at the output of a darlington array?
The PICAXE should switch closely between 0v and supply rail.
The darlington will as much as 1.2v when low and won't go high unless pulled up by a load, eg a resistor.

 

[drewt1983]

Hey Thanks, Im sure im going to be on here quite a bit!

Im looking at the output of the array, i just probed the pins getting nothing, you think i must have fried the chip? I checked a few outher pins (i.e 7 that does not go via the array and its dead to the world).

What puzzles me is that if i pwm out on pin 3 i still get the PWM signal just caped at 600mV?

 

[eclectic]

Im looking at the output of the array, i just probed the pins getting nothing, you think i must have fried the chip?

If you've got a very steady hand, then try measuring the voltage

a. On the INPUT of the ULN2803 Darlington

b. On Pin3 /Leg 9 of the 18X.

e

 

[drewt1983]

Ok...

checked pin 9 on the 18x its high at 4.40V

Checked pin 5 on the array and its also high at 4.40V

checked output of array pin 14 its at 0.6v

checked pin 3 on project board right had side its 0.6v

so the problem is with the darlington maybe?

 

[eclectic]

Ok...

checked pin 9 on the 18x its high at 4.40V

Checked pin 5 on the array and its also high at 4.40V

checked output of array pin 14 its at 0.6v

checked pin 3 on project board right had side its 0.6v

so the problem is with the darlington maybe?

In non-technical terms, the Darlington "inverts".
High in causes a "low" out.
It "Sinks" current.

Have a quick search of the forum, using the terms
Darlington sink
for more info.

e

 

[BeanieBots]

Read post #2 again!

If you have no pull-up on the darlington, there will be no volts!

 

[drewt1983]

Thanks very much, I get what you mean now (and its working!), you sink the current through the darlington by sending a logic high it earths your device.

Thanks very much for the help, its a real lifesaver i need this stuff to work for tommorw.

Most apreciated,

Tom

 

[BeanieBots]

Or, you could replace the darlington with a low value DIL resistor pack but remember the max current will be much less.