problem in ULN2803 connections

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Jpsinghbrar

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I have problem in inverter action of uln2803. Input to it is from 8051 port(with approx 1V as high). I have connected each output to 12V through 10k resistors.But m not sucessful.
Inverter action is abtained only when high input is above 2.5V which is not achievable from 8051 port. Pls help if anyone can.i shall b very thankful.
 

there might be some problem with your 8051 connection

do one thing connect a pull up resistor to 5v ,1k or some resistor
and change logic in your 8051 code sure it will work ie for geting on your inverter write logic 1 to your 8051
and make sure that you are given 12 v supply to your uln
 

Jpsinghbrar said:
.... Input to it is from 8051 port(with approx 1V as high).... abtained only when high input is above 2.5V which is not achievable from 8051 port. .
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this is a very strange comment. Why would you not be able to achieve >2.5v from 8051 ? If your 8051 is powered from +5v, then it will give proper outputs upto almost 5v, unless you have not configured the port as an output port.
 

kripacharya said:
this is a very strange comment. Why would you not be able to achieve >2.5v from 8051 ? If your 8051 is powered from +5v, then it will give proper outputs upto almost 5v, unless you have not configured the port as an output port.
Click to expand...

As far as I know, the 8051 ports don't need to be declared as output ports. They are bidirectional ports. Correct me if I am wrong.
 

Tahmid said:
As far as I know, the 8051 ports don't need to be declared as output ports. They are bidirectional ports. Correct me if I am wrong.
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this sounds unusual. all the uC's i have come across so far require their bi-directional I/O ports to be configured as either input or output.
 

tahmid is right 8051 dont need to be declared as out put or input
if you are taken it as input just connect to your input pin through pull down or pullup resistor depending on your
logic and read the pin
if you want to take it as an output just give the value on the pin(high/low)
 

Tahmid said:
As far as I know, the 8051 ports don't need to be declared as output ports. They are bidirectional ports. Correct me if I am wrong.
Click to expand...

very interesting - you are right to a large extent. It seems that 8051 for port0 has a weak pullup (for 1) on its i/o and a strong pull-down (for 0). So to set it for input you have to write a logic 1 to that pin which deactivates the pulldown. i believe this is automatically done at reset.

then to use it as an output with a decent current source capability you have to connect external pull-ups.


you're right - with the additional point mentioned above. Must write 1 to make it input, and have a pullup externally for proper output use. As you have so rightly pointed out in an earlier post
 



thank you
for your information
 

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