comparator as a switch to 8051

[burassu]

Hi,I have designed a light sensor by using this circuit->>>>>> http://web.sunybroome.edu/~grace_t/m...ab/lm339_d.gif .

As a comparator Ic I am using the Lm339. The circuit is working perfectly but I need it to be connected with an 8051 to act as a switch.

But the only problem is that the output of the comparator when high it is giving 2.5v as an output although it is connected to a pull up resistor to 5v .

How can I switch the output to 5v so that it can be connected to the 8051???

Tnks

 

[jackorocko]

schematic?

The lm339 has an open collector output. This will stop you from ever reaching Vcc.

 

[burassu]

Circuit



Do you suggest any other comparator?

 

[duke37]

You could try dropping R100 to 1k, the 339 will be able to manage this and will provide stronger pull-up.

 

[burassu]

I have connected a 2k2 resistor and still having the same problem

 

[Harald Kapp]

Did you make sure that the port pin of the 8051 is programmed for input? If it is output and pulled low by the 8051, you don't stand a chance of ever pulling it high.

Harald

 

[duke37]

Try disconnecting the 8051 and see if the output goes high. Harald is probably right.
Split the problem, get one piece working at a time.