I need help with the project i am doing in class(teacher wont give any advice) We need to hook up a circuit that will monitor the temp. of water to bring to a certain level. We were told to use a LM35,741 Op amp,A anolog to digital converter 0804, SLN 74Ls buffer, Uln 2803, use pull up resistors then connect it to a PLC using a thumbwheel to input desired temp and display actual temp on a LCD display. Any and all help would be greatly appreciated.

 

The teacher doesn't need to give advice. If you understand how the various components work, you should be able to put this together.

Start with the analog end. Get the data sheet for an LM35. Look at the data sheet for your A to D converter, and see if you can see where and why that op amp might be useful. What might that ULN2803 be good for?

What 74LSxx logic would make a buffer, and what would it buffer? Would you use a binary or BCD TWS?

Try to do a block diagram to get the sense of how the parts interrelate, and then do a schematic. From the data sheets, you will be able to calculate resistor values.

By the way, look very carefully at the data sheet for that 741. Hint: they do not work with single supplies.

 

Well im just wondering if I should use a voltage divider?

 

Let's see your schematic so far.

 

I got it now I am now hooking it into a adc 0804 then into a 74ls240 buffer.

 

Where did that pesky 741 go? Why an 'LS240?

 

I got the 741 to work had to use a +10 -10 voltage with 80k and a 20 k resistor to get my gain of 5. My new stage is hooking up the 0804 adc to a 74ls240 buffer. Then into a ULN 2803 we have to use pull up resistors I'm trying to figure out what resistance to use. This is going to be interfaced with a PLC which is 24v. I dont have a schematic to upload we were not givin one so I am in the stage of doing that.

 

I suggest learning how to draw a schematic. You'll need it anyway for your project.

 

Ya thanks I know how.

 

Care to show the schematic?

After all, anyone searching for help will not "see" what the resulting fix was.