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Questions about Output Voltage Draw ???

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iso9001

New Member
I dont really know how to decribe this but...

I have this sensor that has no return wire. It is grounded to the chassis it is bolted to. The way it is supposed to work is 5V goes into it, depending on what is going on that 5V hits different resistors and then a processor that is sending the signal reads the amount of voltage draw on the wire...

What I want to do is replace that sensor with a 'simulated' one, using a pic and some other stuff... but i want it to operate in the same way. (Its kinda complicated why i need to do this, but alas it does need to be done.

How can I accept a 5V signal with my pic, but only draw specific amounts? Like I want this other processor to see that its output pin has 2.5V being drawn from the 5V its allowing out.

Can I do this with the -Output of a DAC ???
 

Klaus

New Member
It would help if you tell us what this sensor issupposed to 'sense'. There are many different kind of sensors for many different applications.
 

iso9001

New Member
What I should have said is....

Really, its better worded if I say that

I need to sink a specific amount of a 5V signal coming in.
 

iso9001

New Member
No, not the hall sensor.

This isnt 'really' a sensor at all, its a multi-potistion switch that has a series of resistors in it, depending on what switch is pressed, the current feeds into a different resistor. I said sensor cause it would prolly confuse less people.

The problem is it isnt sending a signal on one wire and reading it on another... Its looks like (I cant get to the hardware of this other processor so I have to guess) that they are outputing 5V and then ADC-ing thier own line.

I need a way of faking the switch. So if it is supposed to hit X resistor and then its own adc reads 3.3V instead of the 5V it would ususally read it know it hit X resistor. If I could sink 3.3V of that 5V so that its adc read 3.3, it would be LIKE the switch with all its resistors is still there, I could mimick this I think.

(I cant make a resistor/transitor hardware path b/c the switch has WAY to many options)
 

Optikon

New Member
iso9001 said:
No, not the hall sensor.

This isnt 'really' a sensor at all, its a multi-potistion switch that has a series of resistors in it, depending on what switch is pressed, the current feeds into a different resistor. I said sensor cause it would prolly confuse less people.

The problem is it isnt sending a signal on one wire and reading it on another... Its looks like (I cant get to the hardware of this other processor so I have to guess) that they are outputing 5V and then ADC-ing thier own line.

I need a way of faking the switch. So if it is supposed to hit X resistor and then its own adc reads 3.3V instead of the 5V it would ususally read it know it hit X resistor. If I could sink 3.3V of that 5V so that its adc read 3.3, it would be LIKE the switch with all its resistors is still there, I could mimick this I think.

(I cant make a resistor/transitor hardware path b/c the switch has WAY to many options)

Could you maybe use a potentiometer / divider? You could wire this up to the input A/D on your micro(if you have one built in) and then read in the voltage. When you want to read in another "resistor path" simply adjust the pot to give you the expected voltage.

I'm not sure I fully understand what you are trying to do but so far, it sounds like you dont need much to simulate the switch and resistors.
 

iso9001

New Member
Exactly!

I was just looking at different components on maxims site and saw digtal pots.

That is EXACTLY what I need. I just run my signal wire into the pot, set the resistance via pic, then ground the other end out...

Thats exactly what my switch is doing. But I'm using I2C already and I didnt see anything other then 'serial or SPI-Serial'....

Are there any I2C Digital Potentiometers ???
 

iso9001

New Member
Arg. Well, I found out that YES ther are I2C Digital Pots... Howver, the only ones I can find are SMALL (i'm looking for DIP sizes)...

Any Dip sized signle or dual, I2C, at least 100 tap, Digital Potentiometers ?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
How many positions do you need?, you've never actually said!.

By having NPN transistors fed from output pins of the PIC, with resistors from the collectors of the transistors to the input to your device you could easily have LOADS of switching positions. Using 8 output pins and transistors would give you 256, using 16 output pins would give you 65,536 positions.

If the input would accept a voltage (which it probably would if you could remove it's pull-up resistor) you could use a simple R2R ladder DAC fed directly from the PIC pins.

I would suggest ideas would come quicker and more accurately if you actually tried asking for exactly what you need - rather than trying to simplify it and causing confusion. Up to now we still only have a vague idea what it is you want, and still don't have any idea of how many positions you need.
 

iso9001

New Member
Well...

A digital pot is going to be the most accurate 'simulation' of having the setup that is currently there.

I found out today that depending on some switches I haven't got to test yet i might now even need to use a pot at all, maybe just a dac.

I'm doing all sorts of system crossing (taking things from one system and making it work other systems stuff).

It would take a very long to explain exactly what I'm doing, if I can break it into 'general' questions i find that answers come in MUCH quicker.

Oh, it looks like if i did need a pot it would be at least 100tap and 10k... but I need to go over the diagrams and such before I'l know.

I did notice one thing that confused me about digital pots... Why is there a A, a B, and a W port ? (I think W is for wiper, not sure why you'de need 'a' AND 'b' though)
 

Optikon

New Member
Re: Well...

iso9001 said:
A digital pot is going to be the most accurate 'simulation' of having the setup that is currently there.

I found out today that depending on some switches I haven't got to test yet i might now even need to use a pot at all, maybe just a dac.

I'm doing all sorts of system crossing (taking things from one system and making it work other systems stuff).

It would take a very long to explain exactly what I'm doing, if I can break it into 'general' questions i find that answers come in MUCH quicker.

Oh, it looks like if i did need a pot it would be at least 100tap and 10k... but I need to go over the diagrams and such before I'l know.

I did notice one thing that confused me about digital pots... Why is there a A, a B, and a W port ? (I think W is for wiper, not sure why you'de need 'a' AND 'b' though)

Why is it that you believe a digital pot will be the most accurate resistor simulator? The digital pots have very large tolerances (relatively speaking) and you can fine-tune a potentiometer and avoid the offset voltage that the buffer amp gives you on the digital pot.

If you are willing to go to the length of using D/A then it will be the easiest to change.
 
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