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How do I generate a potentiometer output?

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klick

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I'm looking to generate a 10k potentiometer output, I have a schematic to take a PWM output and generate the potentiometer output, but it isn't exactly working, the source wasn't really that great so maybe it doesn't work. Maybe I hooked it up wrong, don't know. Here is the picture of the schematic I have http://www.transproducts.net/pics/interface.jpg

Really, if you look at that schematic, I don't know what the 2K thing is in the upper right, I guessed it was a potentiometer, for tweaking I guess, if it's not, please tell me what it is :). So I put a potentiometer there and it really doesn't do anything, modifying the potientiometer output doesn't seem to affect the resistance at all on the output, even with a PWM signal being sent to it. The circuit definatly modify's the resistance when you change the PWM output, but it goes from 10K up to a very large number, which it should go backwards.. obviously, 10K being the upper limit.

I've searched the web for other schematics defining how to do this, this is the last thing I need to figure out before I can get my creation up and running, any help or advice would be very much appreciated.

Any other schematic for doing this would be great, and I have looked into Digital Potentiometers, but really haven't seen exactly how I would hook it up with PWM to use it. Really I don't mind ordering them, but i was hoping to test my project out this weekend, there are pictures of my "creation" at http://www.rimmel.org yes, it's scary looking.

Thanks in advance
Ross
 

Roff

Well-Known Member
The circuit will take a PWM output from the BS2 and put out a voltage which is the average, (by virtue of the 1k/10uF low pass filter), of the waveform coming from the BS2, minus 0.7 volts (the Vbe of the transistor). The 2k "thingy" is indeed a potentiometer, connected as a rheostat. It's function is to limit the maximum voltage at pot middle. The transistor is an emitter follower. It's function is to buffer the output of the low pass filter so that the load presented by the 10k resistor and the PWM80 won't load down the low pass filter.
You shouldn't measure the output at pot middle with an ohmmeter, you should measure it with a voltmeter. The PWM80 apparently needs a DC voltage which is normally derived from the wiper of the "10k pot" that you are trying to emulate. This circuit allows you to create that voltage by varying the duty cycle from the BS2.
 

klick

New Member
So you see no reason as to why this shouldn't work? I made the board and hooked it up, doens't really seem to work for me, maybe i'll go play with it for a bit, test it with a voltage meter like you said.

Do you think i should hookup the pot left to anything? ground maybe? since normally the potentiometer is hooked up to all 3 pins of course.

Is there a standard name for this kind of schematic, I did see you mention low pass filter, which in my searching I found some thing refering to that as maybe potential solutions, but as i'm sure you can tell, i'm not an expert at this stuff :)

Thanks so much for the info,
Ross
 

Roff

Well-Known Member
So you see no reason as to why this shouldn't work? I made the board and hooked it up, doens't really seem to work for me, maybe i'll go play with it for a bit, test it with a voltage meter like you said.
Well, not knowing what is inside the PWM80 (I did a Google search and came up empty), I can't say if it will work or not. All I can say is that if the BS2 is putting out a PWM signal at a frequency of at least 1 kHz, you should get a voltage at the emitter of the transistor that is proportional to the duty cycle of the PWM signal, with the maximum voltage being 11.3 volts at 100% duty cycle (BS2 output always high), or 10 volts if the 2k pot in the collector is at its maximum value. The minimum voltage, with the duty cycle at 0%, will be zero volts. If you can come up with a schematic or documentation of the PWM80, we can look at it.
Do you think i should hookup the pot left to anything? ground maybe? since normally the potentiometer is hooked up to all 3 pins of course.
Where is this 3rd pot pin that you are asking about? Are you talking about the 2k pot? If so, it doesn't matter. You can leave it floating or connect it to the wiper (usually the center pin).
Is there a standard name for this kind of schematic, I did see you mention low pass filter, which in my searching I found some thing refering to that as maybe potential solutions, but as i'm sure you can tell, i'm not an expert at this stuff.
The circuit with the dark background is a lowpass filter and an emitter follower.
I'm assuming you are using a Basic Stamp BS2. Where are you getting the PWM80? It looks like an H-bridge motor controller with an added DC speed control circuit.
 
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