led potentiometer question

[kaleaf]

Hi im looking for some help with some led potentiometers that I have ordered. 

http://www.mouser.com/Search/Refine.aspx?Keyword=PTL60-15O1-103B2

I am trying to figure out how I am going to make the led dim at low.  I am thinking about powering the led off the resisted signal side of the pot but it would make my pot nonlinear.  Im running 5v through the pot to a usb hid encoder

This is for a usb-hid project and I have found this forum and it has been very helpful because I am using similar controls (Im building a traktor dj controller).  If I have good luck with this one I hope to build a midibox and some sequencers.   If you guys don't do non-midibox questions let me know,  I have not found a good forum my project is a mix of random stuff (mostly arcade parts so far). 

[madox]

I am trying to figure out how I am going to make the led dim at low.  I am thinking about powering the led off the resisted signal side of the pot but it would make my pot nonlinear.

Hi Kaleaf,

Few questions:

I think you want the intensity of your LED to vary with your potentiometer wiper voltage, right?

You are concerned that the load of the LED will interfere with any other stages connected to the potentiometer wiper voltage?

Are all your potentiometers the same taper (linear/log)?

What other types of loads (inputs of following stages) will be driven by the potentiometers?

The usual way to avoid interaction between loads, is to isolate stages with buffers. Connect your potentiometer wiper to buffer stages, and connect the buffers to the loads. A very easy way to do this, is using opamp buffers, though I think a correctly biassed transistor of some kind should be feasible for your application. You may even wish to introduce some nonlinearity in the LED buffer.

If you are looking for other fora to discuss design issues, there should be something for generic electronics. This type of question is application independant. I doubt too many people would mind you asking this question in the miscellaneous section here, though.

Cheers

[Goblinz]

I'd be more than happy to help with none midi-box stuff if I had the knowledge. I'm interested in any project mentioned on here whether it be mid-box related or not.

There's a few of us here building x0xb0xen, discussing it, organising bulk orders, etc which is not a midibox related project, just something we're interested in.

[kaleaf]

thanks guys

this is the device I have

http://www.u-hid.com/

its a usb-hid board that has 50 pins.  The slide pot with accept 5v from the board and connect to a analog input 0-5v pin on the board.  The board can control leds aswell but not for the analog axis(the slide pot).

I will have the sliders soon so I will come back when I have them to test. 

I think I am looking for a way to split the output on the pot so it can go to the uhid and adjust the led also.  Dual-gang was not available for the led sliders.

[madox]

I think I am looking for a way to split the output on the pot so it can go to the uhid and adjust the led also.  Dual-gang was not available for the led sliders.

Hi Kaleaf,

That's an interesting looking device, and seems to provide a convenient entry for HID experimentation.

What kind of background do you have in electronics? If you are not so confident with design/trouble shooting, then you could use a very simple op-amp buffer to drive the LED. The buffer input will be very high impedance, and will not significantly effect the wiper voltage.

I have attached an image with a basic circuit. However, this may not give you exactly the brightness response you would like. You may need to go to a slightly more complicated circuit, to provide minimum illumination at low voltage, and to give a brightness 'curve' which is satisfying to the user. This should give you something to get you started, though.

[madox]

By the way, what supply voltages do you have onboard your HID?

[kaleaf]

By the way, what supply voltages do you have onboard your HID?

"Power and ground pins can only source/sink 30mA max current per pin.

Total current draw must not exceed 500mA

If you need more current than 30mA then you can use one of the fixed power pins on connector J7. See the U-Config program board image for details of which pins these are. In fact it is good practice to use these pins whenever possible for a 5 volt source as they are powered directly from the USB supply."

thats from the specs.

so I think it can dish out the full amount of the usb spec 500ma. 

My skills are very basic.  I know resistors, pots, and basic circuits, but im still reading about stuff like opamps. I have good soldering skills.

[madox]

Hi,

I just wondered if you may have a -5V rail available, as it can simplify some op-amp designs.

How about the other questions I asked, particularly regarding pot taper?

[madox]

From PM:

Hey, thanks alot for the help.

My project first started here where I saw a diy controller on youtube from djtechtools.com

In this video at about 5min this guy says he powers his led off the wiper (middle pin) of his pot to have his leds dim with the pot. Thats when I started to wonder if the led and resistor would affect the signal to the hid device. Then I noticed the guy has other parts on his board (the yellow one with his pots) and also his pots are on a board in that shot but on his device they seem to be different.

Am I right that the led would make a liner pot non liner.

if you dont have time for the vid no-worries thanks again for the info about the opamps

Hi, I'm moving this back to the forum, where I think it belongs.

edit- I'll try to make time to answer this for you tomorrow...

[madox]

Hi Kaleaf,

The circuit, as described by Ean of DJ Techtools, places a resistor and LED in parallel with the lower section of the resistor divider circuit. This would effect linearity of the output, with respect to wiper position. I expect that Ean is aware of this, and doesn't consider it to be very important for his application.

I think you should consider buffering the wiper voltage with an opamp circuit, and adding DC to bias the LED to the verge of noticeable light emission. This will vary a little, depending on the LED you use, but you will probably be adding a little less than 2V to the wiper voltage.

Most likely, you will be able to use the wiper voltage as an input to both the LED buffer stage, and the HID input stage. The buffer input impedance will be very high, and it is also likely that the HID input impedance will be comparable. If this pair of impedances are both much greater than the potentiometer impedance, then the wiper response remains essentially linear.

Do you know what kind of input is used on the HID? If you don't have a schematic, then you should still be able to follow the circuit trace from an input pin to an IC chip or other electronic device. It would be worth checking this, to make sure the above assumptions will hold true.

I think you will most likely need some help to design the buffer stage. Do you have any friends doing electronics? I can offer you some help from here, but I can't do any fault finding/trouble shooting; you would be pretty much on your own for that. However, this is usually the case with everything, at some point.

Hope you don't mind me asking, but what is your educational background, and what is your level of interest in electronics?

Let us know how you go with this.

[TheAncientOne]

A few thoughts on this.

I note from the Mouser data that these are 10K linear with a centre detent: their target market is consumer tone controls and graphic equalisers

An Op-amp might be overkill here.

First thing to do, once you get them is to establish your required current range. I'd put an old 20K pot, (or so, not that critical), and a 150 Ohm safety resistor in series with the LEDs, put that lot in series wirth the current range of a test meter, wire to a 5Volt supply, and establish 2 current limits:

1) The minimum current that just makes the LED glow

2) The current for the maximum brightness you want

Be aware that few small LED's will be reliable at more than 25mA max current. The 150 Ohm will give roughly 25mA max current with a red LED (1.2 Volt forward drop).

Get that bit sorted, and I think we can come up with an arrangement that should do it.

If you're not sure how to do the steps above, PM me and I'll try to help.

[kaleaf]

Madox and TheProf .... I thank you for your help.

Madox, my background in electronics is just from projects I have done and a few things at work.  I have made lots of rs232 cables at my work and I have good soldering skills.  My interests has been recent after finding diy audio projects like midibox and diy synth kits.  I like synths alot I have taken a course at my local community college on synths and I have always wanted to get my hands on a analogue (I use software at home). 

For me right now I am more intrested in learning what to do to make a dimmable led (from a single gang pot) in general than I am worried about having this exact one do it.  Doing it right is important to me and by that I mean not affecting the linear taper of the pot.  If I have to hold off on this untill I learn more I will still do my project.  I have learned a few things researching this question and from the tip about opamps if I use them or not it all helps.

thanks again I will get back after I have time to do some multimeter tests

[madox]

Hi Kaleaf,

It's always a pleasure to help.

Actually, it sounds like you are in a good position to start on this kind of design consideration. Please don't be discouraged (although one can usually expect some headaches along the way, learning electronics).

I think you should really check out TheProf's suggestion, as he may have an easier/better solution for you. Actually, I would be interested to hear it too  :)

If you are not too concerned about using this particular pot, then you may consider using a dual gang pot, and have one for HID input, and one for LED control. This would not really be doing exactly the same thing, but it should be good enough for this. Remember that the LED is only an indicator. There would still be some things to consider with this approach though. One thing which should be kept in mind with dual gang pots in general, is that one cannot rely on the two tapers to be truely identical (in this case it wouldn't really matter).

After being involved in this thread, I am a little curious to try out something like this design for myself on a breadboard. If I get around to it, I'll let you know how it goes.

If you want to go through a text book to learn more about electronics in general (and cover this topic as well), I think a lot of people like The Art Of Electronics by Horowitz and Hill. I have the book, and find it useful for a lot of practical circuit applications, and design considerations. It is not overly mathematical at all, which is something one finds with the academic approach to electronics. I am OK with both types of text, but I often recommend Horowitz and Hill for people not doing electronics as a degree/career.

[kaleaf]

They don't have dual gang in the bourns brand its not a option.

http://www.bourns.com/ProductLine.aspx?name=sliding_panel_controls  (in the ptl series the options/ganging is blank)

I have found one possibility with another brand but its not on mouser so my chances of getting just a few are very low.

You guys know how it is when you make something and you get picky.  Im going to try and test the taper with the light hooked up and if it makes it more like a audio taper it might be ok.

[TheAncientOne]

I'm with Madox on "The Art of Electronics". I do electronic system repairs for a living, (if you can call it that), and Horowitz and Hill are still a favourite reference. The book is full of working practical designs, and well explained useful theory.

[kaleaf]

Thanks for the book info also.