i need a voltage controlled potentiometer - Page 1
Forum section: electronics / projects

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 11, 2018, 01:51:20 pm
content: Hi, i need a voltage controlled potentiometer,a digital potentiometer does not work for me, i need it smooth without steps and with voltages higher then 5v.I have this LM13700, can i use it as a voltage controlled potmeter ?And what would be the schematic to use 2 transistors ?, 1 NPN and 1 PNP maybe to simulate the potmeter center pin ?btw : i need it in 50Kthanks

author: Yansi
author_rank: Super Contributor
timestamp: February 11, 2018, 01:58:16 pm
content: Please specify more specifically what you are trying to achieve. You seem to be trying to do a stunt that may not be needed at all for the application.Please describe the circuit you need this in, preferably include schematics too.LM13700 can definitely be used as a VCA, but it all depends on the application.

author: amspire
author_rank: Super Contributor
timestamp: February 11, 2018, 02:19:06 pm
content: If you can describe the application, you will get much better suggestions.Otherwise, we have to ask many questions to narrow down what you need. Accuracy, distortion, peak voltage, peak current at low ohms, frequency, DC offset, stability, and so on.It may be that in your application, there are far better ways to achieve the result you want then a voltage controlled resistor or potentiometer. For example, if you want the potentiometer to give you a voltage controlled gain, you may be able to use an IC with voltage controlled gain instead.Active circuits that simulate passive components can work for some very specific applications, but they don't come close to matching resistors, inductors and capacitors in all respects.A simple voltage controlled resistance would be a light dependant resistor controlled by a LED. Not accurate, but it works and it is very cheap.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 11, 2018, 02:25:38 pm
content: Ok, i need to replace 2 potentiometer in this schematic, both should do the same trick.In the schematic its the CUTOFF FREQ. and the ENV.MOD pot,http://www.martianarts.net/web/images/stories/ourgear/roland_tb303_schematics.gifhere a better schematic maybe :http://www.ladyada.net/media/x0xb0x/mainboard_beta.pngThat its type A potentiometers dont matters, i will change the curve to logarithmic in software.I also need to replace the stereo resonance pot, there i plan to use the LM13700 since it need audio passing thru.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 11, 2018, 02:31:25 pm
content: By the way for the ENV.DECAY i used a simple 2N3904 transitor, works the same as the 1M potmeter.Quote from: amspire on February 11, 2018, 02:19:06 pmA LDR ?, yes i still have to buy it to try, then i need a dark enclosure also or something, dont they sell ready to use parts ?,also i need the center pin from the potmeter.Quote from: amspire on February 11, 2018, 02:19:06 pmThey still have to invent something like that.

author: Yansi
author_rank: Super Contributor
timestamp: February 11, 2018, 02:48:55 pm
content: They do not, as there are many solution for voltage controlled circuits, including VCA, filters.Why do you need to replace a mechanical potentiometer with a voltage controlled one in the first place?

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 11, 2018, 02:53:05 pm
content: To make my synthesizer MIDI controlled.

author: ogden
author_rank: Super Contributor
timestamp: February 11, 2018, 03:13:01 pm
content: Quote from: JanJansen on February 11, 2018, 02:53:05 pmMIDI is not "smooth without steps" and it is not voltage controlled.You can use 8-bit digital potentiometer (256 taps):http://www.microchip.com/design-centers/analog/data-converter/digital-potentiometer

author: amspire
author_rank: Super Contributor
timestamp: February 11, 2018, 03:24:21 pm
content: You have two ganged potentiometers. They are both involved in feedback loops in an amplifier. This is one of those cases where simple resistors can behave much better then active circuits. In the case of the left hand gang in the original circuit, the potentiometer is basically acting as a voltage divider. A LM13700 circuit could work, or you could use a voltage multiplier IC like the AD8338 which actually has a log response in the voltage control input (12mV/dB gain).The right gang is more difficult as it is a resistance as part of a peak detector circuit that is very intimately tied in to the quirky way that circuit works. Trying to change it will probably change the effect to some degree. It is modulating the frequency of the voltage controlled filter based on the peak Accent waveform. The actual resistances of the potentiometer and the wiper position are part of the current circuit.I would seriously suggest looking at the 8 bit digital Microchip potentiometers odgen suggested as they probably will work in that circuit and you can get 50K versions..Otherwise, the simplest solution may be to use a motor controlled dual gang potentiometer, if that works for you. They are available, or you could use the existing pot connected to one of the $2 geared stepping motors.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 11, 2018, 03:35:51 pm
content: Quote from: amspire on February 11, 2018, 03:24:21 pmI will take a look at this AD8338, and yes this is a tricky one.Maybe there is some way with 2 transistors ?, 1 NPN and 1 PNP, with in between the center potmeter pin ?,i dont know exact how these NPN/PNP complementary transistors work, and what you can do with 2 complementary transistors.For the decay i used a transistor, and gives some more range then wanted, there i can select the range within software, make a 7 bit range out of 8bit DAC or 10 bit PWM signal,then i dont have the to short and to long decay tones to match the original synthesizer, ofcourse still accessible with another control number.Quote from: amspire on February 11, 2018, 03:24:21 pmA motor controlled potmeter, this is really funny, i bet they also make noise,i am sure there will be a better way, i,m not going for motor potmeters, anyways thanks for the hint, its really funny.Quote from: ogden on February 11, 2018, 03:13:01 pmMIDI will be smooth with a capacitor on the DAC or PWM control voltage.The digital potentiometer will give audible steps, its break before make and not smoothed between the steps, also it works on 5 volt, and cant take any higher voltages because of the gates inside.

author: ogden
author_rank: Super Contributor
timestamp: February 11, 2018, 04:06:45 pm
content: Quote from: JanJansen on February 11, 2018, 03:35:51 pmFETs are voltage controlled resistors:https://www.vishay.com/docs/70598/70598.pdf

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 11, 2018, 04:09:17 pm
content: I have some FETS, only no negative supply voltage, how would i connect them then ?

author: ogden
author_rank: Super Contributor
timestamp: February 11, 2018, 04:22:02 pm
content: Quote from: JanJansen on February 11, 2018, 04:09:17 pmAdd negative supply : ) Other option would be to use NMOS.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 11, 2018, 05:16:26 pm
content: NMOS is N channel MOSFET ?Any suggested part and connection diagram ?Are MOSFETS always very big ?, i need the smallest one.I only have JFETs not MOSFETS, new for me, should do the same i readed.

author: amspire
author_rank: Super Contributor
timestamp: February 12, 2018, 12:06:29 am
content: Quote from: JanJansen on February 11, 2018, 03:35:51 pmMotor controlled potentiometers are pretty quiet. They were huge in the 1990's when remote controls for audio HiFi amplifiers were the rage and they were not using digital control. They were quiet but slow.Professional mixer consoles use slider potentiometers with motors. The idea is that the audio engineer could dynamically change settings during a mix and record the slider positions. Then the engineer can replay the mix and all the sliders move automatically. I think a common way to do this was to have an extra potentiometer track in the slider to sense the position. If you want to search for these, they are often called Motorized Faders.My concern for that circuit is that the potentiometer you want to change is involved in the Voltage Controlled Filter circuit, the Voltage Controlled Amplifier circuit and the Accent control. If you fully worked out what is happening, there may be simple solutions to replacing the control, but otherwise any attempt to replace it with an amplifier circuit may change the behaviour of the device. The voltage controlled filter circuit is interesting. They are using diodes (transistors with base-collector shorted) as variable resistors. They form a chain of RC filters probably resulting in something like a 20B/octave lowpass filter and the frequency is controlled by the current through the diodes (controlled by Q10). The resonance control is feeding back the output of the filter to the input so that it rings and the control is also modulating the lowpass cutoff frequency.This was probably designed with a massive amount of trial and error to get just right. That is why changing the circuit is definitely not trivial.Doesn't mean you don't try.

author: ogden
author_rank: Super Contributor
timestamp: February 12, 2018, 12:51:23 am
content: Quote from: JanJansen on February 11, 2018, 05:16:26 pmRight. N channel MOSFET.Suggested part could be 2N7002.Connection diagrams are all over the internet.Believe me, you don't need smallest one. SOT-23-3 shall be fine for you. You really shall learn parametric search of mouser.com.

author: BrianHG
author_rank: Super Contributor
timestamp: February 12, 2018, 04:53:33 am
content: You can use photo-fet optocouplers:http://www.mouser.com/ds/2/149/h11f1m-185284.pdfBut, I would go with more modern I2C controlled pots as mentioned above if I had to stay in the analog realm.I would rather use a single 4$ DSPic for your app. Have it's ADC sample optional analog pot inputs, send to MIDI. Use it's RS232 for working with the MIDI and use it's DAC to synthesize the output from ADC or MIDI source to a headphone amp.No VCO, no VCA, no mixer, do all of this in software as these things are simple to code today if you are already capable of programming your own MIDI controller.

author: amspire
author_rank: Super Contributor
timestamp: February 12, 2018, 05:02:53 am
content: Quote from: BrianHG on February 12, 2018, 04:53:33 amI have a suspicion that that analog circuit probably has a Resonance control that is extremely touchy and that would mean 8 bit resolution is nowhere near enough. It could also mean a very stable and repeatable solution is wanted.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: February 12, 2018, 07:23:24 am
content: I have looked into this myself for a potential project many years ago (I wanted to automate guitar pedals/stomp boxes). The best/cheapest option, I found, is to go with a FET based solution.Use the opto-isolated variant (as suggestedor other) if speed is not very important (in audio it usually is not) and you want to keep the target circuit and controller circuit separated, which is always a good idea IMO.You need to put a small micro in between the MIDI input and the FET driving outputs in order to correct for small differences in FET sensitivity (and other component variations) and to be able to adjust the curve to be either linear or logarithmic. Adding some extra brains will also give you more MIDI features to play with (invert, offset, curve etc) in the future.My design used two FETs to mimic a potentiometer. The micro calculated how to drive each of the FETs to get a certain resistance value for the pot-center-tab. Depending on how closely the FETs were match, could differ a little bit. Use low-tolerance components for the driving circuit.I faked the DAC you will need (to drive the FETs from software) with a powersupply so I have no advise on that. Will you need a high precision DAC or will a R-2R network do? See if you can find a micro that has on board DACs (not very common, but existing) - or a couple of cheap DAC ICs.

author: Rerouter
author_rank: Super Contributor
timestamp: February 12, 2018, 11:02:27 am
content: For higher voltages / currents, you can use a dac with an external reference voltage pin that allows down to 0V, The using an op amp and a pass element, you can make a constant resistance circuit.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 12, 2018, 02:52:12 pm
content: First of all i am using the PIC16F1708, it has 20 pins, 1 x 8 bit DAC and 4 x 10 bit PWM.Second you cannot make this in digital, else i was not making this.I also made a virtual analog synthesizer, the thing i found out is the digital taking more milliamps.@ obiwanjacobi : how did you connect the FETs ?, any schematic for me ?, i dont have negative power supply, a pedal also dont have negative supply.

author: InterestedTom
author_rank: Regular Contributor
timestamp: February 12, 2018, 04:00:58 pm
content: http://www.ti.com/lit/ds/symlink/lm13700.pdfp16 figure 28If you are getting marked for this it doesn't matter whether your supervisor/teacher sees this page, they will know you don't understand how it works, you are just copying others work.Instead of panicking and being pushy about exactly what to do, use your head, and read the datasheet for the part in question.I know that this is only one half of a potentiometer but hopefully you will think of a simple way to derive a second Control Voltage from your DAC. This way you will have thought of something for yourself and you will learn how to design electronics.The other suggestions will work just as well, probably better: your PIC probably has at least one I2C or SPI peripheral, that would be used to control the 8bit potentiometer. 8bit pot vs 8bit DAC is pretty even, but the LM13700 specifies 0.5% distortion at best, why would you risk the existing sound of your synthesizer getting worse? Try setting a single turn pot to 1/256 using a multimeter, then realise that 8bit is pretty damn smooth even with steps. You can filter out resistance steps too, smooth them out a bit at the very least...One thing to be aware with both circuits I have mentioned will have a limit on input voltage that may be lower than the voltages in your synthesizer. But different part numbers will have different limits, read the datasheets...

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 12, 2018, 04:26:26 pm
content: Quote from: InterestedTom on February 12, 2018, 04:00:58 pmOk, can it be made on single supply ?Are the input and output where the calculation is in between ?The calculation is : RX = 2R / GM RAwhat should i do to get 50K ?, i,m not a professor or schooled.Ok the control voltage = Vc, only they are connected to 2 circles, what is that ?

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 12, 2018, 04:34:46 pm
content: by the way i have this idea :using a PWM from 0 to 5 volt, connect that somehow to the variable voltage and the 12 volt, how can that be done if that is possible ?Quote from: InterestedTom on February 12, 2018, 04:00:58 pmI noticed when building a MIDI controller, thats why i use a 10bit PWM for the tuning, 10bit PWM into hexinverter on 5,333v then smoothed in rail to rail opamp also on 5,333v,there you have it i have done something myself.Quote from: InterestedTom on February 12, 2018, 04:00:58 pmThe problem is the DAC in the PIC16F1708 is not totally linear, so it has to be done with resistors somehow, i will ask in the forum how to do that, i have done enough already and am always busy with stuff that dont need external brain processing.Quote from: InterestedTom on February 12, 2018, 04:00:58 pmYes i use the forum first before thinking and reading and searching, saves so much stress.Please dont start a square wheel discussion ( again ), square wheels dont exist.Do i really need to re-invent the wheel ?, maybe a square wheel will exist then.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: February 12, 2018, 04:54:02 pm
content: Quote from: JanJansen on February 12, 2018, 02:52:12 pmNo schematic, sorry - I experimented a lot with several different options at the time, building and testing parts and pieces. I just remember FETs being the most successful in those experiments. Like I said I think I faked driving the FETs with a separate (bench) power supply at the time. But the problem was not -not having a negative power supply- the project was going to power the pedals (PCBs) and not the other way around, so I anticipated a beefy power supply - also allowing to 'over clock' (supply more than the usual 9v) some pedals that could handle that - gives you more headroom and some pedals react well to that - its a tone thing.Could you not use a negative dc-dc convertor to generate a negative rail from the positive supply you have?Tip:MIDI has double precision (14-bits) commandsyou could use if you need more resolution.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 12, 2018, 05:04:12 pm
content: I used 14 bit pitch bend in my VA synth.NRPNs i think are difficult to edit in Cubase,normally in my synths i use 2 CC values :Code:cutoff = cutoff1 + cutoff2;And if i have 10 bits (PWM) i use another controller to have 4 finetuning settings.

author: InterestedTom
author_rank: Regular Contributor
timestamp: February 12, 2018, 06:09:03 pm
content: Sorry for being brisk earlier. I will do my best to get back to your questions soon.

author: InterestedTom
author_rank: Regular Contributor
timestamp: February 12, 2018, 09:36:15 pm
content: Quote from: JanJansen on February 12, 2018, 04:26:26 pmYou are correct so far. The two circles show that the LM13700 has a current mode output, it's a standard symbol for a current source. gm is the transconductance which is Iout/Vin. In the LM13700 the transconductance is current controlled, the resistor converts Vc to a current and that controls the transconductance. I suspect that reducing the resistor value there would allow you to apply 5V directly as Vc. There should be an equation in the datasheet for calculating that current value and the transconductance from that. The datasheet should also help you choose a resistor for the Vc input resistor.If you use another LM13700 ic in the same configuration but with PWM of the opposite polarity, as I think you suggested in another post then this can function as the other half of the potentiometer.To generate the negative voltage would it be acceptable to use a DC to DC converter?

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 13, 2018, 03:05:10 pm
content: Thank you Tom,i like to avoid those switching converters, not only are they very expensive and sensitive for damage, they also bring switching noise.For most of my projects i use single supply, this Roland synthsizer is also single supply, thats why i like to build it, one of the rare occasions where no negative supply is used and no CEM IC`s are used.I also have some 15V AC adapters from ebay, to make dual supply, still have to get busy with that, the bad thing is the AC adapters are no longer for sale in new condition.Normally there is always a way around dual supply, only for DC currents it wont work, so i,m not sure for this case.

author: InterestedTom
author_rank: Regular Contributor
timestamp: February 13, 2018, 05:02:43 pm
content: The LM13700 won't work below 9.5V according to the datasheet, is that a limitation?I think you would be better off either way using a 1024 tap (10 bit) digital potentiometer, or PWM filtered to control some MOSFETs.You can get digital potentiometers that take an analog supply voltage as well as a digital one, allowing the potentiometer to be connected to a higher voltage than 5V.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 14, 2018, 02:44:22 pm
content: Oh, i need such a digital potentiometer then.I,m going to find out how to use that MOSFET, thanks.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 20, 2018, 01:50:29 pm
content: What is the equivalent for the 2N7002 in normal package then ?, this one is only for SMD it seems.

author: ogden
author_rank: Super Contributor
timestamp: February 20, 2018, 02:43:51 pm
content: Quote from: JanJansen on February 20, 2018, 01:50:29 pmIn 21th century normal package is SMD!2n7000 is more or less similar to 2n7002 with slightly higher Gate Treshold Voltage. When planning to use mosfet as variable resistor- don't forget to study gate treshold voltage versus temperature curve and think what you will do about it, how you ensure that resistance will not float around when temperature of your thing changes (attach).

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 20, 2018, 03:47:36 pm
content: I am watching these movies on youtube now, it seems they are not linear adjustable from 0 to 5 volt.Now i readed something nice on wiki : for artificial intelligence they need VCRs voltage controlled resistors, when will they be in dip package ?

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 20, 2018, 03:56:14 pm
content: For digital potentiometers i cannot find any in DIP package that allow higher then 5volt signals :https://eu.mouser.com/Semiconductors/Digital-Potentiometer-ICs/_/N-4c498?P=1yzmno6Z1z0z63xZ1z0t3csZ1z0jdih&Ns=Pricing%7c0

author: ogden
author_rank: Super Contributor
timestamp: February 20, 2018, 03:57:06 pm
content: Quote from: JanJansen on February 20, 2018, 03:47:36 pmIndeed they are not linearBesides watching youtube videos, you have to read datasheets as well. Input/output relation specs are first you shall try to look for, in the documentation of components like voltage controlled something. If you want repeatable, linear, adjustable resistor without quite complex curve & temp compensation circuit, more or less you have one choice - digital potentiometer.

author: ogden
author_rank: Super Contributor
timestamp: February 20, 2018, 03:59:34 pm
content: Quote from: JanJansen on February 20, 2018, 03:56:14 pmWelcome to 21th centuryYou really shall consider using adapters like those:https://www.sparkfun.com/products/13994

author: ogden
author_rank: Super Contributor
timestamp: February 20, 2018, 04:08:26 pm
content: You shall look into AD5291/AD5292, obviously with according ssop to DIP adapter.

author: JanJansen
author_rank: Frequent Contributor
timestamp: February 20, 2018, 04:51:38 pm
content: Thanks, i found this now :quadhttps://eu.mouser.com/ProductDetail/Analog-Devices/AD5263BRUZ50?qs=sGAEpiMZZMuD%2f7PTYBwKqdeb0s0H1yW0NP0AKzoWprc%3ddualhttps://eu.mouser.com/ProductDetail/Analog-Devices/AD5262BRUZ50-RL7?qs=sGAEpiMZZMuD%2f7PTYBwKqdeb0s0H1yW01n79LQfaP4s%3dVery expensive, i have to try it,how does it work : do they break before make ?, or do they keep the old gates on until the new ones are on ?, else i might have to use 2.i will order one for testing if i have enough money.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 21, 2018, 03:56:43 am
content: Motor-controlled volume control? LDR?