Replace physical potentiometers with digital ones - supply voltage - Page 1
Forum section: electronics / general

author: obiwanjacobi
author_rank: Super Contributor
timestamp: May 02, 2012, 02:21:22 pm
content: Hi,I'm looking into replacing physical potentiometers with digital ones in analog audio circuits (guitar effects).The problem I'm facing is that all digital pots work on a 5V supply voltage - while most guitar effects work of a 9V battery.I foundone Maxim ICthat could handle +/- 5V but that was marked as obsolete :-(I could not find any other digital pots with dual/symmetric supply ranges...As a work around I thought about using two LDR's and two leds to simulate a pot (2 LDRs in series middle is wiper). The advantage of this would be that it would provide separation between the driving circuit and the analog effect circuit, voltage would not matter because of its passive nature and I will use an MCU (Arduino) to drive the two leds is such a way that it would simulate a specific wiper position (using PWM?). Problem with this solution is probably the cost and physical construction as well as the fact that I need a dozen or 2 of these pots...Please let me know what you think.

author: baljemmett
author_rank: Unknown
timestamp: May 02, 2012, 02:52:54 pm
content: Quote from: obiwanjacobi on May 02, 2012, 02:21:22 pmNot sure if this helps, but I've used the AD5260 in a project before -- it's specified for up to 15V on the pot terminals. It certainly worked happily hung between 0 and 12V rails in the design I used it in.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: May 02, 2012, 05:13:29 pm
content: QuoteOuch, they're a bit expensive!But thanx for the tip.

author: LEECH666
author_rank: Frequent Contributor
timestamp: May 03, 2012, 08:12:38 am
content: This might not be the most efficient solution for a battery powered device but why don't you just use a fixed voltage regulator like the LM 7805 to create a 5V supply from the 9V of your battery?If you want a more efficient solution because you want to optimize your battery life you probably would have to look for a step down switchmode regulator (they are also called buck converters / regulators etc.).Florian

author: obiwanjacobi
author_rank: Super Contributor
timestamp: May 03, 2012, 08:30:54 am
content: Oh, the effects are not gonna be powered of a 9V battery anymore - and most effects can be run on a 9V adapter too. I plan to take out the effect's PCB and integrate several of them into one housing. So a normal power supply is possible, I can build all the voltages I need. So far I need +/-15V for some opamp buffers, +/- 7.5V for some analog switches and +5V for the logic ICs.The pot has to be integrated into the existing schema.Click here for an example.Thanx.

author: T4P
author_rank: Super Contributor
timestamp: May 03, 2012, 03:18:19 pm
content: Use a +/-15VDC DC-DC Boost Converter and there just divide the 15v into 7.5v for analog switches and then use 7805 for the logic

author: obiwanjacobi
author_rank: Super Contributor
timestamp: May 03, 2012, 05:13:11 pm
content: - Either you're not getting it - its not a power supply issue/question (in terms of how to build the supply itself).- Or I'm not getting it - I thought that +5V supplied digital pots could not be used in +9V supplied circuits (that is what I get from the datasheet).Which one is it?

author: amyk
author_rank: Super Contributor
timestamp: May 04, 2012, 06:52:53 am
content: Yes, if you look at the datasheet you will see maximum voltage range between H, L, and W pins and ground. Usually GND-0.3V to Vdd+0.3, due to the CMOS structure (parasitic diodes).Digital pots are still much more sensitive and fragile compared to physical ones, and if you need to handle large power dissipation and voltages it may be better to use a DAC and discrete MOSFET.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: May 04, 2012, 08:13:52 am
content: I wondered about that (could I use a transistor to fake a resistor?).I guess I would need 2 MOSFETs and couple them with some kind of an inverter? - to get a wiper 'signal' of the middle...?What would that look like?Thanx!

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2012, 08:03:04 am
content: I don't see why this can't be done with careful design.The simplest solution would be to use the Maxim IC you pointed to and run the whole thing off a+/-5V power supply.Another solution is to use and digital pot, run from the appropriate supply voltage (+/-5V, 5V or whatever) and just make sure the analogue signal passing through it doesn't exceed the maximum ratings. For example, you could use a 5V digital pot, bias the analogue side at 2.5V and AC couple the audio signal through it, which will be fine as long as the peak voltage doesn't exceed +/-2.5V. If you used the Maxim IC you're talking about you wouldn't need to AC couple, as long as you ensure the input voltage doesn't exceed +/-5V.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: May 05, 2012, 02:33:35 pm
content: But I need to use the digital pots in schemas I have no control over.See example of 4 pots.I don't think I could figure out how to do all that for all the pots of all the effects I want to control...