Programable Digital rheostat-potentiometer project help - Page 1
Forum section: electronics / projects

author: looker
author_rank: Newbie
timestamp: May 02, 2012, 05:41:22 pm
content: Hi guys Im new, and a hobbyist. I have been away from electronics about 30 years I’m 61Im looking for any circuit Ideas for a Solar tracker tester. I’m hoping the combined braintrust of the EEV forum can help out a grateful old guy.Objective:Design a programmable digital rheostat to test my Solar Panel Tracker designs(I also have an audio auto-level circuit and a pick and place CNC project I could use this for too. lol)Environment:I live in an area which is a flight path for airplanes large and small and some overcast.I am using CDS cells and may use phototransistors to track the sun’s position. When It gets overcast or cloudy the resistance changes When a bird or plane flys over it is short duration resistance swing.Specs:The circuit can sense at 1 vdc to 9 vdc The resistance range needs to be from 1.5k to 50k, so its low current.Form factor:About the size of an Arduino and protoboard plus batteryOld TTL chip size is best for me I’m not quite ready for SMD microsoldering LOL.Investigation:I have a Arduino and have been looking at Analog Devices Digital Potentiometershttp://www.analog.com/en/digital-to-analog-converters/digital-potentiometers/products/index.htmlFor me the spec sheets have my head swimming. I’m used to Guitar amps and CNC stepper controllers Simpler stuff. I can sort of program in C for Arduino control, but the interface to the variable Potentiometer is where I’m stuck. It could be a stand alone chip too I just need a start stop button.Creeping elegance might include computer interface or LCD screen and/or maybe 5 or 6 buttons for small test segments, like bird, airplane, overcast.Cost factor:I would like to keep it under $150.Any Ideas are gratefully received.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 02, 2012, 06:03:15 pm
content: What is going to be the function of the digital pot? You're not going to send power through it are you?

author: looker
author_rank: Newbie
timestamp: May 02, 2012, 06:11:56 pm
content: HiThe digital pot will just take the place of a photo resistor sensor It will need to sink some power probably worse case 50-100ma. More likely to be much lower in the micro amp range.I thought about using one of the PWM or analog ports on the Arduino and using a transistor to sink the power but for some other sensor circuits like a Pick and Place micro production line I just thought a programable pot might be more useful as a multi use tool, and easier to wrap my head around.For example with a programable pot I'm hoping I could actually measure the resistance with a VOM to get an accurate reading before I connect it to the circuit.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 02, 2012, 06:41:12 pm
content: So the pots will be used to simulate the photoresistors during testing, then?You're going to have trouble finding one that will pass that much current. But assuming you do, there is a ton of code out there on the web for Arduinos. I'd be amazed if you can't find some Arduino code to talk to a digital pot.There's not much to them, really. All you can really do to them from a programming standpoint is set their wiper position and, perhaps, read the current wiper position. The interface for doing this is usually one of the following: I2C, SPI or some proprietary serial interface.I've attached a ZIP of some code I used in a few projects. It is for a specific MCU and a specific digital pot so it won't be of direct use to you but will give you an idea of the complexity involved (very little.)

author: looker
author_rank: Newbie
timestamp: May 02, 2012, 08:50:06 pm
content: Thanks for the zip fileOk, I think I coulda searched better before posting.I found an Aduino digital pot interface herehttp://arduino.cc/en/Tutorial/SPIDigitalPotLooks like a nice 6 channel pot which lights up 6 LEDs in 256 sequential steps.I ordered the chip from Digikey so I can play to my hearts content.description:In this tutorial you will learn how to control the AD5206 digital potentiometer using Serial Peripheral Interface (SPI). For an explanation of SPI see the SPI EEPROM tutorial.Digital potentiometers are useful when you need to vary the resistance in a circuit electronically rather than by hand. Example applications include LED dimming, audio signal conditioning and tone generation. In this example we will use a six channel digital potentiometer to control the brightness of six LEDs. The steps we will cover for implementing SPI communication can be modified for use with most other SPI devices.Thanks for the reply. Thanks to the Holy Internet, it Looks like it wasnt as difficult as I had immagined.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 02, 2012, 08:56:34 pm
content: That's the nice thing about Arduinos: there's always code out there for what you need.