I'm an analog guy and I need for a bit of logic/adc design work done, will pay. The design is essentially to sample the voltage on the wiper of a potentiometer, and use it to drive one of 48 relays, depending on the wiper voltage. The relays need to be driven in a make-before-break style. I don't want to run any clocks in the product if at all possible. Low-speed stuff. I reckon this is digital design 101 as taught in 1980.
Please private mail me for more information if this sounds like no-brainer stuff to you and you have the time and are interested.
I'm located in Carlsbad, CA.
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Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: "Mellow Yellow (The Electrical Banana)"
KG6RCR
How about the wiper drive one of the inputs to N comparitors, the other input of each of those N comparitors goes to a desired point on a voltage divider:
| | | | v v v v C1 C2 C3 ... Cn
If the taps are equally spaced, then set the hysteresis for all of the comparitors the same, and wide for "overlap"; each output controls a relay.
The LM3916 bar/dot LED driver looks like it might just fill the bill. Daisy-chain a few, add some relay driver ICs (what's a good 10-channel relay/lamp driver IC?) and I might just be up and running.
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Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: "Mellow Yellow (The Electrical Banana)"
KG6RCR
Oops -- Thanks! (Wrong chip, right family. The LM3914 is the linear one.)
I don't know how to write code, so programming a uC would require paying someone for that. And buying hardware to program the uC. I can't debug code, either, so I would need to keep bothering the programmer. I've had difficulty in the past with code that needed debugging a couple years later, only to find that the programmer guy was no longer available. A hardware implementation, like this simple IC approach, is something I can understand and debug myself. I guess I prefer to do things myself if possible using technology even I can understand.
Clocks and potential interference. The product will be a vacuum-tube high end audio phono preamplifier. I personally don't think that having an oscillator in this product will be a problem with halfway decent shielding. But the marketplace will have more trust in the product if it is dead quiet (EMI-wise) inside. High-end audio is odd that way. Far easier to not build in a perceived problem then try to defend challenges later.
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Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: "Mellow Yellow (The Electrical Banana)"
KG6RCR
Hello Mike, this chip has a logarithm scale. I don't think it can fit your application. I recommend a controller which only wakes up when the potentiometer will be turned. This would be a one chip plus relay driver solution.
I can't understand why you fear a micro-controller like "the devil fears the holy water".
The LM3914 has overlapping comparators and no hysteresis so for some of the pot you will have one relay on, some of the pot you will have two relays on, and the rest of the pot you will have one on and one buzzing with the slightest noise in the system.
And I thought is was going to be something sensible like a tapped inductor for an antenna tuner ;|
that is very easy one. Use a multiswitch with a lot of contacts that is the trick. You will not have to use any electronics only some 1 % resistors. You also can use a coarse and fine switch as 48 pos switches are rare.
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Of course I would do this if good-feeling 48 position rotary switches with four decks (stereo balanced signals) were available. The fine/coarse idea is interesting but not very user-friendly.
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Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: "Mellow Yellow (The Electrical Banana)"
KG6RCR
Hah. As if. Sensible -- if I wanted to do something sensible I'd have found another line of work. Or maybe not: I'm going back to college to get my Master's degree in social work. How sensible is that?
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Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: "Mellow Yellow (The Electrical Banana)"
KG6RCR
Well high end audio is more of a social than electronic phenomena.
If you insist on a non-processor solution I think I would use a 6 bit up/down counter clocked at a few Hz. Drive a DAC from the counter and slightly tricky dual comparators compare its output with a voltage from the pot. The comparators will produce up/down enables for the counter.
That would take about 5 chips and give you a 6 bit binary code which tracks the pot with no missing counts and a defined maximum slew rate.
The make before break is tricky and perhaps depends on how you can drive the relays.
7 off 3 to 8 line decoders like the 4051 will give you one of 48 outputs from a 6 bit code.
If the relays need drivers then integrating an R/C turn off delay on each driver might be the simplest. If the relays can be driven by 'logic' then it may be simpler to have two 6 to 48 line decoders driving the relays, one from the up/down counter and another from a latched copy of the counter delayed by one clock.
Hewlett-Packard do incremental encoders from 200 to 2000 increments per rotation, and while this isn't entirely clock free, it would probably generate a lot less noise than switching your relays on and off. Farnell list a respectable range of parts, from about $30
Farnell also list the Bourns EAW0J-B24-AE0128 7-bit absolute rotary encoder which encodes 128 different positions as an 8-bit Gray code onto 8 parallel outputs, which is as clock-free as you are going to get, and only costs about $12 in small quantities, but you won't have as much freedom to play with the "feel"of the knob as you would with the HP kit.
The rest of the circuit could probably be a Xilinx CoolRunner PLD - these are available with enough output pins to drive each of your 48 relays separately, though I'd go for a 2-D matrix which could select any one of the 48 with just 14 outputs.
Rotary encoders don't provide visual feedback about there the volume is set. A regular old potentiometer is generally connected to a knob, and the knob generally has a pointer. One can glance at the pointer to see where the volume is set before pushing "play" on the CD player, or lowering the stylus onto the record. All the rotary encoders I've ever seen go 'round and 'round w/o any real reference and w/o any pointer. The only way that I know of to provide visual feedback about where the volume is set when using a rotary encoder is with a display of some sort, generally a VFL dot-matrix thingy, which requires a micro to run and lots of other things to sort out.
As always, I could be wrong.
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Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: "Mellow Yellow (The Electrical Banana)"
KG6RCR
Then you want an absolute rotary encoder, like the Bourns part I mentioned.
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Put a collet knob with a pointer or a dot on the shaft of the encoder, and make sure that the shaft is sitting in the right place before you lock the knob onto the shaft by tightening the collet, and you've got your display and memory.
The running torque is specified as between 0.5 to 1.5 newton-cm (0.75 to 2.5 ounce-inches) so it should stay put if you don't explicitly turn it. You might want to put a sector plate on the shaft with a pin on the panel (under the knob) to prevent the knob rotating through 360 degrees, but you could probably sell the continuous rotation to your customers as a feature of a professional servo-quality rotation sensor .......
There's a problem when trying to do make-before-break with a matrix. Overlap of two relays is ok until the selection of the next relay requires a change to both a new row and column. Two rows and two columns being active will result in 4 relays being energised.
The only solution I can see is to have an R+C across each relay. Around about 100 ohms and 47uF will do a 5-10mS holdup.
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