single voltage source equivalent of multiple voltage sources

[sstimuluss]

bump.. please help, i've been looking for a solution to this problem for weeks now. Granted I can just use a battery per touch circuit, but there Must be a more elegant solution! My reasoning is that, apart from saving money, it would be better to be able to run all the touch circuits at equal voltage, so the current range for the LEDs generating midi controller data is within a defined range for processing on the computer, whereas using batteries results in variable ranges. I can't really use a 5 volt regulator on each touch circuit since 5 volts doesnt give enough current range throught the skin (because I'm using the first transistor as an amplifier rather than a switch) As I understand it, one needs to use a voltage source at least 2 volts higher than the regulator, so 5 volts seems like the highest option when using 9 volt batteries.
Ideally I would like to be able to power multiple touch circuits from a single AC-DC transformer with a regulator to smooth out ripple etc.
If my actual problem isn't explained well enough, please let me know and I'll try to clarify.

 

[Roff]

I finally understand your problem. The only solution I have thought of so far is much more complicated, involving either frequency or time selective circuits. How many pairs of electrodes do you have on the floor?

 

[Russlk]

The key is, that the negative and positive of the battery cannot be common. A transformer with multiple secondaries and rectifiiers (one for each circuit) is all that you need (or multiple transformers). That would be a custom design.

 

[sstimuluss]

The key is, that the negative and positive of the battery cannot be common. A transformer with multiple secondaries and rectifiiers (one for each circuit) is all that you need (or multiple transformers). That would be a custom design.

ah ha.. thanks! i've never designed a transformer circuit so will have to do some research.. maybe a stupid question, but is it possible to use secondary DC-DC converters instead of secondary AC-DC? or would that still result in a common ground?

if a custom transformer design ends up being too much work for me (i have deadlines for show unfortunately), I'll have to use multiple transformers..

i recently picked up seven or so 8.5V 60mA AC-DC adapters at the dump, however when reading their voltages with a multimeter, an open circuit measures 14.2V, a 1M resistor measures 13V, and a 1.5K resistor measures 9V
.. this isn't the same as another adapter i have which measures 9.15V no matter what.. doesnt seem right.. and a bit scared to use them in a circuit designed for 9v.. i'm guessing they're old cellphone chargers, which may use different transformer design to regular adapters

 

[sstimuluss]

I finally understand your problem. The only solution I have thought of so far is much more complicated, involving either frequency or time selective circuits. How many pairs of electrodes do you have on the floor?

I'll be using maximum of 10 pairs, probably around 6 or so would be more managable in performance..

I've pondered over that approach as well, but not yet tried going down that route.. to somehow step through the touch circuits one by one to take readings eh? that would eliminate the need for exclusive V+ and Gnd..
would it be possible to take this approach and not have the LED's flicker on and off? even the slightest ripple causes oscilation in the midi controller data, not good

 

[Roff]

I'll be using maximum of 10 pairs, probably around 6 or so would be more managable in performance..

I've pondered over that approach as well, but not yet tried going down that route.. to somehow step through the touch circuits one by one to take readings eh? that would eliminate the need for exclusive V+ and Gnd..
would it be possible to take this approach and not have the LED's flicker on and off? even the slightest ripple causes oscilation in the midi controller data, not good

Frequency division multiplexing:
Each circuit has an oscillator (relatively low frequency) connected to one electrode, and a tuned amplifier connected to the other electrode, and a synchronous demodulator following that receiver, the output signal being proportional to the resistance between the electrodes. You would then use that output to drive your MIDI transducer. The frequency selectivity would cause it to reject signals from another electrode, which would be at another frequency.

Time division multiplexing:
Generate N pulse trains with duty cycle=1/N, where N is the number of transducer pairs, and each pulse train would be at a different phase (time slot). You could then synchronously demodulate the pulse trains (the demod acts as a sample-and-hold), and feed each output to its respsctive MIDI transducer.

Russ's separate transformer windings and regulators might be easier. Your idea of isolated DC-DC converters should also work. It still boils down to separate transformers (inside the DC-DC converters).

 

[sstimuluss]

Frequency division multiplexing:
Each circuit has an oscillator (relatively low frequency) connected to one electrode, and a tuned amplifier connected to the other electrode, and a synchronous demodulator following that receiver, the output signal being proportional to the resistance between the electrodes. You would then use that output to drive your MIDI transducer. The frequency selectivity would cause it to reject signals from another electrode, which would be at another frequency.

Time division multiplexing:
Generate N pulse trains with duty cycle=1/N, where N is the number of transducer pairs, and each pulse train would be at a different phase (time slot). You could then synchronously demodulate the pulse trains (the demod acts as a sample-and-hold), and feed each output to its respsctive MIDI transducer.

Thanks for the info! fascinating.. will have to do a bit of research to fully understand what you're suggesting, but sounds worth my while to try designing such circuits regardless of which approach i end up using for this particular project.

Russ's separate transformer windings and regulators might be easier. Your idea of isolated DC-DC converters should also work. It still boils down to separate transformers (inside the DC-DC converters).

seems like the quickest fix would be to use isolated DC-DC converters, if all that's required is an IC that is..

 

[Russlk]

The simplist solution is multiple transformers, this is the circuit. Parts are available from Mouser https://www.mouser.com. The capacitor, C1, is probably not needed for your application. You could use the whole secondary but the output voltage would be double (about 18 volts).

 

[sstimuluss]

The simplist solution is multiple transformers, this is the circuit. Parts are available from Mouser https://www.mouser.com. The capacitor, C1, is probably not needed for your application. You could use the whole secondary but the output voltage would be double (about 18 volts).

thanks, i haven't mentioned i live in new zealand.. would this be a good alternative?

12.6V CT, 150mA 2VA Centre Tap - Type 2851 Transformer
**broken link removed**

also, would there be much fluctuation in the output voltage?

 

[Russlk]

Yes, that transformer would work fine.