Looking for a way to remotely measre 12v and 24v with a single measuring wire

A resistor and a diode could provide you a source that is 24 volts if unloaded, and drops to 12 when loaded. On the DVM end, the load could be a resistor and a pushbutton.

Something fancier could alternate the voltages.

But if the 12 volts looks OK, isn't it very probable that the 24 is OK too?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Oh, then you should have marked it OT: :-)

John's resistor + diode suggestion is good.

--
 Thanks, 
    - Win

A timer and a relay to switch between the voltages.

Fundamentally you'e going to need some sort of multiplexing approach. You could do it with a chopper circuit (switching that one line back and forth between the two voltages) and measure the resulting waveform indoors.

It might be simpler to move the measurement out to the gate. There are any number of little flea-bite microcontrollers that have onboard analog-to-digital controllers, and have multiple pins that can select the voltage to be digitized. You could:

- Power one of these from the voltages in question, maybe through a pair of diodes and a 78L05 regulator (the micro would run as long as either voltage is "live").

- Use four resistors to form two voltage dividers, to step down the 12 and 24 volts to a range that's within the micro's limits.

- Write a bit of code to digitize the voltages (alternating between them) and send the resulting values back to the house by using the micro's UART pin to transmit ASCII or binary.

An oscillator (could be a 74HC14 based, but a '555 would work too) can be powered from one supply to generate a 1 kHz tone, and from the other to generate a 10 kHz tone. Let each oscillator free-run, and you can use a filter and an earbud to pick up the tone of choice. from the main power cables if you like (a series transformer that saturates, but doesn't burn up, when major current is drawn would facilitate that).

The old LM567 tone decoder, too, could pick up its programmed frequency from the midst of noise, and flash a 'power good' light, if you want to use that 'spare' wire. You could add 'gate open', 'locked' and other indicators, just arrange a little separation between the different frequencies.

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If you are going to get that complicated, just toss a tiny MCU at it. Have it measure the two voltages and send ASCII data over the wire.

The resistor and diode is probably the best analog solution. It requires n othing to measure the voltages but a volt meter and a resistor. Too bad th e resistor is needed. The MCU approach requires even more circuitry at the receiving end, at least a USB to serial port cable and a phone/computer, b ut way more cooler than a meter.

Or if you want to get a bit fancier, spend another $10 and use an MCU modul e that has Bluetooth or WiFi built in and connect wirelessly! Wifi won't w ork at 150 feet, but Bluetooth will. My car remote is Bluetooth and it has an amazing range (it's needed for summon mode). Pull the extra conductor out of the cable and scrap it to help defray the cost of the MCU board.

You can tie it to your house computer and make it available over the Intern et. Let others see what your day to day gate voltages are. Goolgeads will help you pay your electric bill to keep it all running.

--

  Rick C. 

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How about using an R2R ladder circuit and two opto-couplers. Bias them so that the opto's are ON when the voltage is present.

Then, read directly off the R2R.

This arrangement would reliably read all (4) possible states of the two voltages at the gate, and return the result over one wire back to the house.

yeah, and you can set up a bar-graph LED display with both-good lighting GREEN, one-good lighting YELLOW, and neither-good lighting RED. Those indicators are the key to getting user comprehension of the info on display (the resistor/diode takes some notebooks and a voltmeter; not for everybody).

When half an experiment is locked behind radiation shielding, the general solution is a videocam. Anyone can check positions, settings, and readouts who knows which channel on the site feed covers their 'hot' side, and it doesn't take training for the drop-in users. It also doesn't have to be pre-planned specific to the latest changes. Heck, leave a business card in the field of view with your cell number just in case...

It's a bit long-winded, but if you could take a few turns of the supply wires through a transformer at each end and superimpose A.C. on them, you could return a signal up the power wiring without needing any additional wires. A pair of two-transistor multivibrators, one on each power rail, could generate different audio frequency signals -- the amplitudes of the signals could then be detected by selective amplifiers at the house end.

The cheapness or otherwise would depend on what you had lying around in your spares box.

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk

Or a relaxation oscillator between the +24 and +12

7555 or something like that.

--
  Jasen.

Use a resistor divider between 24V and 12V. You should get 18V. Send the 18V back to the hose and set up to comparators in your house to monitor over18V and under 18V

One approach would be to use a latching 12-V DPST (Form C) relay. Connect the pole and a 1-uF ceramic cap to the long wire, and connect the throws to +12 and +24. Wire the coil as shown.

sense 10k 0----+12V wire

0----*---RRRR---O | \ CCC / 0----+24V CCC 10uF / | / C / O / I L | GND

And on the other end, a couple of pushbuttons and a DVM:

+24V S1 | *-> | |-- *-> | | | 0---------*------------------* | S2 | *-> | DVM |-- DVM *-> | | | GND +12V

Push one button for +12, and the other one for +24. Don't push both at once. ;) (To get fancier, you can use a MOM-OFF-MOM toggle switch instead of the pushbuttons.)

That way, you put a nice beefy pulse on the sense wire that switches the relay, then after a second or so you can get a good measurement.

Make sure the coil is the right way round--you want a positive pulse to make it connect to the +24V side. That way, when it's reading +12 and you push S1, you put +12V across the coil, and it'll switch to reading

+24. When you push S2, there's +24V on the cap, so the coil sees a pulse of -12V and so switches to measuring +12V. You could also just ground S2. That'll overvolt the coil a little going from 24V to 12V, but that shouldn't hurt anything much.

You may need to experiment with the size of the capacitor. A 9-volt relay would get you a bit more margin, which helps when pulling this sort of stunt. Obviously this will stop working at some point when the supply fails or the batteries get too flat.

I once did something like this with two relays so that I could bootstrap the coil-to-contact capacitance in an ultra-high-Z front end.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Since the 12 is derived from the 24 you have 3 states no 24 and therefore no 12

24 but no 12 24 and 12

Take the 24 and run through a 4:1 divider and a diode and say a 10K load in at the house. If 24 is good you read about 6V - diode drop. Take the 12 through a diode and tie the 2 diode K together.

no 24: you read zero across your 10K load

24 but no 12: you read about 6 across your load 24 & 12: you read about 12 across your load

--
Chisolm 
Texas-American

Nice. I was thinking of two current setting R's on the supplies, and then a current measuring resistor back where you do the measurement. (You only measure the sum of the voltages then... but maybe that would be enough.)

George H.

Ahh good. I knew there was at least one difference circuit, but I was thinking the wrong way.

GH

If you use a 3x Form C (3PDT) relay, you can use the other two sections to flip the coil around. Then you'd only need one button.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I actually read the responses, here but not there wherever there was.

We have to assume the OPer has a reason for wanting to see both voltages. W hy else ?

As such I got an idea. First of all that free wire is not likely to have DC leakage. At least not much. So both the 12 as 24 could go through a resist or, and then diodes, the drop of which would be calculated on the other end . (there are ways to build this in)

So you take a flip flop, or "bistable multivibrator" and a set of diodes. E ach goes through the resistors and then to the collectors of the transistor s in the flip flop. Then they go to those diodes. You can get Shottkys if y ou want or just calculate it, or you oculd just take the meter - to the sam e diode drop from common and correct it as it sits.

How this "free wire" always sees an open circuit pretty much just going to a meter. Even a panel meter bare is fairly high input resistance. So you ha ve a switch that takes the line to ground/common to trigger the FF outside and get the other reading.

So if for some reason he wants to stay on one setting he can by simply not pressing the button again. Let's say it is peak time and since there are ba tteries involved it is more important to monitor them. The regulator he has probably has a feedback loop that keeps it constant, so any big changes th ere do indicate a fault. But at certain times one or the other could be mor e important.

All the shit costs minimal. Couple transistors, small caps, a dozen resisto rs tops. A way to build the thing, they got ways for one offs. Those boards that emulate breadboards, five bucks I think for a size that'll probably d o three of these.

If automatic switching is desired it is easily put by the user switch. An o ld style relaxation oscillator would do just fine. It would also facilitate , if desired, showing both by simply multiplexing them.

It is a little unusual I think to refer to such a scheme or DC as that but what other word is there ?

how would you get >18V for either supply failed? The idea looks workable, but the compare levels don't look right...

-bill m

OPEN output failure: If 12V fails you get 24 volts from the divider; if 24V fails you get 12 volts from the divider; if both fail you get 0 volts from the divider.

SHORTED output failure: If 12V shorts you get 12 volts from the divider; if 24V shorts you get 6 volts from the divider; if both fail you get 0 volts from the divider.

If both work, you get 18 volts from the divider.

Unspecified failure = unspecified output from divider.

Ed