Need Help! Measuring 3 battery's voltages with 1 Voltmeter

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Why can't you just use a 4 position rotary switch?

Bat1, Bat2, Bat3, OFF...

Well, to start with, you need to debounce the pushbutton. I'd recommend a '555 for that, probably one set to give a 3-second pulse (this is the 'monostable' configuration). Then, you need a counter to go from 1 to N (the number of batteries); it just counts the number of times the pushbutton has been depressed. I'd think in terms of a CMOS logic counter for that, maybe CD4017 is a good part number. Connecting the '3' output to the RESET input will make it count 0 ... 1 ... 2 ... 0 ... 1 ... 2

Getting the counter to start at '0' when you power it on, is left as an exercise.

Metering the batteries is possibly going to require a load resistor, but if it's just a matter of voltmeter switching (not switching the full load into the resistor, just the trickle into the voltmeter) then analog switches ( a kind of solid-state relay) will work; CD4016 has four of 'em. You have to be careful, though, the CD4016 has to be powered from +15V or so (the voltage it switches must be between the power supply and ground potential).

If you want the voltmeter disconnected after the three-second measurement window, an inhibit-able analog switch (CD4051) is a good choice; it can take the '0', '1', and '2' outputs from the CD4017 counter as its address A0, A1, A2 inputs, and will connect the lines indicated as channels 1, 2, 4. The output from the '555 can run its 'inhibit' input, to disconnect the switches after that 3-second window.

Yes - go to digikey.com and search on "rotary switch"; go through their parametric selector and you'l probably find exactly what you want.

Good Luck! Rich

(...)

These batteries are not connected in series by any chance? If *not*:

I see a dual 4:1 analog mux in your future. ADG509A from Digikey is 5 bux in oneses.

Address it with an RC oscillator-counter like the 7240 and Bob's your uncle.

Power both with 16 V DC to keep the mux happy.

Use a level shifter and decoder to drive 4 LEDs so you know which input it is looking at.

--Winston

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Corporate executive forgets to commit a felony for 24 hours.
Film at 11.

If the batteries have a common negative connection, I'd use three SPDT push buttons - NO terminal of each button to its battery's positive, C terminal to the NC terminal of the next button, with the final C terminal to the meter positive.

If the batteries do not have a common negative connection, you need a similar circuit, but with DPDT push buttons, one pole switching the positive, and the other the negative lead of the meter.

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Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

Go to Harbor Freight and buy three of their digital multi-meters, put one on each battery. These meters go on sale for $3 or $4 each fairly often.

I have them stashed in my cars, tool kits, and sometimes in my pockets. They operate on a small 12 volt battery, you could power them from your circuits easily.

Wilby

No you can't: powering a volt meter from the circuit being measured won't work.

Why not! Yes the cheap digital volt meters normally run of an isolated 9V (PP3) battery supply at less than 1mA.

There are digital meters around that can share negative terminals between operating supply and measured input, but they tend to be quite a bit more expensive than those that require an isolated supply.

How about a design challenge for the group! Design a 11 to 15 volt input to 9 volt (+-10%) output at 1mA load (100V input - output isolation).

1) Lowest cost design 2) Most efficient conversion (at 1mA load) 3) Least number of components / smallest physical size design

How about more conditions. Not more than +-10% output voltage change for a

0.5mA to 2mA output load change at a specific input voltage (say 12volt input to simplify comparison).

1st prize - the happy feeling that a lot of guys will copy your design and possibly make lots of money without paying you a cent ...... and then go bankrupt because everybody is flocking it.

Why 100V isolation? So that you can measure each battery in a stack of 12V batteries (in series), while supplying the meter form one of the batteries in the stack. (using a multi-way 2-pole rotary switch - not part of the design challenge :-).

Gerhard van den Berg CSIR

not always true. depends on the device you have and when it was made and by who. I have some older look alike Harborfreight cheap DMM's that will measure it's own battery with no problems. I got a box of them years ago at a traveling tool sale..

I've seen this meter being sold with many names on it and have many different boards now. You no longer know what is in there when you buy them.. Most of those $5.00 meters i've seen now only support the V-ground, but that does not mean all the others are like that.

It won't always work, but some meters are built for that kind of application. There are 12V (actually 5 to 20V range) panel meters that do exactly this. Apparently, they're made for 12V battery monitoring. (search on eBay for "blue LCD voltmeter" or go to item# 180565202298)

Of course, what one USUALLY does, is just move a probe from one battery to the next... it's no harder than pushing a button or rotating a knob.

Problem is, that's too much like work. What's happening is, we're seeing the unfoldment of the push-button society, as was foretold in the 1960s and 1970s - where people get up, and push a button that says, "Do my life for me."

Thanks, Rich

(...)

We really don't know the whole story, yes?

I can think of a couple ways that Brandon's requirement would be a minimum level of complication to address a particular need.

I *do* see your point though. On another newsgroup, I outlined a procedure that would allow a guy to 'reverse engineer' connections to a 50 pin control panel using his DMM. Instead, he insists on designing a complicated test jig to check those connections.

Who knows? Maybe he really has 1000 differently - wired control panels to document and his 'automated' solution really is the most efficient way to gather that information.

Either way, I figure it's more helpful to ask a question than make a statement when confronted with a mystery. :)

--Winston

[snip]

Its worse than that. Nobody's getting up. That pushbutton has been replaced by a remote control.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Time's fun when you're having flies. -- Kermit the Frog

and a 6 pack of beer in the cooler attached to that nice ezzy chair, crapper in the cushion and your best friend beside you, the dog.

Jamie..

I always figured that you were that lazy. It shows in every post you write.

--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

And once a week your Mexican housekeeper comes in and hoses you down...

;-) Rich