Negative supply generator

Thread Starter


Joined Jul 7, 2006
Hi all,

I want to monitor the secordary battery voltage in my 4WD (run fridge and camping lights). So I purchased a LCD voltmeter module form Dick smith electronics, see .

It has the follwoing warning "Neither input can be connected to the meter negative supply unless a dual power supply is used" and the following circuit was also supplied.

I do not understand the circuit, looks to me that inverters between pins 1-2 and 3-4 are shorted?

I it just my inexperience or how do I interpret this circuit?

I can not tell you anything about what the jumpers J1-3 do as this in not supplied.

Looking forward to your comments.


Joined May 16, 2005
There are typo's. The first inverter should be pins 1 & 2 rather than 3 & 2. The inverters & caps are an oscillator, the four ganged inverters provide more output current.

I found no data on the 7865 (admittedly I didn't look very hard) but it resembles a voltage regulator.

Thread Starter


Joined Jul 7, 2006
The part that I do not understand also is I believe points A and B (I have added them to the circuit) are shorted together. Am I correct or not?


Joined Jun 30, 2006


Joined Jan 10, 2006
The 7865 is another typo, it should actually be a 7805 5 volt reg.
Yep, 4 of the inverters are wired in parallel (your AB short) to get 4 x the output of just a single inverter. The square wave output has it's DC component blocked by the capacitor, and the resultant square wave AC is 1/2 wave rectified by the diodes to -ve 3 volts.
I just checked the specs of your meter, supply volts 7 - 11 volts. (perhaps the supply is the +5 and -3 ??? I think I need more data, will do a quick google....
I need more data, will do some googling....

OK, Both the OZ, and NZ dick smith sites were as much use as a Pocket in your Underpants.... I found a similar (separated supply) meter at Oatleys, and they have a kitset PCB to obtain an isolated supply for their meter, should work with the Dick Smith on as well. (Oatley Kit 212)... I'm sure you can find their web site...


Joined Sep 10, 2007
Thanks Gadget,
I found this thread with Google looking to complete this circuit with the voltmeter
to monitor the charge of a 12 Volt battery that runs a power inverter.

I note your findings of errors on the Dick Smith datasheet (I have the same sheet).

The inverter runs a sat TV installation and lasts over ten hours on one charge cycle,
so it's easy to loose track of the battery "time left" that I'm counting in my head
when you turn it on and off for multiple sessions.

Edit,, I question why as part of the 12 Volt battery metering configuration for the device,
they have a 990K, and 10K resistor running in series. Why not just a single 1M resistor?
990K is an uncommon type not available here. Maybe just to close both resistor links on the PCB.


Joined Sep 10, 2007
Well no luck here the project is a binner :(

I was sucessful in finding more errors in the documentation.
The resistors in the 12 Volt system metering configuration do not agree
with the values given in the table on the other side of the datasheet.

I would be interested if anyone ever got it working from the datasheet provided.
It looks like following that info could result in destroying the product.

I tried the easy way with a 9 Volt battery and got the same results.
it counts and does some other wierd things when voltage in is connected to nothing.


Joined Jul 17, 2007
Nice job, Art :)

I got a blast of nostalgia from your use of the skateboard wheel assemblies. When I was a teenager, I used a skateboard to move a newly-assembled Fiat engine up from the basement via some 2x12's and an assist from a block & tackle ;)

Is the battery you're using rated deep-cycle? If not, you might think about getting one as a standard auto battery will have a short life when used as you are. Deep-cycle batteries are constructed differently; the plates are much thicker.

Looks like you're camping in the woods - peaceful spot. But you'll have a bugger of a time trying to charge the battery with solar panels in the woods. Perhaps you could construct a small steam engine or mini-turbine instead; you could use a DC motor as a generator. Something to ponder, anyway.


Joined Sep 10, 2007
Thanks :)
I can't make noise, and don't really think I want to.
19 hours + of sat TV out of a single charge is enough :D
The only problem is only 4 hours out of my desktop PC. I need a lappy!

The skateboard was only $16 AU, so having occasional back trouble, how could I not :)
I will burn the deck next time I light a fire to get full value out of it too!

I'm looking at spending $800 AU on an 80 Watt panel that should cover one whole side of that green tent.

The battery is 13 plate, but a cheapie.. not too woried about overall service life,
just single charge cycle.
I discharge it no less than 12.0 Volts, but probably won't go that far on most occasions.

Next time I mihgt spend a little more on a number of sealed batteries wired parallel.