Author Options:

Tiny circuit to measure amps? Answered

I am going to use a ATTiny43U to obtain the voltage of a 2vdc solar battery, via self powering (0.8-5.5VDC).
Using ohms law I know I can obtain the amps as I will have the voltage and a resistor.
I want to know what resistor to use?
I will feed the amps result back into the ATTiny43U as well as the temperature from another sensor.

Can someone please advise me what tiny circuit I can use to get the amps as I do not know what size resistor to use?
The operating voltage should be between 1.55 and 2.67 vdc give or take a bit.

Thanks and kind regards,


So you want a current measuring circuit ?

Hi Steve,

Yes. Just getting a big bogged down with the plethora of data on the web. I would like the circuit as small as possible even an IC if such a beast exists? But also a heads up on what size resistor to use to measure the amps if ohm's law is the way I should go?

Thanks for the reply,


The best way to do this is with a very, very low value resistor and a special chip to read the current - that way you don't waste a lot of your energy supply in the resistor, but you also have a very low supply voltage too, which makes it even more difficult. The 43U has a 10bit ADC, whose input is scaled against the supply rail, which is going to vary, unless you force it to run on, say, 1.5 V.

What RANGE of current do you want to sense, and what precision do you want to sense it with ?

Life would be a lot simpler if you added a power supply circuit to boost your supply to say 3V.

Hi Steve,

A multitude of conundrums.
Sorry if this sounds patronising or trite I do not mean it just an attempt to explain the WHOLE problem

I have 24 x 2VDC 1200 Ah Hoppecke Lead Acid Plate Bateeries.
These are wired in 2 banks of 12 to give me 24 VDC output and about 1580 Ah, fed into a Victron Inverter Charger.
The main input into the batteries is photo voltaic and wind.
I have a backup 11 Kva Kubota generator, that is also used to boost and equalise the batteries.
I have attached to each battery a copper lug that houses an LM35 temperature sensor.

Each bank is divided into 2 sub banks (6 batteries) that are to report to an Arduino and consequently into an MySQL server / cloud.
Other arduinos handle the panels and the wind turnine, the inverter and the generator sensors.

Each battery will have a single self powering ATTINY43U "sensor" that will report on polling, power (volts), current (amps), temperature and via interrupt any major deviations. I would like each "sensor" to be in a package as small as possible, with maybe an led or 2.

The main reason for this elaborate set-up is that I had an accident which has left me disabled and partially sighted and I am going down hill which is limiting me manually inspecting and recording data and I cannot ask my wife to do more unless I can say battery XX is not looking well, etc.

I am still trying to work out how to measure the min/max levels of the electrolyte and even obtain the specific gravity.

I have been running (nice word) around in circles for about 18 months now trying to come up with a solution and had hit upon the ATTiny43U as a good starting point.

I have a long background in IT so programming is the easier part of this project. I am learning electronics as I go, which is why I ask dumb ass questions.

Truly grateful for any steerage and advice.

Kind regards,


NOW you're talking.
Forget using tiny voltages to drive this circuit then. You can run all your monitors off a couple of cells - they'll draw a few mAs, and I bet your leakage currents will swamp that

I use the AstroFlight on my solar power system.

I use a "WattsUp" meter to read Amps and Volts and Watts.
It only uses 3mA from the source to power it.
You can find them in Hobby Shops that sell Radio Control stuff.

Google WattsUp or DoctorWattson or Astro Flight.

Astro Flight is pictured.


Hi Steve,

Yes you are right I could add small button battery as used for an RTC, to the circuit.



If you're really going to do this, and its to be self-powered, I recommend you use a supply solution from Linear Technology, who have a whole suite of neat devices. I would be clever, and only boot the circuit when I need to make a measurement - every second, say ? The processor would run contendly on the low voltage available, until you need to make the measurement, then it turns on the big supply, you wait a few msec, and then using a chip like the ZXCT1010, from Diodes Inc to measure the current flowing to the load. The ZCXT1010 uses an ultra low value shunt resistor to measure the current, and it will not take much energy from the system