Measuring voltage from small solar panel

I’m using some solar post lights from Lowe’s and modified them:
https://www.lowes.com/pd/Dekorators-Fits-Common-Post-Measurement-4-in-x-4-in-Actual-6-75-in-x-6-75-in-x-4-85-in-Solar-VersaCap-White-Solar-LED-Plastic-Deck-Post-Cap/3184405

I removed the solar control chip + LED, and install a more powerful LED bulb, power under 12VDC:

Since the solar panel exist already and not used anyways, I thought I will try to use it as an ambient light detector to measure the voltage with the Arduino. Apparenty, under full bright sun, voltage is 4,5V

So i don’t need a voltage divider since max voltage is <5V, but should i put a series resistor ? If yes, what value?

If it's less than 5V you don't need a resistor. For example, it's OK to connect the Arduino's 5V power supply directly to an analog input.

If you want to play it safe (or safer) a 1K resistor should limit the current in case of over-voltage so you don't burn-out the Arduino's internal protection diodes.

As you may know, the Arduino's input impedance is around 100 megohms so you won't get any voltage drop across any "reasonable" series resistor unit you over-voltage and the protection diodes start conducting. However, higher resistance will make the circuit more sensitive to electromagnetic noise.

Wise to use a resistor between solar panel+ and Arduino pin. So the solar panel can't phantom-power the Arduino through the pin protection diodes in case the Arduino is off. 10k is a good value. That keeps fault current under 0.5mA.
Leo..

OK, thanks....will try 10k between Solar panel+ and Arduino pin

french_guy:
Since the solar panel exist already and not used anyways, I thought I will try to use it as an ambient light detector to measure the voltage with the Arduino. Apparenty, under full bright sun, voltage is 4,5V

Two points.

1. Solar cell voltage is usually specified at the nominal operating voltage which is lower than the open circuit voltage. A solar cell array that has a nominal optimum operating voltage of 4.5V might have an open circuit voltage of around 6V.

2. Solar cell output current is approximately linear with light intensity. To use one as a ambient light detector it might make more sense to load it with a suitable load resistor and measure the voltage across the load resistor with the microcontroller ADC. This would probably give a more useful range of light readings.

With small solar cells, one can use a multimeter directly to measure open circuit voltage and short circuit current under bright sun to get a good estimate of these characteristics.

I had to put this project on hold, but I'm back to it now......
I will measure open circuit voltage and current (but I think I did last year, and under full sun it was less than 5V - not sure about the current, but probably low)
I'm also planning to power the Arduino (Nano V3.0) with a 9V wall wart.....I guess it doesn't change anything, and the max voltage the arduino can read directly is 5V, right?

Thanks

Well, after finally having some sun in Michigan, I measured the solar panel (voltage and current) without any load
Sun was high and blue sky (no cloud)
V= 4.8 V
I=80 mA

I will try to measure the voltage drop thru a resistor (like 270 or 330 Ohm), and see what I get...

The best performance point will be about 4V / 70mA I reckon, so try more like 50 ohms. Its likely to be a
9-cell array as open circuit voltage per cell is usually a little over 0.5V. Optimum power point in full sun tends
to be about 0.45V. Assuming its silicon.

This is the solar panel in question:

I was out by one, 8 cell arrays

At 7:30pm......not sunny anymore, but not dark yet
With 412 Ohm, I measure 1.4v
I will have to check the voltage a 8:45pm (time when I turn my lights on with a smart plug)

Well, actually it was a 0.00V well before it was dark …Direct measure w/o resistor was 1.5V though
Than might be the threshold I need to use then

Now under full sun, no resistor: 4.40V