# 12 vcd to 3.3 vdc help

My high school electronics is a little rusty. Need some help stepping down 12 vdc (12.0 to 14.1 range) to 3.3 vcd to connect to a digital pin on an Arduino Due. Lots of options with resistors, transistors, bucks, etc. Looking for a recommendation that has a low current draw on the 12 volt side and does not generate a lot of heat.

Check out Linear Technology they have modules for all that stuff otherwise just use a standard voltage regulator IC you can also find modules at the Robotshop under robot parts power supplies they have a few modules and power conversion boards relatively cheap.

You have not really explained what you are trying to do. If all you want to do is make 14 volts or so read as a digital high and close to zero read as a digital low, then two resistors arranged as a voltage divider will be sufficient.

Yeah, a [u]voltage divider[/u] (2 resistors) should do it. In most situations the resistors should sum-up to about 10K but it's not critical... The ratio is critical... Higher resistance is more prone to noise pickup and lower resistance will waste more power.

Since you're implying the voltage is not tightly regulated (automotive power?) add a [u]protection diode[/u]. Or in some cases, assuming this is digital signal you can just make a protection circuit (a resistor and diode) without the voltage divider.

A 12k and 3.3k divider would give about 3V from 14V.

14V------12k---+---3.3k------GND
|
A0, 3.02V

A 12k and 3.3k divider would give about 3V from 14V.

14V------12k---+---3.3k------GND
|
A0, 3.02V

Or better for overcurrent protection.

14V------12k---+---3.3k------GND
|
------ 3.3k------A0, 3.02V

why not to use 3.3V on arduino board ?

OP probably wants to detect automotive 12v on some line, and wants to just stick it into an Arduino I/O.
An optocoupler might be suitable for simplicity sake.

Thank you all for your suggestions. I will take a closer look at the resister and OP suggestions.

outsider, what happens to the voltage at the 3v side when the input voltage is closer to 12 volts?

On my sailboat, the batteries put out 12.2 to 12.9 volts. When a charging source (solar or alternator on the engine) kick in, the voltage climbs slowly until 14.1 is reached, then that voltage is held for x hours until the batteries are charged.

What I’m looking for on my Digital Pin is 3.3 vdc (Due’s are 3.3 boards) for any voltage between 12 and 14 to siginfiy on or high. The off or low happens when the power is turned off to the input source. The power is controlled with a switch else where on the boat.

For this project, I don’t care about the voltage level in the code, that is why I want to use a digial pin.

Thanks!

You do not need 3.3 VDC to signify a high. I do not have a Due spec handy, but an ATmega328P specifies 0.6 x Vcc, which would be 2.0 volts for a Vcc of 3.3 volts.

A little margin is a good thing, so perhaps you should aim for something more than 2.0 volts, but 3.3 volts seems excessive. A resistor divider dividing by 5.5 would work from 12 to 18 volts for an Atmega328P. Could the Due be much different?