But what about the absolute values? In principle we can pick any magnitude we want, but in practice there are several important considerations: Knowing this, we can construct a divider for any ratio we want simply by determining the relative values of Ra and Rb. The voltage where Ra and Rb meet will depend on the current flowing and the value of Rb - it's Vdiv = i * Rb.

We can calculate this with i = Vin / (Ra + Rb). Current will flow from IN, through Ra and Rb, to ground the amount of that current depends on the voltage at IN. Ignore 'Radc' for a moment, and assume 'IN' is connected to a voltage source. The basic operation of a resistor divider is simple. Simulate this circuit – Schematic created using CircuitLab In order to measure a wider range, up to 24 volts, we need a voltage divider, like this: You're going to want to use it to measure both the voltage across the sensor wires and the current through them if the wires are used for both power and communication, it's likely that the way it communicates is by increasing and decreasing the amount of current it consumes, in which case your most useful information will come by measuring the current waveform.Īs you observed, the Arduino can measure voltages between 0 and 5 volts on its analog ports. It seems like your first task is going to be determining what sort of signalling is being used, so what you need to start with is a 'poor man's oscilloscope' in the form of a microcontroller with ADC. I would really appreciate it if those with more understanding and knowledge could offer their insights and assert my thoughts above and even elaborate on it to further my understanding and clear up some of my misunderstandings. I realise SE prefers Q&A type of posts, but to me the above is context that fits together for the larger circuit - at least that's what I think. Out of my depth, so some insight would be useful. One particular question that I have is about the fact that the ADC and the 24v sensor signal that I'm trying to interface is that they have different power sources, and apparently this is an issue because they don't share the same GND. I'm not certain why this is useful or what it achieves, but I think it seems to help address the mismatch of impedance from the 24v signal and that of the ADC? But I could be wrong.

Voltage buffer / op-amp: I seen references to including this as part of the circuit, but again my ignorance only makes me dangerous at this point.

I'm still lost with the whole impedance thing so I'm still unsure about whether to use R1 and R2 sized as say 4.7Kohm and 1.2Kohm or larger by an order of magnitude or two.

I should choose my resistor values not only to achieve the desired divided voltage, but also to suit the impedance of the ADC.

I can use a simple voltage divider circuit to 'scale down' to 0 - 5v range.

The resolution is good enough for my needs.

The ADC is a 10bit for 0-5v so I have roughly 4.88mV per step (total of 1023 steps).

I've searched various posts and sites regarding conversion of the 0-24v analog signal to the range 0-5v the ADC on the Arduino Uno (actually Freetronics Eleven with ATmega328P) can interpret. The sensor is has only two connections which is used both for power and signalling.

I have a proprietary 24v DC analog sensor signal that I'm trying to interface with using a Arduino based microcontroller. Disclaimer: Please be gentle - I'm a newbie with electronics.

Arduino read analog input