24v pulses as Arduino input to track electricity usage

Other Hardware General Electronics
1

Hi all,

I have a question on how I can measure 24 volts with my Arduino.

The situation is as shown in the image below. I have a simple electricity meter that is connected to a transmitter. This transmitter is used by my landlord to track my electricity usage, thus, I do not want to influence that in any way. At the same time, I am doing quite a bit with domotica and would like to track my energy usage.

What I found is that the electricity meter and transmitter communicate using an extremely simple protocol (DIN 43864). The protocol consists of a 100ms pulse for every 1/1000 KWh used. The meter has an open collector used to create the pulses, the transmitter provides 24 volts.

I would like to count those pulses with my Arduino without interfering on the signal. I would like to use a voltage divider to bring down the voltage to 5 volts and use it as an input for my Arduino. Would this be possible using a 10k and 47k resistor?

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thanks!

Edit: changed the image to have a more readable font.

Edit, edit: Just realized the above is probably not going to work. The arduino is directly connected to the 24v out of the transmitter and will always read a value right?

2

You need to divide by five with your resistors, so the upper section will be four times the value of the lower

3

Most meters would have an IRLED that outputs pulses based on energy. Usually, its part of the opto-com interface on the cover. A cellphone camera viewer could be used to actually visibly see the pulses being emitted. Using this port eliminates the possibility of interference on the pulse output/transmitter circuit.

However, if you already have access to the transmitter circuit, I would suggest using an opto-isolator. It could be weakly driven (because the pulse rate is low and pulse width is 100ms long). Specifications or circuit diagram for the transmitter should reveal what loading the circuit is using and what additional load can handle.

4

There is indeed a LED on the meter as well which blinks at each pulse.

If I would use an opto-isolator, would that not interfere with the transmitter since it would draw some power for the opto-isolator itself? Or is that so little it is negliable?

5

There is indeed a LED on the meter as well which blinks at each pulse.

Using an opto-sensor here would be ideal.

If I would use an opto-isolator, would that not interfere with the transmitter since it would draw some power for the opto-isolator itself? Or is that so little it is negliable?

Need to see the transmitter's specifications and/or schematic.

6

Thanks! I'm going to try to get it working with a light sensor since that removes the chance of influencing the transmitter.

7

There should be NO connection from your arduino 5V and your voltage divider on the 24V line.
Your schematic shows a 47k resistor from +24V to arduino +5V. This is incorrect.

I would use 10k to ground , 43k to +24V and the connection of the two resistors in the middle can go to
a digital input or an analog input.

24V * 10,000/(43000 + 10000) = 4.5V.

This will allow you to use a digital input OR an analog input.

[+24V ]> <43k resistor> | <10k resistor>>[GND]
|
|____________________________> [DIG INPUT]/[ANALOG INPUT]