If the photo transistor T1 registers any light it closes the circuit from collector to emitter and this triggers the T2 transistor to close as well. This detail is written on each of the electricity meter. Every pulse of the LED means certain amount of energy consumed. How it works? Photo transistor is placed in front of the red LED of the meter.
D2 : Removed in order to get the maximum on the output. 
D1 : Normal diode to have some polarity protection. Wiring diagram for pulse detector List of components I am sharing here the schematic diagram of the wiring I successfully tested and implemented myself. But no worries, Photo-transistor, NPN transistor, two resistors, and a diode will do the trick. There is no ideal ready-made one, or at least I was not able to find one. The sensor itself requires some DIY skills. The best results are obviously when the meter is in complete darkness, but then the problems that can cause some inaccuracy is whenever someone opens the cabinet and daylight interferes with the sensor. Sensitivity of the detection circuit is the key here and I spent quite some time optimizing and tweaking the sensor, and then there is the shielding other light sources. Now, reading the LED pulses from the meter is very dependent on few important factors. And still I read some comments about placing the sensor on the acrylic cover of the electricity meter as not acceptable. These meters do have better options how to connect (IR Meter reader, S0 pulse output), but because of the strict rules and restricted access to the meters outputs, the LED pulses are the only “non-invasive” method of capturing the measurements. Optical reading of the pulses will never be absolutely reliable. Before we jump into it, let’s make few things clear. I described the integration of a gas meter in my previous post, let’s focus on electricity meter here. Photo-transistor installed to read the LED pulses Again, not talking about any Modbus meter integration here, I am about to get the data from official utility meters. Continuing the story about the utility meters and its connection to Loxone.
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