Large diameter photoelectric direct reading remote water meter is based on the mathematical model of the word wheel, through the combination calculation, the correct solution is obtained, and then the specific mark is printed on the corresponding position of 0-9 of the word wheel according to the mathematical law, and the fixed photoelectric sensor is installed around the meter according to the mathematical law, so a mathematical problem is realized by electronic circuit method. At the same time, we can get the correct solution, that is, the data on the word wheel. At present, there are mainly two kinds of direct reading meters in this way, i.e. the photoelectric radiation (transmission) type and the photoelectric reflection type. The main difference lies in the difference of mechanical structure and mathematical algorithm.