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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@tage
You may have to read the datasheet for the ACS712. you should feed it with 5V and the output will be half of the supply voltage at no load, and as you apply a dc load you should expect the reading to change by 0.1V per Ampere (may vary depending on the version of the part).
a change of 0.1V equals a change in reading of 0.1/3.3 = 0.0303, if your Espruino adc has 3.3V max range.
Because the output signal has an offset that equals half of the supply voltage (5V) you need a stable 5V supply to get any accuracy.