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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.
@Gordon
I'm not sure what you mean by 'Serial ground'?
So you're saying the SIM800 is the motor here? That's definitely what I'm going on about. When the SIM800 is off, use a volt meter and measure the voltage on the SIM800 GND relative to battery GND.