This is a typical pulse width modulator (PWM) circuit. The oscillating control voltage (green) directs the current through blue and red paths creating an AC current which can be modified to look like sine wave. However this is a bit dangerous circuit.
With voltages around 12V there is no danger, but with high voltages (e.g. standard 230V AC) this becomes deadly. The voltage from battery has to be 325V in order to get the sine to have 230V power rating. The negative side is also taken from the same battery which makes grounding difficult, or even impossible. The device has 325V ungrounded DC and 230V ungrounded AC ... nope, I am not going to do this. And if you see this kind of device near you I suggest a few steps back.
To avoid this problem with deadly electronics, I am turning my head towards DC currents below 120V and AC below 50V. These limits obey the regulation of Protective and Safety Extra Low Voltage (PELV and SELV). The difference is that former is (allowed to be) grounded and latter is not. And further along to regurations... if SELV AC current has a power value lower than 25V there does not have to be any cover for the circuits. This sounds good.
After learning this, the project changes its DC to AC goal to PELV compliant DC with PWM controlled power. The idea is the same as the above circuit. Removing the red connectors (and limiting battery voltage to 12V) makes this circuit safe.
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