To create the particular waveform explained in the previous page for this modified sine wave power inverter, first of all obviously we need a square wave generator.
Primarily, we need to break each square wave into pieces so that they are equally spaced with notches and also to make them grow in size gradually from start to the center and reduce as it ends.
Ideally we may think of a PWM IC for this, but the condition is also very effectively met using a couple of Johnson decade counters. The circuit may be understood as follows:
We know that the IC 4017 produces a continuously shifting sequential logic high pulse across its 10 outputs in the order: 3-2-4-7-10-5-6-9-11.
Here, two 4017 ICs are cascaded to provide the above sequence in 20 continuous steps. D1---D4 are appropriately configured to make both the ICs conduct in tandem.
If you inspect the outputs of each IC carefully, you will find that a few of the alternate pin-out sequences are cleverly skipped, while the remaining are joined together through diodes.
The diode junction goes to the base of the power transistors. These transistors respond and conduct exactly according to the switched pin-out sequences and remain shut-off intermediately due to the skipped pin-outs, producing pulses exactly as shown in the diagram.
The above DC pulses are forced through the windings of the transformer where it is stepped up to the required primary voltage level.
The frequency of the AC will particularly depend on the input clock pulses applied to the ICs.
Since the outputs shift in response to every rising edge of the input clock, one complete sequence from the start of IC1 (pin #2) to the end of the IC2 (pin #9) constitutes a single AC pulse. However, since one complete sequence should happen 50 times (for 50Hz) per second, implies that the input clock should have a frequency of 50 × 20 = 1000 Hz or 1 KHz or 1.2 KHz to get 60 Hz (for 120 volt Outputs).
The circuit incorporates a simple oscillator circuit using IC 4049, however other standard oscillator configurations may also be tried. For example a simple oscillator circuit using IC 4060 as discussed HERE may be employed for driving the above circuit.