Great design! I first looked at the Power Integration devices some time ago when I was trying to understand why my wireless thermostat receiver had stopped working (o/p cap gone high ESR). The original design from Power Integrations was for a Bluetooth enabled switch, but this is a nice simplification. It's worth looking at the original design notes to understand how this uses 'leakage current' through the load to drive the switch mode power supply. As far as I can see. the TI TPL7407 device driving the relays was not originally designed as a touch switch, but with its CMOS compatible inputs I presume it works OK with the leakage current from the live mains.
Best posts made by tony1tf
Latest posts made by tony1tf
RE: 💬 No neutral power supply/relay board for in wall switch
RE: What's the "best" UPS for a Raspberry Pi 3B?
Carrying on with the "Super Cheap" theme, I have successfully used two Poundshop Lithium ion batteries. They have a 5V USB input and output which can only be used one side at a time. However, connect the two batteries in parallel (+ve to +ve, and - ve to - ve), and you get 2 charger/discharge circuits, so you can power the micro USB side from a USB 5V supply, and the other USB 5V o/p to the Pi. The cheaper ones (1pound) will supply 800mA, but they also have higher power ones for a bit more, especially as they are sometimes on offer.