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Reverse engineering RF RC Wall Socket... dodgy way to switch relay?
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formatc1702:
After years of watching the EEVblog, finally posting for the first time!

Backstory
I got myself some cheap RF (433 MHz) wall plugs/sockets like these, to switch some lamps in my living room.
One of the applications does not require switching mains voltage, just a TTL high/low signal.
To be able to control this from the same remote (supports 4 addressable receivers), I am taking one of the receivers apart, reverse engineering the schematic, tapping the IC's output pin that switches the relay, and removing the mains plug since I'll be powering it from a 5V supply anyway, together with the electronics to be controlled.

The findings
You can see the PCB and my reverse engineered schematic here: https://imgur.com/a/xEXbE66.
Fairly simple stuff. What was new for me (Mech.Eng. by training, electronics hobbyist) was to see the arrangement of two Zener diodes in series to generate the separate 5V for the electronics (5.1V zener Z1, between VDD and VSS), and the 24V for the switching relay (23.5V zener Z2, between VSS and V-).

If I traced/measured correctly, the relay is switched off by essentially SHORTING Z2 across Q2 (Vss to V-) and relying on R4 (200 Ohm, the only low voltage THT resistor on the board) to dissipate the excess power? :wtf:

Did I reverse engineer this correctly? If so, is this technique a common thing? If not, where did I go wrong? Also, what's the purpose of D5 near Q1? (Mistake in the drawing: anode of D5 goes to VDD, not VSS)

Note: In my case, I will directly apply 5V between VDD and VSS, so Z2 will remain unpowered and no current will flow through R4, either. However, I am still curious about the original intent behind the design.

Thanks in advance!
amyk:
I did not look at the schematic (please attach the images here instead), but if it is based on a capacitive/resistive dropper then shunting the output to switch off the relay is normal because those supplies are essentially constant-current sources and always need a load to prevent their output voltage from rising up to the mains.
formatc1702:
Thanks! The keyword "capacitive/resistive dropper" was a new one for me.
I found an app note that explains it nicely and even includes an example with multiple stacked zeners as in my case.

Still not quite sure about the roles of R4 and D5, however... any hints?
Why is Q1's emitter connected to VDD (via D5) *and* V+ (via R9)?

I have attached the images below as well.
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