Code for STC15F104W MCU which was installed in cheap ESP-01 relay module marked "LCTECH".
I bought few such modules from ebay but they had broken MCU firmware and not works. It must accept on UART commands (hex): A0 01 01 A2 for Relay ON and A0 01 00 A1 for Relay OFF
UART interaction code I got from STC15F100 docs http://www.stcmicro.com/datasheet/STC15F100-en.pdf
By the way this MCU has very similar architecture to 8051 MCU so some code which works on 8051 MCU also may work there.
sdcc -mmcs51 --iram-size 128 --xram-size 0 --code-size 4096 --nooverlay --noinduction --verbose --debug -V --std-sdcc89 --model-small "relay.c"
stcgal -p /dev/ttyUSB1 -b 1200 -D -t 11059 relay.ihx
disconnect +5V (red wire on image) and connect it again for start flashing.
Relay board MCU is connected to relay by pin 3.2. This pin has high level on cold reset or power on so device based on this board will click each boot time. To avoid this issue I added 2K resistor between pin 7 of MCU and GND and changed code by configuring push-pull mode on P3.2ch MCU pin 3.2. See article "How to Make I/O Port Low after MCU Reset" in STC15F100 docs.
