Specific question about Sonoff 4ch Pro - adding an external switch

[fdemees]

Dear Community,
I have a good old Sonoff 4ch Pro with all 4 outlets used and the corresponding buttons associated to their relative outlet.
I would like to add an additional input switch (a reed contact for a door sensor) using a free GPIO, not interfering with the existing 4 circuits.
In Tasmota I have declared the device as a 4Ch Pro, module type=23.
In addition I have declared GPIO3 (Serial In) as "Switch8" , hoping to connect my switch to the Rx pin.
I have read here an there that I have to disable the serial console
22:29:03 CMD: seriallog
22:29:03 MQT: stat/sonoff5/RESULT = {"SerialLog":"0 (Active 0)"}

The problem is that it toggles relay1. This is documented.

I have read discussion #8302 and https://tasmota.github.io/docs/Buttons-and-Switches/#switchmode
I have tried the following command
22:44:57 CMD: switchmode8 15
22:44:57 MQT: stat/sonoff5/RESULT = {"SwitchMode8":0}
in order to decouple Switch8 from Relay1.

It does not work as expected. Relay1 is still triggered.

Should I declare the device as a Sonoff Basic and declare all the GPIO ?
Or add a rule as explained in #3166 ?

Thank you for your expertise.

Additional documentation:

22:14:48 MQT: stat/sonoff5/STATUS = {"Status":{"Module":23,"FriendlyName":["Sonoff","Sonoff2","Sonoff3","Sonoff4"],"Topic":"sonoff5","ButtonTopic":"0","Power":1,"PowerOnState":3,"LedState":1,"SaveData":1,"SaveState":1,"SwitchTopic":"0","SwitchMode":[0,0,0,0,0,0,0,0],"ButtonRetain":0,"SwitchRetain":0,"SensorRetain":0,"PowerRetain":0}}
22:14:48 MQT: stat/sonoff5/STATUS1 = {"StatusPRM":{"Baudrate":115200,"GroupTopic":"sonoffs","OtaUrl":"http://thehackbox.org/tasmota/release/sonoff.bin","RestartReason":"Software/System restart","Uptime":"0T00:00:53","StartupUTC":"2025-03-05T21:13:55","Sleep":0,"BootCount":34,"SaveCount":862,"SaveAddress":"F6000"}}
22:14:48 MQT: stat/sonoff5/STATUS2 = {"StatusFWR":{"Version":"6.3.0","BuildDateTime":"2018-10-30T17:33:53","Boot":31,"Core":"2_3_0","SDK":"1.5.3(aec24ac9)"}}
22:14:48 MQT: stat/sonoff5/STATUS3 = {"StatusLOG":{"SerialLog":0,"WebLog":2,"SysLog":0,"LogHost":"","LogPort":514,"SSId":["lunix4","lunix3"],"TelePeriod":300,"SetOption":["00008009","55818000","00000000"]}}
22:14:48 MQT: stat/sonoff5/STATUS4 = {"StatusMEM":{"ProgramSize":489,"Free":512,"Heap":14,"ProgramFlashSize":1024,"FlashSize":1024,"FlashMode":3,"Features":["00000809","0FDAE794","000183A0","23B617CE","00003BC0"]}}
22:14:48 MQT: stat/sonoff5/STATUS5 = {"StatusNET":{"Hostname":"sonoff5-1419","IPAddress":"192.168.68.21","Gateway":"192.168.68.1","Subnetmask":"255.255.255.0","DNSServer":"192.168.68.5","Mac":"60:01:94:85:65:8B","Webserver":2,"WifiConfig":5}}
22:14:48 MQT: stat/sonoff5/STATUS6 = {"StatusMQT":{"MqttHost":"192.168.68.25","MqttPort":1883,"MqttClientMask":"DVES_%06X","MqttClient":"DVES_85658B","MqttUser":"DVES_USER","MqttType":1,"MAX_PACKET_SIZE":1000,"KEEPALIVE":15}}
22:14:48 MQT: stat/sonoff5/STATUS7 = {"StatusTIM":{"UTC":"Wed Mar 05 21:14:48 2025","Local":"Wed Mar 05 22:14:48 2025","StartDST":"Sun Mar 30 02:00:00 2025","EndDST":"Sun Oct 26 03:00:00 2025","Timezone":1,"Sunrise":"07:23","Sunset":"18:39"}}
22:14:48 MQT: stat/sonoff5/STATUS10 = {"StatusSNS":{"Time":"2025-03-05T22:14:48","Switch8":"ON"}}
22:14:48 MQT: stat/sonoff5/STATUS11 = {"StatusSTS":{"Time":"2025-03-05T22:14:48","Uptime":"0T00:00:53","Vcc":3.152,"POWER1":"ON","POWER2":"OFF","POWER3":"OFF","POWER4":"OFF","Wifi":{"AP":1,"SSId":"lunix4","BSSId":"20:28:18:A1:FE:56","Channel":10,"RSSI":100}}}

[sfromis]

You should not skip numbers when assigning gpio components, but start with 1 and increment with each occurrence of the same type. The numbers are only for ordering, not for you to plan a numbering scheme.