Hi,
(sort of) good news.
The device only sends the payload when activated by smoke or by the test button and it’s the same code always. However, the length is different based on the voltage: starts at 513 at 9 V and increases up to 561 at 4 V (4 V is the cut-off voltage and I assume it takes a longer time to drive the circuit with a lower voltage). A sensor can be set to monitor the length topic in addition to the payload topic to trigger when its value is something like above 540.
Anyway, when below 6 V the sensor emits a faint beep after each 40 seconds.
9V:
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/protocol', ... (1 bytes))
1
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/bits', ... (2 bytes))
24
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/length', ... (3 bytes))
513
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT', ... (7 bytes))
1168723
7.5V:
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/protocol', ... (1 bytes))
1
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/bits', ... (2 bytes))
24
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/length', ... (3 bytes))
533
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT', ... (7 bytes))
1168723
6V:
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/protocol', ... (1 bytes))
1
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/bits', ... (2 bytes))
24
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/length', ... (3 bytes))
546
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT', ... (7 bytes))
1168723
4V:
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/protocol', ... (1 bytes))
1
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/bits', ... (2 bytes))
24
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT/length', ... (3 bytes))
561
Client mosqsub|5593-hass received PUBLISH (d0, q0, r0, m0, 'home/433toMQTT', ... (7 bytes))
1168723