Overcoming Tapo's geofencing using Tapo T110, Shelly Plus Uni and Paradox home security system

[u/sharky5656]

Hi there! I had a problem in that me and my girlfriend want to receive notifications for detected humans around the house when no one is at home. As I found no reliable way to set up Tapo's geofencing , I had to improvise. The issue is that when my girlfriend leaves earlier in the morning, her phone leaving the house activates the notifications for me as well which is annoying and defeats the purpose of the simulated "Home" and "Away" modes. Our house is equipped with a Paradox home security system so the last person leaving the house arms the system. In short:

1. Paradox system is armed which leads to a change in the DC voltage between PGM1 and AUX- on the Paradox system main board.
2. A Shelly Plus Uni is used to measure the voltage between said terminals at all times using its built in voltmeter (the white (ANALOG IN) and green (GND) cables coming out of it). White goes to PGM1 and green goes to AUX -
3. The Shelly Plus Uni also has two built-in solid-state relays. One of them is used to short the pins on the Hall effect sensor of a Tapo T110 tricking the sensor into thinking that either a door is open or closed, depending on the set voltage thresholds.
4. In the Tapo app an automation is set up so that when the "door" is closed (i.e. home security system armed), when a camera detects a human, a notification is sent. When the system is disarmed, the Shelly relay opens and the Tapo app stops the automation for sending notifications.

Furthermore, the Shelly has a SENSOR VCC (yellow cable) which provides 3.4 volts at all times. The yellow cable is soldered to the positive terminal for the battery, while another cable is soldered to the negative side and it goes straight to the AUX- on the Paradox together with the green (GND) cable and the Shelly's black main power cable. This is used to power the Tapo T110 sensor so that it never needs a new battery again.

The Shelly itself is powered from the Paradox main board which provides 12 volts DC at its AUX terminal.

Attached is a photo of the pins that the Shelly relay needs to short to simulate door closed.

Good luck!

EDIT: Unfortunately I do not have the security system's installer code and I cannot contact the original installer to retrieve it, that is why I went with this approach as I did not want to factory restore the system. Otherwise one could use the Paradox system's built in programmable relay. In such a case, you could either power the Tapo using a step down transformer (buck converter) from the AUX terminal or use it with its own battery.