Domoticz

I want to create a virtual signal light on th Domoticz dashboard who will light on and off when a set of contact set are closed or opened.
Its only function is to let a signal light burn only on the Domoticz dashboard, not a real light

Just change the icon of the door sensor to light icons (edit button) or create your own icons. https://www.domoticz.com/wiki/Custom_ic ... binterface

Thanks for the reply, but that's not what I mean.

I would like 2 or more sensors to signal 1 light, when they meet a certain condition, say: when S1 and S2 ar on and S3 is off, (virtual) light on dashboard should go on to "signal" that a condition is true or false

Ok, then you need scripting like Blockly or dzVents (more complex).
https://www.domoticz.com/wiki/Blockly
https://www.domoticz.com/wiki/DzVents:_ ... _scripting

I already use Blockly. The problem is I want a virtual light, same as a virtual switch.
In Domoticz I can create a virtual switch wich appears on the Home Screen and I can use the functionality to switch other devices.
But I want I virtual light bulb on the dashboard wich lights up when conditions are met (some switches closed)

I do not understand your problem.
A switch or a light is for Domoticz almost the same so you can change the icon of your virtual switch to a bulb and then the bulb icon will show up on your dashboard.

That if you are using the standard dashboard and not something like Dashticz.