Jim Andrews, KH6HTV developed a circuit to control the PTT line for his repeater so when the receiver detected a valid signal it would turn on the transmitter.   This is the same one used in the W0ATN repeater.  The PTT line is connected to an open collector transistor which could turn on other things like a relay, another light or in this case, a visual and audio alarm.   Refer to the schematic and PCB layout.  

 The alarm “beeping” level may be controlled by simply blocking off the hole on its miniature “Sonalert” type of audio indicator.  The alarm is available from eBay and Amazon for a few dollars.  A switch was included to “disarm it.”  A low cost plastic enclosure would work fine too.  The interface between the Green LED and the alarm is made with a small PC Board mounted inside the receiver.   The 2N3904 Bi-polar NPN transistor will handle up to a couple of hundred mills at a nominal 12vdc.  Refer to the docs and pictures for building one for your Hi Des HV-110/120 or 122 DVB-T receiver.   A bare PCB is available free to SLATS club members.  Thanks to Bob, KD0JIY for help with the PCBoard design developed using open source KiCaD.