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We use out TT4? as a digipeater ZL1ROT-6? ?and to monitor our repeater solar system electrics? its over an hours drive to and from our 2m Linear repeater site? so when the TT4 locks up its a real? pain.? ? After it doing it a number of times despite latest firmware upgrades? I decided to build a watchdog timer to reset it if it ever locks up . I monitor the Tx red led? ?not the DCD led? as I have never come on a situation where the red red had locked on? ?solid,? but many times when the DCD orange like is locked on . I use the red led to trigger a reset on the timer? so long as the resets continue to happen the timer never reaches its predetermined count? and wont? do a reset on the power to the TT4 . If the tt4 locks up,? it will stop sending packets,? the red Tx led wont reset the timer chip (cd4060) and the timer? will time out? and reset the power.? Im sure there are more sophisticated ways i could have built the circuit? but its what i had at the? time? when the device was urgently? required? ...so far? over 6 months? ?we havnt had to go up into the bush 4X4? ?and do a manual reset? cheers? mike ZL1BTB

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http://www.amalgamate2000.com/radio-hobbies/radio/Watchdog%20timer%20.htm

I like it! I did sort of a similar thing with my MT-AIO in my truck-- I didn't want to have to remember to turn the APRS on/off as I drove or parked and I wanted the unit at the rear of my vehicle, farthest away from my other transceivers in the cab and I didn't want to run additional wiring. My camper shell had a live full time 12Vdc line for the dome light and an intermittent 12Vdc line for the roof top (3rd) brake light. Using those two circuits, a 555 timer IC, and a relay I made a ≈20 minute timer to turn on the APRS and keep it on as long as I step on the brakes at least once within the time period. It made for an easy installation and hands-free automatic turn on & off as I travel.

I can now never forget to turn it on when I go nor off when I arrive. ??