A Mobile Radio Power Controller

[Pete], a.k.a. [KD8TBW] wanted to install his Yaesu radio in his car. From experience, he knew that having a radio in a car inevitable led to leaving it on once in a while, and this time, he wanted a device that would turn his rig on and off when the key was in the ignition. He ended up building a mobile radio power converter. It takes the 12V from the car when the alternator is running, and shuts everything off when the engine has stopped.

The Yaesu radio in question – an FT-8800 does have an automatic power off feature, but this is a terrible way of doing things. There is no way to turn the radio back on, and the radio must be left in a non-scanning mode.

In what he hopes to be his last design in EagleCAD, [Pete] whipped up a board featuring an ATtiny85 that measures the voltage in the car; when it’s ~14V, the alternator is working, and the radio can be switched on. When it drops to ~12V, it’s time to turn the radio off. It’s a great project, and with the 3D printed case, it can easily be shoved inside the console. Video below.

24 thoughts on “A Mobile Radio Power Controller

    1. ACC turns off when the starter is triggered… Causing your power to turn on, turn off, then on. You need to use the RUN circuit as it does not switch off during starting.

      The best bet would be a time delay RELAY on the RUN circuit, to avoid power line noise during starting. While key is off, no power draw, ever. But what is the fun in that? Why solve a problem when you can over-engineer a solution? Attaching a device to an already switched power source is boring.

      I am curious as to the power rating of the MOSFET. My FT7800 can pull some amperage while at 50W mode. I’d use the mosfet to drive a relay if I couldn’t find an switched power source.

      1. That might vary from car to car. I use the ACC line + relay method in my Jeep Cherokee and never had that problem. i was planning on doing the same in my Hundai Accent soon. Maybe I will have this problem there and end up needing this circuit…

      2. During testing I was able to pull around 15 amps at 13.8 v or ~200 W while still passing the “can I keep my finger on the mosfet without burning myself” test. I have found that limit of comfortability to be about 50C previously.

    2. I’ve used a 30A ignition-switched relay for 10+ years in my last two vehicles without a problem running 2 radios, APRS, a laptop boost power supply and a multi-port 12V outlet setup. It works even when my battery is low, which let me use my radio to call a friend for help when my battery failed (happened only once).

      It’s still a neat idea and a nice application of an ATtiny. But devices like these have been available commercially for years:

      The commercial version adds an override button, over voltage sensing and AC sensing of starter pulses (helps detection with some cars).

      I know, not a hack, just pointing out it’s not a new idea.

  1. Ohhh, okay. It’s a time delay digital relay he built. That’s the secret sauce.

    I suppose if you’re pumping a lot of watts out, you might want a direct-battery connection, but I never needed one myself. I’ve personally got a Yaesu FT-8700, and all I did was hook it up to an ignition-controlled fuse in my car. The radio returns to its last power mode after a power-loss event. In other words, if the radio is on when I turn the car off, the radio turns on when I turn the ignition back on. Easy-peasy.

        1. Yup, there are a couple solder points on the board for this. PB1, PB2, PB3, and PB5. Just wire up a switch between one of those points and ground. Then, add a few lines of code to make the switch do something! I thought about this, but didn’t implement it because I figured I wouldn’t use it ( or even install said remote switch ).

      1. Well, yes and no. I’ve got it wired into the same circuit as the FM radio, so its powered pretty much any time the key isn’t in the off or start position. But I do try to not transmit when the motor is off, as that little sucker does pull some amps. :-)

    1. All the radio manuals I have ever read plus many other ARRL books, online blogs, etc… I have read on the subject have been very emphatic about running a dedicated line straight to the battery. This is both due to the amperage needed as well as noise. Supposedly a direct line to the batter will have less alternator noise than a “cigarette lighter” line.

      That being said I have used a tap into the accessory line which has a cigarette lighter type plug in my Jeep cherokee for years. I have had no problems. It isn’t the actual cigarette lighter line though, it is an always-on line marked 20 amps. There is a separate, smaller gauge line for the lighter.

      For comparison my Hundai Accent has wire that could almost be used as dental floss going to the cigarette lighter. I never even attempted to use it for transmit, I just ran the dedicated line right away.

  2. I have been thinking along similar lines for a project – an auto shut off based on voltage or a manual “off” switch. It would have a mode to turn on for one minute every hour to put out an APRS location beacon. In my F-150 I have a secret hiding spot under the dash that the microphone and control head go into at night. If the truck was stolen it would still be able to beacon out position.

  3. I can’t remember where I bought my board and circuit. It powers on (through a relay) when car starts, but stays on for ten minutes after car turns off. I like the delayed turn off.

  4. “There is no way to turn the radio back on, and the radio must be left in a non-scanning mode.”

    Since when? My FT-8900, which is overwhelmingly similar to the 8800, has an APO that works just fine, as do all of my other Yaesu radios.

  5. Am I correct in thinking that I could make a dead simple charge controller out of this by hooking the output to a SPDT relay, and making the code switch it on at on at a low voltage and off at a higher voltage? This looks way simpler than my failed attempt to duplicate the 555 charge controller.

  6. As convenient as this sounds, I want to be able to run the radio without a key on the ignition. For example if I am hiking with an HT and using the car as a cross-band repeater.

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