Updated: November 10, 2003
I have a couple of Motorola Talkabout T5420 FRS radios. I bought them primarily for use when driving in a group of cars, so we could keep in constant contact with each car so no one would get lost or miss an exit or something. But I also didn't want to worry about draining the batteries, or about the rechargeable NiMH batteries not having any juice. So I thought about making some sort of car adapter.
Motorola sells a drop-in type charger for use with battery packs with the Talkabouts. The battery pack has metal contacts which touch the metal contacts on the radio's batteries cover. This allows you to just drop the radio into the charging cradle and forget about it. I originally wanted to make a cradle of some sort along with a "dummy" battery. The dummy will contact the same contacts of the battery compartment cover that the Motorola battery pack touch. The radio would sit in the cradle, the cradle would provide power via the cover contacts, and I'd use the radio with a Push-To-Talk headset. But after trying to heat-form some acrylic, I decided against the idea. So then I decided to simply use the dummy battery idea and instead of a cradle, I'd make a strap-on adapter. I have a 5 dollar car power adapter that's volatage-adjustable and has a bunch of connector sizes so you can plug it into all your crap. I chose the 9-volt style snap because I had the connectors on hand.
Introducing: The Strap-On
Here you see the completed radio Strap-On setup. The Strap is made from velcro, a piece of acrylic, a 9volt battery snap, and couple sections of spring from a ball point pen. Ghettoriffic.
Here you can see how the strap on touches the cover contacts.
Ok, great. It works. But it's clumsy as hell to strap that damn thing on. So what now?! Redesign! Something, not neccessarily less ghetto, but a bit cleaner. I thought I could do away with the whole Strap-On.
Now introducing: The Suppository
So it's a couple pieces of copper wire bent to fit inside there and touch the internal battery contacts. But it works. I did have to notch out the battery cover to accomodate the wire hanging out. But other than that, NO modifications to the radio in both versions were made. So that's it for now.
Oh by the way, the T5420 radios have VOX voice activated transmission. This is done with a headset plugged into a seemingly normal 2.5mm headset plug. The same size as your typical cell phone headset But the nazi's at motorola decided that they wanted you to buy their special FRS radio headset. So they made they made the plug a few millimeters longer than usual, and they flipped the microphone and the speaker wires around. So to ghetto-rig your own headset, just take you regular run of the mill cellphone headset, and swap the mic and speaker wires. And grind away some of the plastic from the headset jack on your Motorola radio, so your regular plug will sit deeper into the jack. The long Motorola brand headsets reach all the way in there, but the relatively stubby cell-headset needs a little help.
If you do that mod, and turn on the radio while the headset is plugged in, the 5420 will go automatically into VOX mode, and it'll start transmitting when you talk loud enough. If you want to use a push-to-talk (PTT) type of headset, just put a normally-open pushbutton switch in series with the microphone wire. And then you could put a toggle switch in parallel with the PTT button, and you can have your VOX and eat it too.. or something. Anyway, just make sure you "reboot" your radio whenever you flip the toggle button, and the radio will set it in the appropriate mode for you.
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Can you hear me now?