Portable Racing Transponder

[BrianG]

I'm between projects right now, so as usual, I got bored and decided to do a little tweaking to something. I have a total of 9 running vehicles and two AMBrc personal transponders. Now, I don't do much racing at all, but it sucks to have to uninstall a transponder from one vehicle to another disrupting my carefully routed wiring in the process. I could use the track's loaner transponders if I forget to swap mine out, but I have my own, so why not use it?

So, I thought I'd make my own portable transponder.

First, I went to RadioShack and picked up a rechargeable 9v battery, and a tiny project box. I then gathered the transponder, a spare female servo plug, and some extra servo wire (not pictured):





A NiMH 9v battery consists of 7 tiny cells arranged in series for a nominal 8.4v, but that's too high. I cut open the battery casing and removed two of the cells so I'm left with 5 cells, which is 6v nominal. Perfect. Don't let the size of the picture fool you, there are small! After all 7 of these fit into a 9v battery case:





Since the cells will not fit as-is in the box I have, so they must be reconfigured. The cell sheathing was coming apart as I disassembled them, so I reheatshrunk them to make sure they were insulated well. I then made a little side-by-side pack using spare servo wire as the bus "bars".:





I cut a rectangular opening on one end for the servo plug and CA-glued it in place. I attached the wires to the pack and servo-taped the pack in place. I added a little hot glue for some extra insurance:





This box is normally held together via a screw right in the middle, but since the pack is there, I cut it out. So, I measured the voltage at the plug was right and then CA-glued the case together. No going back now!

And the finished product:





Just gotta remember to unplug the transponder when not in use or it'll drain the batteries. Charging the batteries is simply a matter of making a servo plug adaptor and using a regular NiMH charger.

The transponder is held on with velcro in case I decide to attach it directly to a receiver for a specific vehicle.

Now, some techy data. These transponders can take between 4v and 8v, and draw 15mA @ 5v. Assuming the current is linear with voltage, it will draw 18mA @ 6v. The cells I used are rated 150mAh, so that should give me well over 8 hours of use between charges.

Useful? Maybe not, but like I said; I was bored.

[E-Revonut]

You still have to take it out of a vehicle and put it in another. Now you have a bigger item to find space for, an extra battery to charge, and extra weight. Slap a piece of velcro on the PT and a matching piece in your race vehicles. As soon as your done racing remove it and you won't have to look at wires that aren't perfectly neat!

[BrianG]

Yeah, it's kinda big, but I have room in any of my conversions. As far as battery to charge, it lasts over 8 hours, which should be plenty for a day racing. Extra weight? Maybe in 10th scale 2WD, but for larger 8th scale b/truggies, a couple oz on a 8lb+ car is not gonna matter much. And I can easily detach the 'ponder from the battery case if a setup has easy receiver access.

My issue is that my receiver boxes are already pretty tight and usually requires moving wires around. And if I'm using two vehicles, that means tidying up the wiring in two vehicles when it is switched.

No biggie, I didn't think this was gonna be a big hit. But maybe someone might find it useful.

[lincpimp]

I like it Brian! Good stuff.

Nothing like finding a good use for a 9v battery.

I have a few of the lipos out of a pc battery laying around. I was wondering what to use them for, but I think they will be made into a pack for my newly acquired nintendo ds. It has a lipo in it already, so it must have a cutoff too. Plus the lipo cells were free so who cares!

[Snipin_Willy]

Wow those cells are small if the transponder is just as wide as them. But 8.4V? I had a couple of rechargable "9 volt" batteries that are 9.6v nominal. Guess they figured out how to cram another cell in them.

[BrianG]

Yeah, they are tiny cells. I felt like I was a giant working on a sub-C pack.

An 8 cell, 9.6v nominal pack would be around 11v fresh off the charger (~1.4v/cell). Some electronics may not like that much overvolting. But I don't really like a 7 cell 8.4v nominal value either - some devices see that as getting close to a dead regular battery. But, fresh off the charger, they read about 9.6v, which is safer IMO. Anyway, there was easily enough room in the 9v case for another cell, they just spaced the cells so it was filled up.