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.