For about the same price as a transponder for other systems, you can get the complete Zeppin Voice Lap set-up. It works very well, but isn't well suited for racing with others.
You can get multiple transponders (only $12 each) and bind them to the receiver/recorder, but they're all on the same channel... great for outfitting different vehicles you want to time one-at-a-time, but useless for timing multiple vehicles at once.
In a multi-competitor racing scenario, each racer would have to have his own Voice Lap Rx/Tx and lap times would have to be compared manually at the completion of the race. Workable, but not at all convenient.
The device works by picking up an IR beam from the transponder. There are dual emitters on the full-size transponder and they offer a Mini-Z size transponder with a single emitter.
They claim the emitter can work through Lexan bodies but I've read of mixed results. My dual-emitter transponder easily shoots through the front wheel well of my Rival MT10. You have to pick one direction; if I run the car the other direction around the track, I have to aim the transponder out the appropriate side or move the receiver/recorder to the other side of the track. I haven't tried it through Lexan. It has worked perfectly so far.
I have the transponder Velcro'd to the top of the MT10 receiver box. It plugs into the receiver for power and has an on-off switch on it.
Edit to add: To clarify, the receiver has several channels and you bind the emitter to the receiver's current channel. That allows you to operate several Voice Lap systems at one time.
