Restoring an old RC Lap Counter

Thread Starter


Joined Sep 12, 2021
Hi everyone, I'm new in this forum and this is my first post. I'm a software developer but electronics has always been one of my favourite hobbies. I don't have experience in RF circuit design, and for this reason I'm here trying to find someone that can help me in this project. I'll try to be coincise as much as possible.

I have an old RC Lap Timing system which has been built by someone 20+ years ago. This transponder system is composed by the usual small plastic cubes which are installed onboard (on rc cars) and are recognized by the receiver station when the rc car passes over the antenna loop (a wire which is layed on the track). These cubes are died because of the leaky battery that after all of these years has ruined all the internal circuits, but the receiver still works pretty well.

What I'm trying to do is to rebuild the transponder cubes using an alternative solution, maybe using modern oscillators, or in the worst case reusing the existing crystals. It's hard to figure out which are the components due to a glue which covers everything, but after taking a look to the circuitry of transponders and receiver station, I can say the following:

- The transponders are transmitting just a plain signal on the crystal frequency, or multiples, since there are not particular components apart from a JFET transitor and an inductance. Crystals are in the range of 5,3 to 5,6 MHz.
- The receiver is just an array of narrow filters tuned on the frequency of each transponder cube, so no decoders or particular things. This is compatible with my hypothesis that the transmitted signal is just a sine wave or at most a square wave.
- I tried to generate an signal using an RF generator but I didn't manage to trigger the receiver, even in sweep mode, I didn't get any "transponder" pass. The generator I have is capable just to generate signals from 35 MHz, so I'm assuming that the used frequencies are below this value, even if multiple of the crystal base frequency.

I'd like to build something that can completely replace the existing cubes, and after some research I saw that there are programmable oscillators or the easiest adjustable ones like the LTC1799 which I think could fit my needs. Now, even if these transponders are working in a very close range (max 60cm) I guess that an amplification stage is still needed. This is the only missing part for me, so I'd like to ask you guys some help in this way.

Obviously if you have any comment, question or remark, feel free to reply!

Thanks in advance!