My 3 year old son tends to over-steer and over-throttle his E-Revo (he has somewhat fuzzy understanding of the word "gentle" :) so to make the vehicle more kid-friendly I needed an "Exponential control" (Expo) feature and Dual-Rates - something not available on the cheap stock Traxxas remote control. Naturally, I turned to Spektrum and their DX3S surface radio in particular. The kit came with telemetry capable SR3300T receiver and sensors - such nice functionality should not go to waste.
The SR3300T has inputs for a temperature sensor, a RPM sensor and a LAP sensor. The voltage sensor is internal to the receiver (more on this later...)
The RPM sensor install turned out to be a bit more involved. The stock spur gear cover has a mount for RPM sensor but Traxxas telemetry is using magnetic sensor - a small magnet is placed in the spur gear and the sensor on the cover picks up the magnet's rotation. Spektrum, on the other hand is using optical (infra-red) sensor. IMHO this is a more universal solution - a reflective or non-reflective sticker (depending on the situation) can be placed on almost every spinning part and it will not cause off-balance issue like the heavy magnet will.
This picture shows the optical sensor mounted on the spur-gear cover. I drilled a small hole for the opto-couple and secured the little sensor PCB inside the magnetic sensor bed, using the screws provided by Spektrum. A word of caution - the clearance between the inside wall of the spur-gear cover and the spur gear is very small - I had to shorten the screws so they are now flush with the inside wall and don't catch on the spur gear.
I used a thin aluminum sheet (roof flashing), which I polished with polishing compound to a mirror surface and covered the polished side with transparent Scotch(tm) tape for weather protection. Then I cut a few "pizza slice" pieces - different sizes to fit on the 3 different spur gears I might use. I epoxy glued the metal slice to the spur gear as shown on the picture. To minimize any potential balancing issues caused by the added eccentric weight, I selected a spot opposite of the magnet's cavity to glue the reflector and filled the cavity with a mixture of epoxy and metal shavings - approximately the same weight of my mirror + glue combo used (I used very precise micro-gram scale). This is probably not needed at all and it is just me, obsessing with accuracy. The spur gear is small enough and light enough that balancing is not a concern - Traxxas certainly didn't provide a way to balance the much heavier magnet in their setup.
Speaking of Spektrum shortcomings - why on Earth there is no external voltage sensor input? The voltage reported by the Telemetry module is the internal receiver / servo voltage and not the actual battery pack voltage. On electric models, the ESC supplies power to the receiver and it is usually 6v regulated. Only explanation is that it probably never occurred to the designer that the receiver might be used with electric models too.
Too bad that Spektrum never thought of this! I'd take battery pack telemetry reading on any day instead of the stupid LAP timer. What good is lap time for if you don't know that you should be preserving power just to finish?
A possible work-around is to power only the servos from the regulated ESC and have the receiver powered directly by the battery pack. The price to pay is when the setup is used with 3S or 4S LiPo packs or dual NiMH packs connected in series - such solution needs a proper preset voltage drop circuit at the input as the receiver can not handle more than 9.6V. Going that route, one needs to mentally add the amount of the voltage drop to the displayed value to get an actual reading.
I might try to open up the receiver and see if I can hack into the internal voltage sense input. Then I can re-purpose the useless LAP sensor connector as a external voltage sensor. Not sure if it is possible - wish I could find the schematics. If successful, I'll eliminate extra wiring and connectors to separate receiver and servo power and can have just a couple of voltage sensor pigtails for different power sources.