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Old 07-10-2011, 08:11 AM   #1 (permalink)
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Default How Telemetry Sensors Work on TM1000

I'm an engineer and have been keenly interested in learning how the Spektrum TM1000 telemetry system works. This weekend I learned how the temperature, voltage and RPM sensors work and am posting that info here so others can benefit.

First, this information is based purely upon observation of how my system works. Unlike most electronics companies, Spektrum does NOT publish the specifications for their radio systems which means we have to reverse engineer them to understand what they can really do. I would very much like to know things like: rf power output & spectral purity, TX and RX power drain with and without telemetry, telemetry update rate, telemetry resolution per channel, telemetry sensor range, telemetry sensor accuracy and so on and so on.

What I have found so far:

RPM port. When looking into the TM1000, the left-most connector is for the RPM sensor. The three pins going from left to right are: +, ground and signal. The + power supplied to the sensor by the TM1000 seems to be around 3.5 volts on my TM1000 when using a BEC with a 6V output (TREX 450SE). I think this is because the TM1000 can actively turn on/off sensors if you have them enabled/Inhibited in the DX8. I saw that power to the RPM sensor does not come on immediately when you plug in the battery, it becomes active a few seconds later.

RPM Signal. The good news is that the type of signal fed to the TM1000 from the RPM sensor is a simple square wave pulse where you get one pulse for each trigger of the sensor (crank pin swing, gear tooth, optical reflection, etc.) This means a whole host of sensors can be used with the TM1000 as long as the signal is within the voltages on the + and ground pins. Many magnetic (Hall effect) and light sensors are available that would work right out of the box with the TM1000. Typically they are 3-wire devices which can plug directly into the TM1000 (power, common and signal). Just make sure they work at 5V or lower power supply.

My first RPM sensor. I was able to plug-and-play with a brushless motor RPM sensor from Eagle Tree Systems.
The ETS sensor comes with a full size servo-type connector and so an adapter will need to be made. I used a Spektrum satellite RX cable and servo extension to make a patch cable. IMPORTANT: The ETS sensor wires do NOT follow the typical convention used for servos! On the ETS RPM sensor, the black wire is +, red is ground and white is signal. If you swap the wires correctly the brushless RPM sensor works very well with the TM1000.

Temperature Sensor. The TM1000 temperature sensor is a NTC thermistor, or "negative thermal coefficient" resistor. I measured it's resistance at two temperatures (room and in hand) compared against a 10K NTC thermistor I had on hand and the two agreed within 5%. But I only had the one and it was consistently slightly lower in resistance than the 10K. The next lower standard NTC size is 8.5K which would be considerably lower that what I measured, so I conclude that the TM1000 expects a 10K NTC thermistor.

Temperature & Voltage Connection. The temperature & voltage sensors are connected to the TM1000 in the center port. The Y-harness should be used in this port if using both temperature and voltage sensors simultaneously. The port pins (and color from Y harness) from left to right are Voltage (red), Common (black) and Temperature (orange).

Voltage Sensor. This is the easiest as the TM1000 relays the voltage measured between the common and voltage input wires.

What's missing. I still need to determine what the min/max values are for the RPM pulse rate, voltage and temperature measurement capability. The NTC thermistor has more precision at room temperature that at engine cylinder head temperatures (so BEWARE! and check the temp sensor against a known thermometer before putting your $$$ aircraft on the line).

I have found both by research and my own experience that if you plan to use telemetry you MUST be sure your BEC can supply enough current. A ship with lots of digital servos, gryo, RX, sat RXs and a TM1000 will use a LOT of current. I am near the limit already on the TREX 450 and am thinking about a stand-alone BEC for safety. If your RX power drops too low during flight you can loose control of the aircraft!

Good luck and I hope this helps!
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