Hosim S911 2.4 GHz RC car
These $35 - $75 toys have been cropping up in the past year or so, highly capable, repairable, and hackable for the price. I was curious as why I was getting much less than the specified 100 meter range from control to car. I found some surprises and some questions as well as answers.
The control is made by Lansu, a known manufacturer of inexpensive RC servo control systems.
There are two chips inside, one had illegible labeling and was connected to handheld control inputs, while the other was connected directly to the antenna with model LT8912SRC, which I could not find on the internet.
There is no FCC ID that I could find anywhere on the unit, despite copious approval logos.
One leg attaches directly to a 4.5 cm wire monopole antenna. On the remote, this wire was tucked down amongst other wires instead of sticking up into the antenna pipe–probably why I was getting 20 m range instead of 100 m range! Also, for 2.4 GHz we would expect about 3cm for a monopole antenna, so we are probably fairly non-resonant, perhaps some high > 3:1 SWR affecting range.
As compared to a helicopter controller spec’d at 3dBm output power, this controller, was about 6dB stronger, so the S911 controller may be just under 10 mW EIRP. The fix was sticking the wire up from the controller. The factory tucked the antenna wire inside because the antenna pipe was in the wrong location! Some disconnect between mechanical engineering and electrical engineering departments is apparent!
Frequencies
Hosim Car Radio beacons every 49 ms at 2404.3 MHz when in pairing mode, stops transmitting until no sync heard from control (out of range or control turned off). Looks like FSK.
Hosim S911 controller at 2404.3 MHz--long time 33ms period.
Hosim S911 controller-short time 100us preamble + 1400us message.
Hosim Controller Radio does not transmit until receiving pairing signal from car, then transmits indefinitely even without control input
| Freq (MHz) | length (us) | cadence (ms) | Function |
|---|---|---|---|
| 2404.3 | 1400 | 48 | car sync / pair |
| 2404.3 | 1500 | 33 | ctrl sync |
| 2422.7 | 1700 | 4,60 | ctrl |
| 2432.7 | 1700 | 20,28 | ctrl |
| 2442.7 | 1700 | 4,13 | ctrl |
| 2452.7 | 1700 | 4,20 | ctrl |
| 2462.7 | 1700 | 4,16 | ctrl |
| 2472.7 | 1700 | 4,20 | ctrl |
Secret unpopulated controller switch: all it does is reverse the left-right steering, perhaps to allow for different steering potentiometers.