Developed by SlimeVR in Rotterdam, Netherlands, the SlimeVR Butterfly Trackers are open-hardware, IMU-based, ultra-slim, full-body VR trackers for virtual reality gaming, motion capture, VTubing, and more. These trackers provide full-body positional tracking without base stations, cameras, Wi-Fi, or Bluetooth.
Each tracker employs an IMU to measure absolute rotation and sends data over a custom 2.4 GHz ESB protocol via a dedicated dongle supporting up to 10 trackers. This setup eliminates occlusion issues and router setup. Based on Nordic Semi’s nRF52833 wireless MCU, the innovative “butterfly” design places the PCB and 90 mAh battery side-by-side, achieving a thickness under 7 mm and a weight below 10 grams. A flexible interconnect enhances comfort under clothing. The system offers a 100–200 Hz refresh rate, 15 ms latency, over 48 hours of battery life with USB-C charging and OTA firmware updates.
Specifications of SlimeVR Butterfly Trackers include:
– Wireless MCU: Nordic nRF52833
– CPU: Arm Cortex-M4F microcontroller @ 64 MHz
– Memory: 128 kB RAM
– Storage: 512 kB flash
– Wireless Connectivity:
– 2.4 GHz proprietary wireless via Enhanced ShockBurst (ESB) protocol (requires a dedicated USB dongle on host)
– Up to 10 meters of range
– 100 Hz to 200 Hz refresh rate
– 15 ms latency
– USB: USB Type-C port for charging and firmware upgrade
– Sensor: 6-axis IMU for absolute rotation tracking (TDK ICM-45686)
– Misc: RGB status LED
– Power:
– 5V from USB-C is used for charging
– 90 mAh battery with 48+ hours of active use
– Deep sleep support for extended standby
– Optional 10-tracker charging dock
– Dimensions: 56 x 35 x 7 mm
– Weight: 10 grams per tracker
– Certifications: FCC, CE (WIP)
– Mounting: Hook-and-loop (Velcro-compatible) strapping option, straps included in sets
– Enclosure: Injection-molded polycarbonate with flexible butterfly bridge for body contouring
These are not the first VR trackers from SlimeVR; the original SlimeVR trackers (which raised over 9 million dollars) were based on Espressif ESP32/ESP8266 modules. They were suitable for DIY users but had high power consumption due to Wi-Fi overhead. The Butterfly Trackers use the more efficient Nordic Semiconductor nRF52833 SoC and Enhanced ShockBurst (ESB) protocol, running over 48 hours on a 90 mAh battery. They require a dedicated USB dongle instead of connecting directly to a home router.
Notably, the 7mm thin design has PCB and battery positioned side-by-side. The flexible bridge connecting the two halves allows the tracker to conform to the user’s body, enhancing comfort when using VR while lying down or wearing tight clothes.
The software stack includes firmware, a central server, and third-party integrations. The Butterfly trackers operate on the Smol Slime firmware, originally a community-driven experimental project. The Smol Slime Firmware and Smol Slime Dongle Firmware are available on GitHub.
The SlimeVR Server, a cross-platform application, acts as the “brain,” receiving raw rotation data and using forward kinematics and a virtual skeleton model for limb positioning. Features include Autobone (automatic skeletal calibration) and Stay Aligned (drift compensation). The software supports up to 20 trackers, aiming for extreme precision.
To visualize movement in games like VRChat or Minecraft, the server integrates with the game. It employs OpenVR Driver, making SteamVR recognize the trackers as official HTC Vive or Tundra devices, and supports OSC protocol for standalone Quest users.
Currently available on Crowd Supply, the SlimeVR Butterfly Trackers are offered in various bundles, with a $279 Core Set (6 trackers + dongle) and up to a $788 17-tracker Motion Capture Set. A $499 Sakura Edition includes a charging dock. Upgrade packs, individual trackers, and accessories like a charging dock, straps, patches, and belt clips are available. Shipping is scheduled for August 31, 2026.