Drum kits are essential in many genres of music, ranging from classic rock and roll all the way to modern dubstep. While many use drum machine VST packages (virtual studio technology), there are millions of musicians who use real drum kits and for good reason. While drum kits are a great instrument they are not the most portable kit. This is where a Kickstarter project, Freedrum, hopes to change just that! Their small device is attached to either drum sticks or feet (or even both) and allows you to play drums wherever you are.
The Main PCB
The main PCB consists of a few ICs and surface mount components. The PCB clearly demonstrates production techniques that are helpful for low-volume production such as placing all surface mount components all on one side.
Using all surface mount helps to keep production costs to a minimum as there is no hand soldering needed. The use of all surface mount parts on one side also means that, once all the parts are placed onto the PCB, the whole board is soldered in one go using a reflow oven.
The underside of the PCB shows the use of a large ground plane which helps to remove unwanted electronic noise in signal and power wires. The use of the ground plane and stitching via also helps with compliance with EMC regulations that are imposed by CE and FCC.
One other interesting feature of the Freedrum PCB is how, instead of being V-Scored, the PCB is milled with drilled tabs. While the tabs, themselves, are a dead giveaway to milling, there is one other subtle clue, the angled cut marks on the board edge. This shows the tool rotation as the milling bit cut out the PCB border.
The underside of the FreedrumPCB
The Freedrum can connect wirelessly to tablets, computers, and even Android devices using Bluetooth. The IC responsible for this is the N51822QF, which is a Bluetooth system-on-chip produced by Nordic Semiconductor.
The SoC is built around a 32-bit ARM Cortex M0 CPU which 256kB flash ROM, 32kB RAM while integrating a 2.4GHz Bluetooth protocol stack. The chip, itself, has an event-driven API which is crucial in embedded designs as signals that can arrive at any time need to be processed immediately (for low latency).
The Bluetooth controller N51822QFA
The Main Controller
The main controller for the Freedrum device is the INVESENSE MPU-6050 which is a microcontroller with inbuilt 3-axis gyroscopes and 3-axis accelerometers.
The advantage of such a controller is the use of inbuilt sensors that removes the requirement of external sensor ICs which can drive up production cost, increase PCB complexity, and use precious PCB space. The controller also has generic modules such as I2C and SPI so other sensors and devices can easily be connected to the system.
The main controller IC, the INVESENSE MPU-6050
Wi-Fi Antenna and Other Components
Interestingly, the Freedrum PCB uses a custom PCB antenna trace which makes commercialization more difficult. A complete Wi-Fi module that contains its own antenna should already be certified for EMC standards which means that it can mostly (take it with a pinch of salt) be ignored and does not require testing.
Using a custom antenna means that RF testing is needed which can cost a small fortune (especially for small start-ups). The Wi-Fi company I used to work at was charged close to $2000 per day for hiring a RF testing room to confirm that their products conformed to European standards!
The custom PCB antenna
Near the Bluetooth IC is an SMD oscillator which is most likely needed for the precise generation of the 2.4GHz carrier wave. It is most likely that such precise control is needed not because the receiver won’t pick up the signal but to prevent the emitter from interfering with other radio equipment which is a big no-no from regulations such as CE and FCC.
It is also likely that the oscillator, itself, is of a much lower frequency (such as 150MHz) which is then multiplied to get the 2.4GHz carrier signal while lower frequency allows the controller to use the oscillator as its main clock signal.
The oscillator SMD package
The back of the PCB shows a few marking including the fact that the PCB is lead free (Pb).
Markings on the back of the PCB
The Freedrum PCB is a very small, compact, and neat design. The use of SoC devices and controllers with built-in MEMS devices shows a desire to keep both the cost low as well as the overall PCB dimensions. This is imperative in a design that is meant to be attached to a pair of drumsticks: user friendliness demands that the board must not distract the drummer or make it more difficult to use the drumsticks (more weight on the stick may make it feel less natural).
The use of components on one side show an intention to keep the production line as fast as possible while the use of tab routing suggests that the designers of Freedrum may build the first shipments of units, themselves. If Freedrum becomes a commercial success, the designers may move to a v-grooved design and shift production lines to China.