Hardware teardown: the Naran MicroBot Push
In the beginning of this year, we pre-ordered the MicroBot Push from Naran Inc, and we’ve been itching to try it out. Due to some manufacturing hiccups, the Push was only shipped out recently – perfectly on time for this month's teardown!
What is the MicroBot Push? Naran calls it “the wireless robotic button pusher”, which is a pretty accurate description. The idea is actually as genius as it is simple. With a small robotic lever – let’s call it the “pusher” – this device can push (almost) any button for you.
That might sound odd initially, but of course this means that you can make any device with physical buttons remotely controllable, without having to take it apart and hack the electronics inside.
With the rise of the Internet of Things and connected devices, many players are creating solutions to add some digital smarts to older, non-connected devices. You’ve probably seen smart sockets around, which let you turn the power of any device plugged into it on and off; this robotic button pusher takes that idea to the next level.
So before we’re starting to experiment with this device in our offices, we first took it apart to see how it works.
As always, we used only a basic set of tools to dismantle this MicroBot: a Torx screwdriver T5 and a regular screw driver, pliers and tweezers, a USB microscope, and a utility knife.
I started by removing the four Torx screws on the bottom. Then, by removing the black rubber pad on the “pusher” I noticed a 5th, regular screw. After removing this one, I could open the entire enclosure. On the picture below, you can already see a battery, a PCB and a gearbox.
The MicroBot contains two separate PCBs: one for battery management and a second one for capacitive touch, bluetooth and motor control. These two PCBs are connected by a simple 2-pin connector and its female recipient.
First things first: Battery management
Let’s look at the battery management PCB first.
According to its specs, the MicroBot has a battery life of about one year. The battery is rechargeable, using a regular 5V charger with a micro USB connector.
In the picture below you can see two main circuits: a battery charging circuit (marked in red), and a battery protection circuit (marked in blue).
The lower circuit (red) has a 5-lead SOT-23 packaged IC just above the micro USB port which is in charge of, well, charging the battery. The transistor on the left side probably controls the red and green LEDS next to the USB port, indicating whether or not the battery is fully charged. It’s also interesting to note that each LED has a current-limiting resistor (marked in green) – which, essentially, ensures the LEDs don’t blow up.
On the other hand, the upper circuit (blue) consists out of a battery protection IC and a switching MOSFET. I could not find a datasheet for the battery protection IC (the 6-pin IC on the bottom), but I’m assuming it’s an alternative for the DW01-P. These types of ICs are used to “sense” the current; when the specified current limit has been reached they trigger a switch that will break the current path to the battery. The switch that is triggered in this case is a Dual N-Channel Enhancement Mode Power MOSFET, in this case the 8205A. Finally, there’s a simple on/off switch to make sure the user can keep the device from consuming unnecessary energy.
Next up: the main PCB
Now let’s move on to the second, larger PCB, starting on top. In the picture above, you can see three main circuits: the capacitive touch button and matching circuitry (marked in red), the bluetooth chip and antenna (I had to remove a metal shield to reveal the chip; marked in green), and a voltage regulator (marked in blue).
The big gold circle you see in the middle of the PCB is a capacitive touch button. It’s simply an exposed piece of copper – the real magic happens in the TTP223B chip (blue). This chip is a one-key touch pad detector that can detect the touch of a human finger on the touch pad. It does this by using the “Frequency Change” method, which you can read more about here.
On the top right of the PCB, you can see the voltage regulator, which is used to bring down the 4 volts of the battery to 2.5 volts.
Moving to t he bottom right of the PCB, you can see the bluetooth chip, in this case the nRF51822 from Nordic Semiconductors. It’s a 2.4GHz ultra low-power bluetooth chip built around a 32-bit ARM® Cortex™ M0 CPU. If you read our first teardown, you might recognise this one: Withings used the same chip in their activity tracker.
To the right of the nRF51822, you can see two oscillators, one 16MHz and one 32.768kHz (marked in orange), which generate the clock waveforms for the CPU. On the bottom (marked in yellow) you see the antenna circuitry and the PCB antenna itself. As you can see, Naran did not use a balun chip like Withings did, but instead created a matching network using some capacitors and inductors. You can read about balun networks in more detail here.
Let’s turn to the back of the PCB!
Next to some current-limiting resistors and other passive components, there’s only one circuit here: the one that controls the DC motor.
Unfortunately, I could not trace back the datasheets of the actual components used, but after probing the upper IC with an oscilloscope I noticed that this is also a voltage regulator. This one boosts the voltage from 2.5 volts back to 4 volts to power the motor and the motor controller.
Of course, we saved the most interesting parts for last: let’s dive into the Microbot’s motor and gears.
Opening up the gearbox was fairly simple: I just had to remove two screws to expose the gears and the motor, as you can see in the pictures below.
For readers who are less familiar with these devices, we might first dive into some basic info regarding gears.
What are gears? Gears are toothed wheels that work with others to alter the relation between the speed of a driving mechanism and the speed of the driven parts.
This relation of speed (indicated in RPM, Revolutions Per Minute) can be calculated by the number of teeth on each gear: this gives us the gear ratio.
As you can see in the example below, if the pinion or drive gear has 9 teeth and the driven gear has 20 teeth, then the ratio is 20:9 or 2.22. That means that the driven gear rotates 2.22 times slower than the drive gear.
Using this formula, we can create complex sets of gears to alter the speed and torque of the complete system. Such a system of gears is called a gear train (or gear chain).
Back to our button-pushing microbot! The gears that are used in the MicroBot are called “spur gears”: their teeth are straight and parallel to the shaft. As space is scarce in most electronic devices, manufacturers also often opt for compound gears. These gears have two different sets of teeth, as you can see in the image below. This can seriously minimize the space needed for the gear train.
With this info, we can calculate the overall ratio of the gearbox; you can see my results below. It’s important to note that gear A is the pinion gear attached to the shaft of the motor. You can test an animation of the gear train here.
From the calculations above, we can conclude that when the motor has a speed of eg. 12.000 RPM, and the total gear ratio of the gear train is 948.41, the output gear will have an RPM of 12.65. That is a huge reduction in speed, and an increase in torque.
The latter is interesting to take note of: the gears are not only capable to alter speed, but also to transfer torque. Calculating the transfer of torque is simple: it is the inverse of the speed ratio.
Unfortunately, I couldn’t find the datasheet for the motor, so I won’t be able to check its performance graph. Additionally, Naran advertises that the MicroBot has a “torque” of 1.6kgf (or 15.7 Newton); however, kgf is a unit that indicates force, not torque, leaving me unable to do any proper calculations there.
And with that, I can conclude this teardown! I hope you found it interesting – we’re certainly looking forward to test this Microbot around the office. And if you’d like to suggest a subject for a future teardown, let us know on Facebook or Twitter.