SkyBell Video Doorbell

Looking through a camera at what’s happening outside was first done by Nazis in 1942, to keep an eye on the immense V-2 rockets. Now, you too can look outside, and using your smartphone. Something that Nazis could not do.

And your camera will be inside the doorbell, so you can see people at the door, particularly their index finger. Video doorbells, the WiFi ones, have been attempted for several years now. DoorBot raised $1 million from VCs over two years ago, and is finally shipping, having been re-branded as Ring. And there’s been crowdfunded Chui, i-Bell, Smartbell, etc.  They look different but somehow they all cost $199 and work with an iPhone.

A big reason for placing an outdoor surveillance camera in the space of a regular doorbell is that the two wires coming out of the wall carry 16-24V AC, from a transformer rated at 10-30VA. Just think of the doorbell buttons that glow yellow. While WiFi provides wireless data connectivity, getting power outside does take an effort. Bringing power from inside requires drilling the wall. An outdoor outlet with a wallwart and a long cable would at best be unsightly and potentially hazardous. Batteries would run out eventually or get destroyed in particularly hot or cold weather, and the unit would have to be removed to install new batteries.

SkyBell HD Silver WiFi Video Doorbell

One camera doorbell is actually shipping and was well  received – SkyBell. On Indiegogo they raised a hefty $600 thousand two years ago. So, let’s look at what’s inside.

The plastic cover comes off after removing one tiny Allen screw. Not surprisingly, the PCB is round and has a camera setup hanging off it. The main processor is ST Microelectronics STM32F407 in LQFP 100 pin package.  This little embedded monster sport a 32-bit ARM Cortex-M4, running up to 168MHz, with 1MB of Flash. Some of the peripheral options include USB and Ethernet MAC, in addition to the usual UART, SPI, I2C.

The STM32F407VGT6 part is around $10.* They could have used a lower end part, with an ARM M3 processor, at 120MHz and no FPU, like STM32F217VG. But in the same package and memory size, the price quote is about the same between the two devices. As to power consumption, SkyBell is running wired. But if power consumption were a concern, these parts are not particularly low power. Full on, at 168MHz the ‘407 takes 93mA and ‘217 at 120MHz takes 61mA. A low power ARM processor Stop mode, that retains SRAM but has peripherals off, can get below 0.5 mA nominal.

Despite having all kinds of power from the doorbell transformer, SkyBell includes a 300mAh Lithium Ion battery (about $1 on Alibaba in quantity). If the idea behind the backup battery was for the device to stay up through a short power outage, the WiFi AP and the router would likely be down during an outage anyway and SkyBell has no local storage.

Included is a Linear Technology LT3990 power regulator, over $3. WiFi is implemented via a leadless QFN package with no clear manufacturer markings.  A U.Fl cable connects it to Taoglas FXP74 external antenna, $6.

Video is implemented via OmniVision OV7740 1/5″ CMOS VGA image sensor, which is $5-10 with a lens on Alibaba. Plus Conexant CX93510-11Z VGA JPEG encoder with 128K frame buffer, $6. A flex cable connecting the two says 650nm. The lens is marked ZXZ001.

The PCB is unevenly smeared with some goop, which would offer no protection against the elements. The back plate attaches to the (stucco) wall with two screws. This $199 device is conveniently located within easy reach, to be pried off with a screwdriver and ripped out in two seconds flat. The plastic case with loosely connected lens assembly and the button will not withstand a good punch. The rain can get in from several places in the back, as there is no provision for even a simple moisture seal. I suppose, a good amount of silicon caulking inside and around the rim of the unit could offer some protection.

* Prices are planning estimates, according to Octopart, quantity 10K.