I bought a wireless doorbell at Lidl branded SILVERCREST for unbelievable EUR 5.99. Really amazing! I have been asking myself how this price can be achieved and decided to buy a package and do a tear down of the devices. The product is labeled “JAN 271697”, the transmitter has the model number HG01041A-TX, the door bell has the model number HG01041A-RX. Key features are:
- Wireless at 433.92 MHz, OOK
- Battery operated, batteries are included in package
- 1 push button can operate several chime box receivers
- 36 different tunes and one visual LED indicator
- self learning code system
In the package we find the Transmitter with a Label for the name, the receiver box, a manual, batteries and some mounting material. Inserting the battery cell (CR2032) into the transmitter part is a bit tricky – you need to turn the cover of the battery compartment with a coin, but the cover does not fall out. In my case I had to use a knife as a lever to get it off. After insertion of the batteries, transmitter and receiver are not yet paired.
Pairing is straight forward. Press the pairing button, then press the door bell button on the transmitter and its done. The melody can be selected using the Melody button. They say there are 36 chimes available – I have tried a few, most of them I didn’t like, so I choose a classic “ding-dong” sound. The Volume can be adjusted with the VOL button. However, for normal use I would recommend the highest available volume.
Next to pairing I made a few quick range tests. The range is remarkable, it certainly will satisfy most people’s needs. In my case it ranges easily from my house over the backyard to my lab, which is a separate building.
Lets open the transmitter part. The mechanical parts are designed well in a manner to support the PCB from underneath so you can’t break the PCB even if you brutally press the bell button. However, it does not look as if its water proof. The transmitter is labeled “IPX4” which means it is resistant to splashing of water. Wikipedia explains IPX4: “Water splashing against the enclosure from any direction shall have no harmful effect“.
To my surprise there are very few parts mounted on the PCB. The BOM is:
- 1 Integrated circuit CMT2150A
- 1 Crystal 26 MHz
- 9 pcs SMD components (R,L,C)
- 1 Antenna
- 1 LED
- 1 rubber ring (to tighten the LED)
- 3 pcs metal (battery holder, push button)
- 1 PCB, double sided ~ 2.4 x 5.9 mm
- 6 screws
- 4 plastic parts (2x housing, battery cover, wall mounting plate not shown here)
- 1 rubber part (the push button)
It is difficult to estimate the BOM cost and manufacturing cost, but we can do some research and estimate. What I can say as a electronics engineer is that the electronic parts all together account for less than 1 EUR per piece. The IC in low quantity accounts for ~ 0.40 EUR, the Crystal for ~ 0.18 EUR. If I assume high quantities (> 10k) the price is about half of it. The PCB is certainly in the same ball park. Its dimensions are 2.43 X 5.86 mm, this are approximately 2.2 square inches. For a 2 layer PCB in a specialized PCB house costs are approximately 5 cents per square inch. So a PCB price of 10 or 11 cents is realistic. The SMD parts, the LED and the metal parts are less than 1 Eurocent each. Plastic and rubber parts can be manufactured at very low cost, the main cost are the toolings. So at the end of the day the real cost depend on manufacturing volumes. Since this is a Lidl product, I’d estimate the volume to be somewhere between 100k and 250k. There are around 10000 Lidl shops in Europe. If every shop gets 10 of those doorbells, the minimum volume would be 100k. My very rough estimate of material cost end up below 1 EUR, plus manufacturing let it be around 1 EUR.
So what is the CMT2150? Datasheet from HopeRF says: “The CMT2150A is a true single-chip, highly flexible, high performance, OOK RF transmitter with embedded data encoder ideal for 240 to 480 MHz wireless applications“.
Reading further, the CMT2150A has some interesting features. It actually can detect up to 8 push buttons, it has a EEPROM which is programmable and it has a high output power of advertised 10 to 13 dBm. This would actually be too much for Europe, a maximum of 7 dBm (5 mW) is allowed in the 433 MHz ISM band. Anyway – this maybe explains why its range is so remarkable long. Digging more into it, there is even a USB programmer available from a company called CMOSTEK for approximately $ 50. The PC app for it can be downloaded from HopeRF : RFPDK software. I am not going to purchase the USB programmer, but I downloaded the RFPDK software, because it can give valuable hints on how the Lidl wireless doorbell has been configured. There is one strange observation – the push button is connected to K4 of the IC, wich is the fifth button (of 8). This means, following the programming logic, that the wireless protocol needs to send at least 5 data bits (probably 0 0 0 0 1). We will see later that this is indeed the case.
Receiver (Doorbell) Teardown
Opening the Doorbell, we see on the top side of the PCB the 3 buttons, a switch, two battery contacts and a Crystal. The red wire is the antenna. The buzzer is glued to the plastic shell.
The PCB is mounted to the plastic shell with one single screw . The buzzer wires are soldered to the PCB, so the PCB cannot be detached completely from the housing
This is the BOM of the door bell:
- 1 Integrated Circuit CMT2210LCW
- 1 Processor, type unknown, COB with globe top
- 1 PCB, one sided
- 1 16cm wire (Antenna)
- 1 Buzzer
- 1 Crystal
- 3 push button switches
- 1 switch
- 16 pcs SMD components (R,L,C)
- 1 LED
- 2 metal clips (on PCB, Battery contacts)
- 1 metal clip (mounted in housing)
- 5 screws
- 5 plastic parts (2x housing, 2x switches, 1x battery compartment cover)
BOM cost are difficult to estimate again, but it is definitely higher than the transmitter BOM. The µC alone may account for 0.50 $, the buzzer – I really only can guess it – costs less than 0.25 $. Such a speaker can be bought for less than 50 cents per piece in small quantities. The PCB is one-sided, measures 3.3 square inches ( 40.5 x 52.6mm) which is bigger than the PCB of the transmitter. All in all the door bell itself may cost, including manufacturing, 2 EUR. So the two electronic components of the Wireless Doorbell product account for approximately 3 EUR.
RF Signal Analysis
The CMT2210 receiver chip is the counter part of the CMT2150A. It receives the RF and feeds the digital output signal into the on-board processor.