Smart lock

Locking without logging

Sometimes you don't want to record when people come and go. Maybe you're organizing a surprise birthday party and don't want to spoil the surpise?

With the Candle smart lock you can easily disable logging of movement at any time. In the example shown here, it's a matter of pulling down the switch.

Be safe

Don't broadcast the status of your home's locks to third parties.

Keep a secret

Sneak in and out of your own home - it's a healthy thing to do.‌ 

Open your door without the cloud

When you need it, enable unlocking via a temporary SMS password. 

Control 2 doors 

Manage both your apartment door and front door with one device.

Password protected

Set a password and which phone numbers it will respond to.


Lock and unlock the door from anywhere by sending an SMS.

Disable remote control

Don't need to open your door remotely or a while? Then disable the feature for extra safety.‌ 

The user manual

Ignorance is bliss
While data transmission is disabled the lock can still be used, but it won't reveal when it's opened or closed via SMS or using the buttons on the lock itself.

SMS for everything
The smart lock acts as a general SMS gateway, allowing you to trigger all kinds of automations. Similarly, automations could notify you when something happens.

 "When the kids promise to be home on time we want to be able to trust them without being temped to check the logs"

Why it matters

Smart locks can be very useful, but they also come with risks. Tracking when people come and go can feel restricting and inhospitable.

On top of that it can be a serious security issue. Cloud connected devices are regularly revealed to be insecure through poor design, coding errors, or advances in hacking.

Wifi is a risk

Devices that use WiFi to connect almost always use this to directly connect to the internet. This makes it very important to develop strong barriers, but because good security is expensive this doesn't always happen. Even if the manufacturer follows best security practices, sometimes holes are discovered in software that they build upon.

The producer is a risk

The producer of a smart lock can be a weak spot. Your lock will only receive security updates while the manufacturer is in business. Similarly, if their servers are hacked your lock is at risk, no matter if the lock itself is up to date.

Candle's smart lock is not safe either

Our smart lock is not connected to the internet directly, which makes it much harder to hack. But someone, like a neighbour, could ‌try to hack the lock locally.

If you connect your Candle Controller to the internet, then if it is taken over your locks could also be unlocked.

Our recommendation? Don't replace a normal lock. Add a smart lock next to it. Then only rely on the smart lock at times when you really need its flexibility.

A word on safety

Always consult with your home insurer before installing any smart lock or thermostat. Please do not rely on the Candle lock as your only lock. Candle is a research project that focusses on developing new privacy protections, not improved security.

Creating the lock

A‌pproximate build time: 60 minutes.

Connect the relay(s)

Connect the IN pin of the relay to signal signal pin 5. 

If you would like to control two electric locks, connect the IN pin of the second relay to signal pin 6.

Electric locks can either be locked when they are not powered ("fail secure") or unlocked ("fail safe"). In the example above, the flow of electricity to the electric lock is "normally open" (N.O.), which means no electricity flows to your lock unless the relay is activated. 

If you prefer that when the smart lock activates it interrupts the flow of electricity, then use the "normally closed" (N.C.) port of the relay instead. 

Our advice: make sure that in an emergency, where the power might be out, you can open the door.

Connect the GSM modem

Connect its U_TXD (Transmit) pin to signal pin 3, and connect its U_RXD (Receive) pin to signal pin 4. 

Connect a GND pin on the modem to any GND (ground) pin on the expansion board.

Finally the VCC_IN pin on the modem should be connected to any VCC (+) pin on the expansion board. Connect this wire after you've uploaded the code to the device. Read the "understanding the power flow" section below if you want to learn why.

Make sure the antenna is screwed on securely.

Understanding the power flow

In most Candle devices power is provided by plugging a USB cable into the Arduino. It then powers all the sensors and displays attached to it.

The smart is different. The modem can require about 2 amps when it connects to the GSM network, and the Arduino's power regulator cannot provide this much energy. That's why the situation is reversed: the Arduino is supplied power by the modem through its GND and VCC_IN pins. 

In normal operation this works great. But there is one exception: when you want to upload code to the device you'll need to temporarily disconnect the VCC_In wire. This disconnects the Arduino from the modem. You can then connect the Arduino to the Candle controller as normal.

Important: make sure your charger can supply at least 2.4 amps.

Alternative option: using a USB splitter cable

If you plan on changing the code on your smart lock a lot, or if you just want to avoid having to think about that VCC_IN wire, you could purchase a USB splitter cable instead. If you use this USB cable you shouldn't connect the VCC_IN (+) wire at all.

- If you purchase the recommended cable, the short prong can only provide power. Plug that into the modem's USB port.

- The longer prong of this splitter will be able to carry data. Plug this into the Arduino's USB port. 

This way you can easily upload code to the arduino by connecting a USB cable to the input of the splitter wire. Most laptops should be able to provide enough power for this situation.

Plug in the Arduino

Plug in the RF-Nano Arduino

Plug the RF-Nano into its socket. The USB connector should be on the outside of the main board.

Open the Candle Manager

Select the Candle Manager from the menu, and follow the steps. It will help you upload the code to your new creation.

Add your new device

Visit the Candle controller and on the things page press the (+) icon. Your new creation should be available. Optionally, select which of its properties you'd like to have in the spotlight. Click on "save" to complete the proces.

Shopping list

This shopping list on Aliexpress will get you everything in one go.
USB splitter cable (optional)
Jan 14, 2021

Power two micro USB devices with one plug. While optional, this is useful if you would like to place two devices in a single…

Aug 27, 2019

‌A basic relay that can toggle the power or a connection to another device. The central port is ground. Besides it you…

RF-Nano Arduino
Aug 10, 2018

The RF-Nano is an Arduino Nano with built-in wireless connectivity. It's the brain of all Candle devices.

Dupont wire
Aug 09, 2018

These wires allow you to easily connect the main board to sensors and actuators. They snugly fit over the pins. No soldering…

SMS modem
Aug 07, 2018

This A6 GPRS modem allows you to send and receive SMS. It has a Micro-USB port which makes it easy to provide it with the…

Expansion board
Aug 06, 2018

This expansion board makes it easy to connect everything by just plugging in wires.