IoT is spreading everywhere and many products are being modified to become smarter, intercoms are no exception.

We will add a remote door opening function to a well known intercom via an external microcontroller. e.g. use your smartphone to open the door from outside, letting it open for some time, more generally avoid having to press the key on the device.

Caveat: make sure you understand the risks of dealing with power supplies and if applicable that you discuss this with your landlord as the intercom case will be opened (only adding two wires, no soldering required).

Supplies

Siedle HTA 711-01 - https://www.siedle.de/xs_db/DOKUMENT_DB/www/Produ...
P2N2222A transistor - https://www.onsemi.com/pub/Collateral/P2N2222A-D....
330 Ohm resistor
Development board with e.g. ESP32 WROOM-32 - https://www.espressif.com/sites/default/files/doc...

Step 1: Electronic Components Selection

Before turning on the soldering iron, let us have a look at the electronic components selection to better understand what we are doing.

Intercom specifications

From the Siedle HTA 711-01 datasheet:

The "Terminal assignment" section gives us the pins of interest: "6.1/I Contact for door release button".
The "Specifications" section gives us: "Door release button potential-free, contact load 24 V, 1 A".

Intercom voltage measurement

Open the intercom case, take a multimeter and measure the voltage between "6.1" and "I" (on the circuit you can read "Tö" which is the German abbreviation of "Türöffner" i.e. "door release"), you should get something like:

open contact: 18.5V AC

closed contact: 0.0V AC

Experimentation

Shortening "6.1" to "I" with a wire, will make the door open.

As most of the time our microcontroller will have a 3.3V output on its GPIO, we need a specific electronic component acting as a switch on/off letting the current flow from "6.1" to "I": a transistor.

Transistor choice and montage specifications

You can refer to explanations on transistors under https://en.wikipedia.org/wiki/Bipolar_junction_tra... or https://www.electronics-tutorials.ws/transistor/tr...

A widely used and small general purposes low power transistor for microelectronic is the 2N2222A. This is what we will use.

From the transistor datasheet, we know that (~25°C):

Collector Emitter Breakdown Voltage: BVceo = 40 V (we are dealing with 18.5V)
Collector Current Continuous: Ic = 0.8 A
Base Emitter Saturation Voltage *Vbe (Sat) = 0.6V

The ESP32 WROOM-32 GPIOs can output 3.3V @12mA (many forum threads are arguing 12mA vs 40mA, let us go the safe way as it is working).

Rb calculation: Vb - Vbe_sat = Rb * Ib

Numerically: Ibmax = 12mA = (3.3V - 0.6V) / Rbmin => Rbmin = (3.3 - 0.6) / 12*10^(-3) => Rbmin = 225 Ohm.

For safety, we will take a resistor greater than 225 Ohm. 330 Ohm is a common value from the E24 series.

Using different components

If you are to use another intercom, microcontroller with different GPIO characteristics and/or another transistor, have a look at their respective datasheets and plug the numbers into the above equation. Adapt Rb value if necessary.

Step 2: Schematic

Grab your soldering iron and do (after you verified the components characteristics apply) as per the schematic.

note: the two wires going to the intercom are not soldered, ease the screws and add them to the already existing ones used for the device primary purpose.

The programming part is not described here and is left free for implementation.