Gateway Design


The OpenTherm gateway circuitry consists of five major parts:
Power supply (X3, TR1, D1, D2, D3, D4, C1, C2, and IC3)
Two voltages are needed: An unregulated voltage of around 24 Volts for the Opentherm slave interface and a more accurate 5 Volts supply for the PIC and the MAX232.

OpenTherm master interface (X2, D5, D6, D7, D8, D9, D10, D11, Q1, OK1, R1, R2, R3, R4, and R8)
The master interface behaves like an OpenTherm Room Unit / Master device (thermostat). The master interface controls the voltage on the OpenTherm connection and measures the current. The interface is galvanically isolated from the rest of the device.

OpenTherm slave interface (X1, Q2, Q3, Q4, Q5, R5, R6, R7, R9, R11, and R12)
The slave interface behaves like an OpenTherm Boiler Unit / Slave device. The slave interface drives the current on the OpenTherm connection and measures the voltage.

Control unit (IC1, R15, SV2, SV3, and JP1)
The PIC16F88 or PIC16F1847 is the central intelligence of the OpenTherm gateway. The reset line is normally held high by a resistor connected to VCC. Several of the I/O pins are made available via headers. These can be used to drive LEDS, hook up an outside temperature sensor, or connect a reset button.

RS232 line driver (IC2, C4, C5, C6, C7, and SV1)
For communicating with the outside world the TTL level serial signals from the PIC have to be converted to proper RS232 levels. This is done with a standard circuit around a MAX232 chip.

OpenTherm Gateway schematic
[High resolution version (2134x1640)]


  • The actual voltage on the points marked "+24V" is not very critical. It should be somewhere between 20V (maximum Opentherm line voltage + a small margin for the line driver) and 35V (the maximum allowed input voltage for the LM7805).
  • Q4 is necessary for keeping the voltage on pin 3 of IC1 below VDD.
  • The schematic doesn't explicitly show it, but when designing your own print layout don't forget to connect the VCC and GND pins of IC2.
Three headers have been provided to attach some external components.

Part List

QtyDescriptionValue ReferenceConrad Part No
1PIC processorPIC16F88PIC1 165233-89
1RS232 interfacemoduleMAX232IC2 152281-89
1*Voltage Regulator TO220MC7805CTIC3 175030-89
1Dual opto-couplerPC827OK1 140235-89
4PNP TransistorBC558AQ1, Q2, Q3, Q5 1262971-89
1NPN TransistorBC547BQ4 1262957-89
1Electrolytic Capacitor, Radial 63V220µFC1 445902-89
1Electrolytic Capacitor, Radial 16V100µFC2 445928-89
4Electrolytic Capacitor, Radial 50V1µFC3, C4, C5, C6 445724-89
4Diode1N4004D1, D2, D3, D4 162248-89
4Diode1N4148D5, D6, D7, D8 162280-89
1Zener Diode4V7D9 180084-89
1Zener Diode15VD10 180203-89
1Zener Diode4V3D11 180076-89
11/4 Watt 5% Resistor39R12 1417702-89
21/4 Watt 5% Resistor100R2, R11 1417639-89
11/4 Watt 5% Resistor220R3 1417693-89
21/4 Watt 5% Resistor330R1, R4 1417730-89
11/4 Watt 5% Resistor1k2R8 1417712-89
21/4 Watt 5% Resistor4k7R6, R9 1417695-89
11/4 Watt 5% Resistor15kR10 1417711-89
21/4 Watt 5% Resistor33kR5, R7 1417647-89
1*Transformer EI3015V 1.2VATR1 710691-89
1Header10 pinSV1 741648-89
3Screw Clamp2 pinX1, X2, X3 731091-89
* In case you want to power an ESP8266-based device, such as a NodeMCU devkit board or Wemos D1 mini, from the same power supply, you may want to order these alternatives instead:
1Transformer EI3015V 2VATR1 710564-89
1DC/DC convertorR-78E5.0-0.5IC3 157954-89

Some more links to parts I used from Conrad:

The PICs can also be ordered from Van Ooijen Technische Informatica (shipping within NL: €3.50). As a service for people who don't have their own programmer, this web shop offers to program the PIC for a small fee (€3.00).

It may even be possible to get a programmed PIC directly from Microchip (Enter PIC16F88-I/P or PIC16F1847-I/P, then upload the gateway.hex file for the selected PIC). They appear to charge €3.17 for a PIC16F88, €1.90 for a PIC16F1847, €0.14 for programming. Shipping to the Netherlands used to be reasonably priced at around €5.76, but is currently listed as €22.05.
If you try this method, please share your experiences so I can update the above statement to a more definite version.

(Prices recorded Mar 9, 2022)

Assembly Sheet

An opentherm gateway user has prepared a drawing to assist with assembling the circuit board. You can print the image below to use as a reference when soldering the components.

OpenTherm Gateway assembly sheet

Print Layout

The printed circuit board for the gateway was ordered from a PCB manufacturing web site. The picture below shows how it came out.

OpenTherm Gateway circuit board