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,
- 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 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.
resolution version (2134x1640)]
Three headers have been provided to attach some
- 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
- 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.
||Reference||Conrad Part No
|1*||Voltage Regulator TO220||MC7805CT||IC3
|4||PNP Transistor||BC558A||Q1, Q2, Q3, Q5
|1||Electrolytic Capacitor, Radial 63V||220µF||C1
|1||Electrolytic Capacitor, Radial 16V||100µF||C2
|4||Electrolytic Capacitor, Radial 50V||1µF||C3, C4, C5, C6
|4||Diode||1N4004||D1, D2, D3, D4
|4||Diode||1N4148||D5, D6, D7, D8
|1||1/4 Watt 5% Resistor||39||R12
|2||1/4 Watt 5% Resistor||100||R2, R11
|1||1/4 Watt 5% Resistor||220||R3
|2||1/4 Watt 5% Resistor||330||R1, R4
|1||1/4 Watt 5% Resistor||1k2||R8
|2||1/4 Watt 5% Resistor||4k7||R6, R9
|1||1/4 Watt 5% Resistor||15k||R10
|2||1/4 Watt 5% Resistor||33k||R5, R7
|1*||Transformer EI30||15V 1.2VA||TR1
|3||Screw Clamp||2 pin||X1, X2, X3
|* 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:
|1||Transformer EI30||15V 2VA||TR1
Some more links to parts I used from Conrad:
The PIC can also be ordered from Van Ooijen
Technische Informatica. As a service for people who don't have their own
programmer, this web shop offers to
the PIC for a small fee (€3.00).
€4.72 (shipping within NL: €3.50)
It may even be possible to get a programmed PIC
directly from Microchip (Enter PIC16F88-I/P, then upload the gateway.hex
They appear to charge €2.49 for the PIC itself, €0.14 for
programming it, and around €5.76 for shipping to the Netherlands.
If you try this method, please share your experiences so I can update the
above statement to a more definite version.
(Prices recorded Jan 29, 2020)
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.
The printed circuit board for the gateway was ordered
from a PCB manufacturing web site. The picture below shows how it came out.