Gateway Design
Schematic
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.
[High
resolution version (2134x1640)]
Notes:
- 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
Qty | Description | Value |
Reference | Conrad Part No |
1 | PIC processor | PIC16F88P | IC1 |
165233-89 |
1 | RS232 interfacemodule | MAX232 | IC2 |
152281-89 |
1* | Voltage Regulator TO220 | MC7805CT | IC3 |
175030-89 |
1 | Dual opto-coupler | PC827 | OK1 |
140235-89 |
4 | PNP Transistor | BC558A | Q1, Q2, Q3, Q5 |
1262971-89 |
1 | NPN Transistor | BC547B | Q4 |
1262957-89 |
1 | Electrolytic Capacitor, Radial 63V | 220µF | C1 |
445902-89 |
1 | Electrolytic Capacitor, Radial 16V | 100µF | C2 |
445928-89 |
4 | Electrolytic Capacitor, Radial 50V | 1µF | C3, C4, C5, C6 |
445724-89 |
4 | Diode | 1N4004 | D1, D2, D3, D4 |
162248-89 |
4 | Diode | 1N4148 | D5, D6, D7, D8 |
162280-89 |
1 | Zener Diode | 4V7 | D9 |
180084-89 |
1 | Zener Diode | 15V | D10 |
180203-89 |
1 | Zener Diode | 4V3 | D11 |
180076-89 |
1 | 1/4 Watt 5% Resistor | 39 | R12 |
1417702-89 |
2 | 1/4 Watt 5% Resistor | 100 | R2, R11 |
1417639-89 |
1 | 1/4 Watt 5% Resistor | 220 | R3 |
1417693-89 |
2 | 1/4 Watt 5% Resistor | 330 | R1, R4 |
1417730-89 |
1 | 1/4 Watt 5% Resistor | 1k2 | R8 |
1417712-89 |
2 | 1/4 Watt 5% Resistor | 4k7 | R6, R9 |
1417695-89 |
1 | 1/4 Watt 5% Resistor | 15k | R10 |
1417711-89 |
2 | 1/4 Watt 5% Resistor | 33k | R5, R7 |
1417647-89 |
1* | Transformer EI30 | 15V 1.2VA | TR1 |
710691-89 |
1 | Header | 10 pin | SV1 |
741648-89 |
3 | Screw Clamp | 2 pin | X1, 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: |
1 | Transformer EI30 | 15V 2VA | TR1 |
710564-89 |
1 | DC/DC convertor | R-78E5.0-0.5 | IC3 |
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.
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.
|