Reliable low-power domotic module designed to enable/disable loads saving energy consumption, zeroing the standby power.

It uses 3 latching relays, 15A 250Vac, that does not consume anything when are active, and can be used to enable loads or generators up to 3kW that are OFF for long time. Also, removing power to loads during lightning storms helps prevent damage from electrostatic discharges.

It also have 1 AC input to sense 230Vac voltage (detecting power outage), that acts also as a zero-crossing detector that permits to enable relays when voltage is zero (to minimize in-rush current, in case of capacitive loads) and disable relays when current in zero (to minimize overvoltage in case of inductive loads).

4 low voltage inputs, that can be configured as analog or digital, permit to connect pushbutton switches, NTC thermal probes, alarm sensors and energy / gas / water meters with pulsed output.

The 3 latching relays can be used to enable/disable:

• heat pump, that normally consumes more than 5W in standby
• boiler, heater or other appliances consuming power in standby
• gate and garage door, that normally consumes more than 5W/each in standby, and it's also good to disable them when away alarm is active to prevent thefts.
• small photovoltaic systems, less than 3kWp
• outdoor lights

Features

• 3x 15A SPST latching relays, that permit to manage up to 3kW loads or generators 230Vac
• 1x AC input, 100-250V, that is used for two functions: zero-crossing detector + voltage presence
• 4x configurable low voltage inputs, analog or digital: each input, using GND as common voltage, can be configured as pushbutton, twinbutton (UP/DOWN buttons that uses a single input), alarm sensor (magnetic, PIR, ...), NTC 10k thermal sensor, counter / meter with pulsed output.
• only 15mW power consumption, even if one or more relays are ON.
• available with two firmwares (you have to choose the best one for you!):
  • DomBus firmware, supporting the proprietary DomBus protocol, working with Domoticz
  • Modbus firmware, supporting the standard Modbus RTU protocol, working with Home Assistant, OpenHAB, NodeRED, ...
• RS485 bus connection, industrial grade, 115200bps, permitting to use up to 500m of standard alarm cable (2x0.5mm² + 2x0.22mm² + shield) for connections.
• Very compact size: 59x42x21mm, fits everywhere, expecially in switchboxes and junction boxes.

DomBus21 ports capabilities (for the DomBus version)

Default address: 0xff21

DomBus21 Modbus RTU capabilities (for the Modbus version)

At power-on, the module shows on red LED the current Modbus slave address (register address=8192) in decimal format, on green LED the serial baudrate (reg. 8193), and finally on red LED the serial parity (reg. 8194).
If a value is zero, a long flash is emitted.

For example, if reg(8192)=33, reg(8193)=0, reg(8194)=0, at power the following led flashes will be shown:
3 red flashes, pause, 3 red flashes (slave address= 0x21 = 33 decimal), pause, 1 long green flash (reg(8193)=0 => baudrate=115200bps), pause, 1 long red flash (reg(8194)=0 => parity=None).

Device will be operative only when address/baudrate/parity parameters have been shown: then module will accept commands by Modbus RTU, and periodically shows output status for all ports, from 1 to max port: green flash means that port status is Off, red flash means that port is On.

Default slave address: 33 (0x21)

It's possible to activate one or more outputs for a certain amount of time (monostable/timer output) as indicated in the table. The parameter corresponding to the needed time can be computed using the following rules:

From 0 to 60s => 31.25ms resolution 2=62.5ms, 3=93.75ms, ... 1920=60s => value=time_in_milliseconds/31.5
From 1m to 1h with 1s resolution 1921=61s, 3540+1920=5460=1h => value=(time_in_seconds-60)+1920
From 1h to 1d with 1m resolution 5461=1h+1m, 1380+5460=6840=24h => value=(time_in_minutes-60)+5460
From 1d to 1500 days with 1h resolution 6841=25h, 6842=26h, and so on => value=(time_in_hours-24)+6840

The following tables show some Modbus commands examples.

Modbus protocol can be tested easily using a modbus program, like mbpoll for Linux:

mbpoll -v -m rtu -0 -1 -a1 -b115200 -Pnone -r 0 /dev/ttyUSB0 32 0 64 128 0 0 0 65280

to activate RL1 for 1s, R3 for 2s, RL4 for 4s and RL8 forever.

mbpoll -v -m rtu -0 -1 -a1 -b115200 -Pnone -r 255 -c 1 /dev/ttyUSB0

to read all port states.

Application notes

Home automation system controlling a heat pump for climate / hot water

Heat pumps may have a standby power consumption of 5W or more: they stay On for long time, but also stay Off for very long time, so it makes sense to enable heat pump power supply only when needed.
The following diagram shows how to manage the heat pump by DomBus21 module, to get the following features on your home automation system:

• use a cheap energy meter with pulsed output to measure the power and energy used by the heat pump
• supply the heat pump only when needed (for climate, hot water, or anti-freeze protection)
• enabling the 3-way valve only when needed
• having your home automation system to send the climate On/Off signal to the heat pump (thermostat input)
• in case that home automation system is down, permits to manually toggle On/Off the 3 relays by pushbuttons (this feature is available only with DomBus firmware, using the DCMD commands).

The energy consumption of the DomBus21 is about 15mW even with the relays On: considering the heat pump On for 66% of the time, the energy saved is about 25kWh/year compared to a system where the heat pump is always powered and using inefficient home automation relay modules!

Also, having the heat pump disconnected may prevent damages by 230V voltage fluctuations or electrostatic discharges.

Be careful with wirings: DomBus21 handles loads with max 15A (3.5kW), which could overheat or burn out if connections are not made correctly. High power connections must be monitored with an IR thermal camera to ensure they do not overheat when delivering high power.