MQTT - Dual Relay Bricklet

This is the description of the MQTT API bindings for the Dual Relay Bricklet. General information and technical specifications for the Dual Relay Bricklet are summarized in its hardware description.

An installation guide for the MQTT API bindings is part of their general description.


The example code below is Public Domain (CC0 1.0).


Download (example-simple.txt)

# Change XYZ to the UID of your Dual Relay Bricklet

    # Turn relays alternating on/off 10 times with 1 second delay
    for i in 0 1 2 3 4; do
        wait for 1s 
        publish '{"relay1": true, "relay2": false}' to tinkerforge/request/dual_relay_bricklet/XYZ/set_state 
        wait for 1s 
        publish '{"relay1": false, "relay2": true}' to tinkerforge/request/dual_relay_bricklet/XYZ/set_state 


All published payloads to and from the MQTT bindings are in JSON format.

If an error occures, the bindings publish a JSON object containing the error message as attribute "_ERROR". It is published on the corresponding response topic: .../response/... for .../request/... and .../callback/... for .../register/....

Basic Functions

Request payload:
  • relay1 -- bool
  • relay2 -- bool

Sets the state of the relays, true means on and false means off. For example: (true, false) turns relay 1 on and relay 2 off.

If you just want to set one of the relays and don't know the current state of the other relay, you can get the state with request/dual_relay_bricklet/<UID>/get_state or you can use request/dual_relay_bricklet/<UID>/set_selected_state.

All running monoflop timers will be aborted if this function is called.

The default value is (false, false).

Response payload:
  • relay1 -- bool
  • relay2 -- bool

Returns the state of the relays, true means on and false means off.

Advanced Functions

Request payload:
  • relay -- int
  • state -- bool
  • time -- int

The first parameter can be 1 or 2 (relay 1 or relay 2). The second parameter is the desired state of the relay (true means on and false means off). The third parameter indicates the time (in ms) that the relay should hold the state.

If this function is called with the parameters (1, true, 1500): Relay 1 will turn on and in 1.5s it will turn off again.

A monoflop can be used as a failsafe mechanism. For example: Lets assume you have a RS485 bus and a Dual Relay Bricklet connected to one of the slave stacks. You can now call this function every second, with a time parameter of two seconds. The relay will be on all the time. If now the RS485 connection is lost, the relay will turn off in at most two seconds.

Request payload:
  • relay -- int
Response payload:
  • state -- bool
  • time -- int
  • time_remaining -- int

Returns (for the given relay) the current state and the time as set by request/dual_relay_bricklet/<UID>/set_monoflop as well as the remaining time until the state flips.

If the timer is not running currently, the remaining time will be returned as 0.

Request payload:
  • relay -- int
  • state -- bool

Sets the state of the selected relay (1 or 2), true means on and false means off.

A running monoflop timer for the selected relay will be aborted if this function is called.

The other relay remains untouched.

Response payload:
  • uid -- string
  • connected_uid -- string
  • position -- char
  • hardware_version -- [int,int,int]
  • firmware_version -- [int,int,int]
  • device_identifier -- int (has symbols)
  • _display_name -- string

Returns the UID, the UID where the Bricklet is connected to, the position, the hardware and firmware version as well as the device identifier.

The position can be 'a', 'b', 'c' or 'd'.

The device identifier numbers can be found here. If symbolic output is not disabled, the device identifier is mapped to the corresponding name in the format used in topics.

The display name contains the Dual Relay's name in a human readable form.


Callbacks can be registered to receive time critical or recurring data from the device. The registration is done with the corresponding .../register/... topic and an optional suffix. This suffix can be used to deregister the callback later.


Using callbacks for recurring events is always preferred compared to using getters. It will use less USB bandwidth and the latency will be a lot better, since there is no round trip time.

Request payload:
  • register -- bool
Response payload:
  • relay -- int
  • state -- bool

A callback can be registered for this event by publishing to the .../register/dual_relay_bricklet/<UID>/monoflop_done[/<SUFFIX>] topic with the payload "true". An added callback can be removed by publishing to the same topic with the payload "false". To support multiple (de)registrations, e.g. for message filtering, an optional suffix can be used.

If the callback is triggered, a message with it's payload is published under the corresponding .../callback/dual_relay_bricklet/<UID>/monoflop_done[/<SUFFIX>] topic for each registered suffix.

This callback is triggered whenever a monoflop timer reaches 0. The parameter contain the relay (1 or 2) and the current state of the relay (the state after the monoflop).