SMART Relay (DPDT Latching)



The Model Train Technology™ SMART RelayDPDT LATCHING board provides a simple way to use the MTT Precision Sensor to trigger a DPDT (Double-Pole, Double Throw) latching relay.

Latching means that the relay keeps its position after a trigger without using continuous power to the coil.  As a result it is very efficient.  And it also allows some unique behaviors that can use to create automations on the layout without computers.

Unlike basic relays. the latching relay DOES NOT change when power is turned off and does not use current to hold the coil.  Current is only used momentarily to flip the relay and latch it.


There are THREE distinct behaviors of the SMART Relay that can be selected with the pushbutton switch.  They are:

  1.  Follow the Input.  In this mode, it behaves similarly to our standard relay. When the signal is ACTIVE from the Precision Detector the relay is on, and  when the signal is NON-ACTIVE (Unoccupied) the relay immediately shuts off.
  2. Toggle ON and OFF.  One trigger turns the relay on , another turns it off.   The Relay will not change state until the signal from the PD is released and then triggered again.
  3. Trip On with Input 1 and Trip OFF with Input 2.  If either Input 1 or Input 2 trips the relay, the “other” input will have no effect until the first input is release

The relay board also remembers the state of the relay and will activate the onboard GREEN LED to match the relay state on power up- giving you a visual clue of the state.

Lastly, there are three ways to change the state of the relay:

  1. tripped by a Precision Detector
  2. push the onboard push button
  3. connect a SPST momentary remote pushbutton between the GND (center terminal) and the DETECT terminal.

When you press the button in either scenario #2 and #3, it will flip the relay.  You may need to use this initially to get the relay (and turnout if applicable) to the correct state (CLOSED/THROWN) for your application.

There are many application uses but here are two:

  1. Protecting turnouts.
  2. Reverse the polarity of the track for a back and forth trolley style automation or for a REVERSING LOOP.  Im this case you want the track voltages to stay set even after the detector is cleared.
  3. Turn one or more Tortoise-style Turnouts (which require a DPDT to reverse the polarity of the motor) but you want the switch to stay in that position until another/different Sensor trips to turn the switch another way.

This relay circuit is purpose build for our detector and may not work with other sensor systems unless they have an ACTIVE LOW signal.

The relay contacts are rated to 1A or 5A.  Use the 1A version for LEDs and lower voltages/current applications.  Use the 5A version to switch TRACK power.

Use the same 12VDC power source to power the Precision Detector and the DPDT Relay Board.  You may connect the Precision Detector yellow signal wires to BOTH this relay board AND any Signal Controller in our product lineup.

Weight N/A
Current Rating

1A – Low current, 5A – Track power

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