Operation of the MRD2 dual model railroad detector
How to use the direction sensing and automatic switch control capabilities.
The MRD2 combines two infrared train detectors into one circuit module. Each detector consists of an infrared LED light source and a phototransistor light receiver.
Each IrLED-phototransistor pair may be placed in the track roadbed for reflective sensing, or mounted above and beside the track for across-the-track sensing. See the MRD2 instruction sheet and the detector installation examples page for more about reflective and across-the-track train sensing.
The detectors control two on-board LEDs plus two relay contacts that can be connected to lights, signals, switch machines and other devices to automate a model railroad layout.
The relay contacts are rated for 3 amps. See the note about relay contact ratings.
You control how the relays respond to the sensors by setting the two mode switches.
Two Independent Detectors
Set mode switches to N and S. The circuit will operate as two independent infrared train sensors. While detector 1 is sensing a train, relay 1 will be closed and LED 1 will be on. While detector 2 is sensing a train, relay 2 will be closed and LED 2 will be on.
This is the only operating mode that allows both relay contacts to be closed at the same time.
3-Light LED Block Signal Control
The MRD2 circuit can operate a three-light LED block signal. Set mode switches to N and M. Place the two IrLED/phototransistor pairs along the same track (see below). The three-light LED signal head is located next to sensor 1 and faces left, toward trains that are moving from left to right.
When the train triggers sensor 1, the signal changes from green to red. The signal stays red for up to 10 seconds after the train passes sensor 1, waiting for the train to reach sensor 2.
Three seconds after the end of the train passes sensor 2, the circuit will change the signal from red to yellow. Ten seconds later the signal will change to green.
If a train moves from right to left, sensor 2 will activate first. The circuit will turn the signal red while the train is in the block. Once the end of the train clears sensor 1 going to the left, the signal will change to green.
This diagram shows how to wire the MRD2 dual detector circuit to the signal head. When both relays are open, the signal will show green. When relay 1 is closed, the yellow LED will be on. When relay 2 is closed, the red LED will be on.
This signal head in this circuit example is wired in common-anode configuration (the positive side of all three LEDs are wired together). If your signal is wired as common-cathode (negative side of all LEDs wired together), then reverse the polarity of all diodes and the power supply.
Start with a value of about 1,000 ohms for resistor R. Use a higher or lower value to change the brightness as desired. Diodes D1 and D2 are 1N4001, 1N4148 or similar.
A resistor and two diodes are included with the MRD2 dual detector.
MRD2 kit includes circuit board, two IR LEDs, two IR phototransistors and hardware
Dual Infrared Train Detector
Automatic Turnout Control / 'Non-derailing' Switch Control
Trains approaching a track switch will automatically set the switch points to avoid derailments.
For Snap-Action or Twin-Coil switch machines, set the mode switches to TO and S.
The MRD2 detector will momentarily turn on either Relay 1 or Relay 2 when a train is sensed by either detector.
For slow-motion switch machine motors (Tortoise or SwitchMaster), set the mode switches to TO and M.
The MRD2 detector will turn on either Relay 1 or Relay 2 when a train is sensed by either detector. The relay will stay on until the MRD2 senses that the turnout position should be changed.
Automatic Track Switch Operation:
In this example, two trains are approaching a track switch from the right. The green train arrives at sensor pair 1, and the MRD2 aligns the switch points to the straight track.
Now the red train arrives at sensor pair 2. The MRD2 will not throw the turnout points because sensor pair 1 is still sensing the green train.
Four seconds after the green train has cleared sensor pair 1, the MRD2 will respond to sensor pair 2 by aligning the turnout points with the siding.
To avoid derailments, either keep trains moving fast enough that the rear end will move from the sensor past the points in less than four seconds, or stop the second train before it reaches its sensor. The MRD2 will flash its LEDs to let you know when a train is on a sensor.
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