Simple Red-Green LED Model Train Signal Circuit

How to wire the MRD1 and MRD8 Infrared Train Detectors to two-aspect model railroad signals.

Here's how to use an MRD1 infrared train detector to operate a basic red/green trackside signal or control panel indicators.

When the MRD1 detector senses a train, it closes its switch contact between terminals Q1 and QC, and the red LED will illuminate. When no train is sensed, the switch contact opens and the green LED will illuminate.

Two extra components are required:

R1 - 1/4 watt resistor, 800 to 2000 ohms
D1 - diode, 1N4001, 1N4936 or similar
(RadioShack 2761101)

The IR sensor installation page shows how to install the infrared train sensors.

How it works: When the switch on the MRD1 is open, the red LED is removed from the circuit and the current from the power supply flows through the green LED.

When the MRD1 senses a train and the switch closes, current flows through the red LED. But why doesn't the green LED also light? Diodes have a "forward bias voltage" that must be overcome before they will conduct much current. A red LED typically has a forward bias voltage of 1.5 volts, a green LED forward bias voltage is 2.0 volts, and a rectifier like D1 has a forward bias voltage of 0.6 volts.

So, D1 and the green LED, being in series, have a combined forward bias voltage of 2.6 volts. When the switch is closed, the voltage across D1 and the green LED is the same as the forward bias voltage of the red LED, or 1.5 volts. This is less than the forward bias voltage of D1 plus the green LED, so only a very small leakage current will flow through the green LED.

Depending on your LEDs, you may be able to do without D1. If that is the case, connect the green LED directly to the power supply.

On the other hand, if your green LED is very efficient and noticealby glows when it should be off, add a second diode (same as D1) in series with D1 and the green LED.

The LEDs in most commercially made model railroad signals are wired as "Common Anode (+)."

In this video Rob McCrain shows how to connect an MRD1-V to simple 2-light European style signals.
With Rob's LEDs he did not need to use diode D1.

Control multiple two-aspect model railroad signals with the MRD8 Octal IR Train Detector

detectors and LED signals An MRD8 infrared train detector can control up to eight red/green trackside signals or panel board indicators.

As in the example above, when the MRD8 detector senses a train, it closes its relay contact between terminals Q1 (or Q2, Q3, etc.) and C14 (or C58), and the red LED will illuminate. When no train is sensed, the relay opens and the green LED will illuminate.

Extra components are required:

R1, R2... - 1/4 watt resistor, 800 to 2000 ohms
D1 - diode, 1N4001, 1N4936 or similar
(RadioShack 2761101)

Depending on the signal LEDs, you may be able to delete diode D1. Connect the green LEDs directly to the power supply.

On the other hand, if the green LEDs are very efficient and noticealby glow when they should be off, add another diode (same as D1) in series with D1.

Note that each signal has one resistor in the common lead. Some signal manufacturers include resistors in the individual lamp leads. To use the circuits on this page, each signal should only have a resistor in the common lead. Any resistors in the individual lamp leads should be removed.

Signals with incandescent light bulbs:
To operate a two-lamp signal with incandescent lamps, an MRD1 or MRD8 detector requires a relay in addition to the detector circuit. See the diagram at the right.

When a train is detected, the detector circuit energizes the relay coil. The relay contacts 'pick up' and send power to the red bulb. When the train leaves the detector, the detector turns off the relay. The relay contacts drop down, disconnecting the red bulb and sending power to the green bulb.

Be sure the relay and the signal lamps can operate from the same power supply. Azatrax MRAPR relays will work on AC or DC power.

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