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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:
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 up? 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 has 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 about 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 about 1.5 volts. This is less than the forward bias voltage of D1 + 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. Connect the green LED directly to the resistor. 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 the green LED.
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