Re: Tortoise Position Indication with Bi-color LEDs
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Connect the anode of one diode and the cahtode of another diode together. Put a resistor in series with each diode. Tie the resistor free ends together. Put the results in series with the Bi-color LED. You could use trim pots until a final resistance value is selected. The circuit will conduct through one diode and resistor with one polarity and through the other diode and resistor with the other polarity. You can vary the resistance for the desired brightness.
From: "daphilli@... [WiringForDCC]"
Sent: Sunday, December 14, 2014 7:31 PM
Subject: [WiringForDCC] Tortoise Position Indication with Bi-color LEDs
I am using two-lead bi-color LEDs to indicate the position of Tortoise switch machines. The LEDs are in series with the Tortoise switch motor. But the red and green indications do not appear equally bright to me. I can see no way to adjust the brightness of one color of two-lead LEDs, but it appears possible with three-lead LEDs by using different resistors in the green and red legs of the LED. The three-lead LED circuits I've seen are wired to the Tortoise auxiliary contacts, but I really like the simplicity of the LEDs in series with the switch motor.
I have seen a circuit by Don Vollrath that simulates a two-lead bi-color LED by connecting the anodes of two external diodes to the common cathode of a three-lead LED, and connecting the cathodes of the external diodes to the red and green leads of the three lead LED. Those two leads are then connected in the same way as a two lead bi-color LED in series with the switch motor.
If I do that, can I place a small trimming resistor in series with one of the external diodes to lower the brightness of the brighter color? I'm thinking that a resistor value that would drop the current 5 ma or so in the brighter color leg should do the trick. Also, is the voltage to the Tortoise reduced by the forward voltage drop of both the bi-color LED and one external diode, i.e., approximately dropping the voltage across the switch motor by about 4 Volts, for each simulated two-lead LED? This drop would need to be compensated for by increasing the voltage of the DC power supply.
I'm hoping this approach will work as I would really like to avoid wiring the LEDs to the auxiliary contacts on the Tortoise.
Thanks in advance.