Long life, fail safe traffic light

1. A method for detecting a failure in a signal light, comprising:

monitoring operation of one or more light emitting diodes (LEDs) of the signal light coupled to a constant current driver;

detecting a short circuit in at least one of the one or more LEDs, wherein the detecting the short circuit comprises:

setting a voltage threshold for the signal light via a zener diode;

measuring a voltage across the one or more LEDs of the signal light;

and

determining the short circuit has occurred when the voltage falls below the voltage threshold of the zener diode causing the zener diode to stop conducting, which causes a first transistor in communication with the zener diode to turn off; and

signaling the failure in the signal light when the short circuit is detected, wherein the signaling comprises placing the signal light in a high impedance state by blowing a fuse in response to the voltage falling below the voltage threshold, wherein the fuse is blown by a second transistor in communication with the first transistor that turns on in response to the first transistor turning off.

2. The method of claim 1 , wherein the detecting is performed by a controller.

3. The method of claim 1 , wherein the detecting is performed by an under-current and under-voltage sensing circuit on a circuit board of one of the one or more LEDs.

4. The method of claim 1 , wherein the detecting is performed by an under-current and under-voltage sensing circuit that is external to a circuit of the signal light.

5. A light signal circuit comprising:

an alternating current (AC) to direct current (AC-DC) converter for coupling to an AC power supply;

a control circuit in communication with the AC-DC converter;

one or more LEDs coupled to the AC-DC converter;

an under-current and under-voltage sensing circuit coupled to the one or more LEDs, wherein the under-current and under-voltage sensing circuit comprises:

a zener diode having a voltage threshold that causes the zener diode to stop conducting;

a first transistor in communication with the zener diode, wherein the first transistor turns off when the zener diode stops conducting in response to a measured voltage falling below the voltage threshold of the zener diode; and

a second transistor in communication with the first transistor that turns on in response to the first transistor turning off; and

a failed state impedance circuit coupled to the under-current and under-voltage sensing circuit and the control circuit, wherein the failed state impedance circuit goes into a high impendence state by blowing a fuse in the failed state impedance circuit when the second transistor turns on.

6. The light signal circuit of claim 5 , wherein the under-current and under-voltage sensing circuit comprises:

a programming resistor to set a current threshold.

7. The light signal circuit of claim 5 , wherein the AC-DC converter comprises at least one non-electrolytic capacitor.

8. The light signal circuit of claim 5 , further comprising:

one or more terminals for connecting to a thermistor or a resistor mounted on each one of the one or more LEDs to set an output current that is compatible with the one or more LEDs.