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You can also use this circuit from ERR and substitute the MOC3033M for the S101V05V that is no longer available.

Is there a simple explanation for what determines whether the relay is on or off in the above schematic?  I realize the relay should respond to a horn button being pressed, but I just don't see how.  Is it that the DC offset current is needed to overcome the total 32K resistance provided by resistors R1 & R3 so that transistor T1 will allow current to pass between pins 3 and 4 of RL1?

Also, is there a reason for using two '2-position' connectors rather than one '3-position' connector since position 1 on both connectors is connected together on the schematic?

@GregM posted:

Is there a simple explanation for what determines whether the relay is on or off in the above schematic?  I realize the relay should respond to a horn button being pressed, but I just don't see how.  Is it that the DC offset current is needed to overcome the total 32K resistance provided by resistors R1 & R3 so that transistor T1 will allow current to pass between pins 3 and 4 of RL1?

Also, is there a reason for using two '2-position' connectors rather than one '3-position' connector since position 1 on both connectors is connected together on the schematic?

The positive voltage offset on the track for the whistle control is what determines if the relay solid-state relay is energized or open.  D1, C2, and R2 are just power for the input internal LED in the solid-state relay, the circuit comprised of R1, C1, and R3 drive the base of the transistor positive when there's a positive DC offset on the track.  When the transistor conducts, the solid-state relay is energized and track power is enabled on the output connector JP2.

No burning reason for using two connectors, that's just the way the ERR board was laid out.

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