Positive Triggered RS Flip Flop

Thread Starter


Joined Dec 6, 2011
I am trying to use the 12 volt (AC or DC) momentary switch that changes the rail switches on an HO layout to change and hold a red or green LED to indicate whether the train can proceed, but can find only RS circuits that respond to momentary switching to ground. Can anyone direct me to a positive voltage trigger circuit that can set and hold a 12 volt DC relay using a positive momentary trigger?


Joined Feb 11, 2008
In the old days when relays were king it was done by using one of the relay contacts as a "holding contact".

So when the relay turns on that contact connects power to its coil and it stays turned on.

Then a normally-closed switch can be used in series with that as a "turn off" contact.

So one normally-open switch turns the relay on, and the relay keeps itself on. Then the normally-closed button or switch breaks the current and turns the relay off. That means two buttons and a relay can be used for ON and OFF latching. :)


Joined Jul 17, 2007
Why not look at CD4013B dual-D flip flops instead?

You can operate them on 8v-16v. They have set and reset inputs that set and reset the outputs.

They have a Q and a Q\ output which are complementary; which means that when Q is high, Q\ is low and vice versa. One exception is when both the SET and RESET inputs are high; then both Q and Q\ are high.

So, one CD4013B has two complete D-type flip-flops. You don't need a relay at all.

You will need to use a resistor in series with the LED to limit current.