simple circuit to limit the number of times a twin coil will fire

The information you need is all in this OGR thread.

Here is the original 2004 method by Bob Nelson:

lionelsoni posted:

Let me try to describe the circuit in more detail--it's not that complicated:

First, identify the common connection that both coils have to the center rail. This should be obvious because one wire from each of the two coils is connected to it. Cut them loose from that connection, connect them together and to a new wire, to bring that node of the circuit out from under the switch-motor housing. The coil wires are enameled magnet wire; so you have to carefully remove (scrape) the enamel from the ends before soldering.

Get a 4700 microfarad electrolytic capacitor with at least a 25-volt rating. Radio Shack has them rated at 35 volts, which gives a nice safety margin. Connect the common coil wire that you brought out to the positive terminal of the capacitor. Connect the negative terminal of the capacitor to the outside rails. The negative terminal is probably the only one marked, with a minus sign. You can solder directly to the track or use a lock-on or use the terminal on the switch that is attached to a shiny metal plate. If you solder to the track, be sure not to use the non-derailing rail sections on the switch. The capacitor is now connected to be a power supply for throwing the switch, if it were charged.

Get a 10-watt 100-ohm resistor. Radio Shack sells one. Connect one end of this charging resistor to the positive terminal of the capacitor. The other end will be connected to a positive charging supply.

Get a small rectifier diode. Radio Shack sells 1N4001's, with a 50-volt rating, which is plenty. For the charging supply, use an accessory transformer voltage. If your transformer is properly hooked up, one side of the accessory voltage is already connected to the outside rails; so the accessory terminal is providing a constant AC voltage relative to those rails. Convert this to DC by connecting the anode of the rectifier diode to the accessory voltage. The cathode of the diode is then the positive DC supply to which you can connect the charging resistor for all of your switches. The cathode is at the end with a stripe around it. The fact that this DC supply is unfiltered and pulsing 60 times a second is unimportant. It will charge the capacitor every bit as well as any more complicated supply.

I normally put the capacitors and resistors under the table, as close under each switch as is convenient, and run the charging supply wire from switch to switch and back to the transformer, where the rectifier diode is located. If you already have the AC accessory voltage distributed around your layout for other purposes, you could keep things simple by using multiple rectifier diodes located close to the switches and each tapping individually into the existing accessory power. They don't cost that much. Let me know if any of this is unclear or if you need any more explanation. Good luck!

Bob Nelson

He later advocated using a 14 volt bulb as the resistor - gives an indication of the recharge time for the capacitor.

Here is a YouTube video outlining the circuit with regard to an O-27 switch... the circuit is essentially the same for NJI. This will keep the machines from being activated when a train is parked on the trigger rails.

If you search the web for "capacitor discharge unit" you can buy built-up modules.

If going the DIY route and don't mind waiting a couple weeks for eBay parts shipped free from Asia, you can get the components to build 2 units for less than the price of shipping from a U.S. supplier like DigiKey or Mouser as mentioned in the video!  For example, for the 3 parts in the video:

1.  I suggest 1N4003 (instead of 1N4001).  Same price and functionality in this application, but as you'll have extras the 1N4003 is the "go-to" diode for OGR applications.  

2. Or the 14V bulb as Rob mentions.

3. The so-called "radial" package has both of the capacitor terminals on 1 side.  The "axial" package is getting harder and harder to find and comes at a substantial price premium.  Same functionality in this application - may have to fuss a bit more to bend the wire-terminals when interconnecting components.