Electronics - Point Motors and Relays
Note: the section on Point Motors and Toggle Switches has been expanded and moved to its own page.
Here are some little-known tricks you can do with conventional point motors or switch machines, adding additional switch contacts for switching frogs, controlling signals, etc., and also a method of controlling point motors from simple toggle switches instead of the usual momentary contact push buttons - just the thing if you want route indication on a control panel or want to simulate a lever frame.
Since small 2-pole 12V relays can typically be bought for between $1 and $5, they can work out cheaper than using proprietary point motor accessory switches.
Alignment Problems with Point Motors and Switching Contacts
I have used Peco and Seep motors with their associated switches, and have had problems with both. The Peco switches are add-on units that have to be installed and adjusted, while the Seep motors come with one built-in set of switch contacts. I use code 55 N gauge track, and the point blades do not throw as far as on code 80 or OO points. The design of both types of point motor switches appear to predate the code 55 range, and are only just able to cope with the shorter throws. The result is that installing the point motors is a very fiddly task, with almost no margin for error. Worse, the Seep switches can drift out of alignment over time and stop working (they use a sliding washer held on the end of a spring).
If you only need a simple on-off switch function, you can easily install the point motor as there is plenty of adjustment margin. This is also largely immune to long-term drift. The problems occur when you need to use the full function of the changeover contacts.
The simple fix for this is to use the switch contacts in their simple on-off configuration to control a relay which provides one or more sets of full changeover contacts. This is identical to adding extra contacts, which is described in the next section.
Adding Switch Contacts to Point Motors With One or More Existing Switches
If you already have a point motor with one or more sets of contacts and need more sets, you can reassign one set (probably the only set) to control one or more relays with as many contacts as you need. One set of relay contacts needs to be used to duplicate the original function.
This approach is simple and straightforward. The only real drawback is the need to have a power supply for the relays.
Here are some little-known tricks you can do with conventional point motors or switch machines, adding additional switch contacts for switching frogs, controlling signals, etc., and also a method of controlling point motors from simple toggle switches instead of the usual momentary contact push buttons - just the thing if you want route indication on a control panel or want to simulate a lever frame.
Since small 2-pole 12V relays can typically be bought for between $1 and $5, they can work out cheaper than using proprietary point motor accessory switches.
Alignment Problems with Point Motors and Switching Contacts
I have used Peco and Seep motors with their associated switches, and have had problems with both. The Peco switches are add-on units that have to be installed and adjusted, while the Seep motors come with one built-in set of switch contacts. I use code 55 N gauge track, and the point blades do not throw as far as on code 80 or OO points. The design of both types of point motor switches appear to predate the code 55 range, and are only just able to cope with the shorter throws. The result is that installing the point motors is a very fiddly task, with almost no margin for error. Worse, the Seep switches can drift out of alignment over time and stop working (they use a sliding washer held on the end of a spring).
If you only need a simple on-off switch function, you can easily install the point motor as there is plenty of adjustment margin. This is also largely immune to long-term drift. The problems occur when you need to use the full function of the changeover contacts.
The simple fix for this is to use the switch contacts in their simple on-off configuration to control a relay which provides one or more sets of full changeover contacts. This is identical to adding extra contacts, which is described in the next section.
Adding Switch Contacts to Point Motors With One or More Existing Switches
If you already have a point motor with one or more sets of contacts and need more sets, you can reassign one set (probably the only set) to control one or more relays with as many contacts as you need. One set of relay contacts needs to be used to duplicate the original function.
This approach is simple and straightforward. The only real drawback is the need to have a power supply for the relays.
Adding Switch Contacts to Point Motors with No Existing Switches (First Choice)
The easiest way to do this is to use a special type of relay known as a latching relay. There are two kinds: single coil and double coil. The double coil type works just like a point motor - when you apply power to one coil, the relay changes one way; when you apply power to the other coil, it changes back again. It stays in whatever state you set until you apply power again. The single coil type is similar, except you apply positive and negative voltages to the one coil.
The advantages of using latching relays are that no existing switch contacts are required, nor is a separate power supply needed. One drawback is that the relay cannot know if you change the points by hand - you will then have a conflict between the track and the electrics until you move or fire the point again. Another drawback is that latching relays can be hard to find - not all electronic stores carry them, will probably only have one or two types even if they do, and will be fairly expensive. They can be obtained at lower cost from the Model Railway Electronics Group (MERG) if you are a member, or from various internet suppliers.
The next three circuits show how to use latching relays with point motors. It is simple if you use a CDU, and a bit more complex if you use AC for your points. The single coil circuit relies on using the other point motor coil as a return path. At the moment, I cannot see a simple way of using a single-coil latching relay with an AC point supply.
These circuits all include one or two voltage-matching resistors. The relays will likely be either 5V or 12V parts, while the point supply is likely to be 15-16V AC or 20V DC for a CDU. You will have to calculate the value of the resistor based on your actual supply voltage and the specific relays you use. The following calculations are somewhat simplified, and you can use whatever resistor value is closest to the calculated value. Standard 1/2W resistors and 1A diodes are OK.
For the CDU cases (example: 15V AC and a 12V relay with a coil resistance of 500 ohms),
resistor voltage = AC supply voltage * 1.33 - relay voltage (e.g. 8V = 15V * 1.33 - 12V)
resistor value = relay coil resistance * resistor voltage / relay voltage (e.g. 330 ohm = 500 ohm * 8V / 12V )
For the AC case do the same but omit the "times 1.33" step.
If you need multiple relays, the simplest way is to just graft another relay+resistor+diode combination onto the point motor. There are more efficient ways, but they involve recalculating resistor values, etc. For example, I use two 5V single-coil latching relays in series with a single resistor.
Adding Switch Contacts to Point Motors with No Existing Switches (Last Choice)
It is also possible to use a conventional relay even when there are no switch contacts on the point motor, but there are a number of serious drawbacks. The points must be powered by a CDU, one pole on the relay is required to provide a self-latching circuit, and there are likely to be synchronization problems when powering up the layout.
Caveat - the relay will not be turned on until the point motor button is pushed, so if the point was left in the powered state when the layout was powered down there will be a mismatch. This can be corrected by any movement of the points, but you do have to remember to do do. On the other hand, If you use a lever frame control system and always put your levers back to neutral at the end (or start) of an operating session, then this would not be a problem.
Caveat - this circuit relies on some tricky timing - the CDU provides a power pulse to drive the point motor, and this needs to be long enough to also activate the relay. This circuit works best if the CDU is powered from 15-16V, or is a heavy duty type (something like a 4700uF capacitor instead of the more common 2000uF), or when using medium or low power point motors (e.g. Peco rather than Seep). Better still, any two of these options.
This type of circuit normally requires two relays, with the second one used to turn off the first. This circuit sneakily avoids that by using the activation of the second point motor coil to reverse bias both diodes and cut power to the relay. The +12V DC relay power supply must share a common ground with the CDU (i.e. be driven from the same transformer). If the CDU is powered from 15-16V, the same 15VDC supply can be used for the relay but then a voltage dropping resistor (R1) may be needed. The extra capacitor (C1) may be needed if you use fast relay to ensure that it latches properly (the DC supply is a series of pulses, and the relay can drop out or chatter in the gaps between these pulses).
It is also possible to use a conventional relay even when there are no switch contacts on the point motor, but there are a number of serious drawbacks. The points must be powered by a CDU, one pole on the relay is required to provide a self-latching circuit, and there are likely to be synchronization problems when powering up the layout.
Caveat - the relay will not be turned on until the point motor button is pushed, so if the point was left in the powered state when the layout was powered down there will be a mismatch. This can be corrected by any movement of the points, but you do have to remember to do do. On the other hand, If you use a lever frame control system and always put your levers back to neutral at the end (or start) of an operating session, then this would not be a problem.
Caveat - this circuit relies on some tricky timing - the CDU provides a power pulse to drive the point motor, and this needs to be long enough to also activate the relay. This circuit works best if the CDU is powered from 15-16V, or is a heavy duty type (something like a 4700uF capacitor instead of the more common 2000uF), or when using medium or low power point motors (e.g. Peco rather than Seep). Better still, any two of these options.
This type of circuit normally requires two relays, with the second one used to turn off the first. This circuit sneakily avoids that by using the activation of the second point motor coil to reverse bias both diodes and cut power to the relay. The +12V DC relay power supply must share a common ground with the CDU (i.e. be driven from the same transformer). If the CDU is powered from 15-16V, the same 15VDC supply can be used for the relay but then a voltage dropping resistor (R1) may be needed. The extra capacitor (C1) may be needed if you use fast relay to ensure that it latches properly (the DC supply is a series of pulses, and the relay can drop out or chatter in the gaps between these pulses).