This circuit is for a switch machine controller for reversing loops. The circuit is light activated and designed to drive stall type switch machines.
The circuit is designed to operate from a regulated, 12 volt DC power supply.
Additional phototransistor sensors can be used with the circuit for more complex track arrangements.
In the fixed route reverse loop track arrangement, the position of Q1 can be placed near the turnout or farther away if desired. If needed, both Q1 sensor positions shown could be used.
In the Alternating route reverse loop track arrangement, the position of Q1 and Q2 should be placed so the the switch motor has time to travel before the train is on the turnout.
The switch machine will stay at its last position when power is applied to the circuit. If this is not needed the contacts of the switch machine can be disconnected from terminals 4, 5 and 6 of the circuit.
LEDs can be added to the switch machine circuit to indicate which way the turnout is lined.
The switch machine can be controlled manually by adding push button switches as shown on the following schematic.
An optional relay can be used to control the phasing of the loop for AC and DCC systems or the polarity of the lead track for DC systems. The relay should have a 12 volt coil.
Below is an installation schematic for the reversing loop circuit. The optional relay is also shown on the schematic.
| Qty | - | CIRCUIT PART | - | DigiKey Part |
| - | - | Semiconductors | - | - |
| 1 | - | IC 1A, B | - | LM393NFS-ND |
| 1 | - | IC 2A, B | - | 497-1965-5-ND |
| 3 | - | Q1, 2, 3 | - | 160-1988-ND |
| 4 | - | D1, 2, 3, 4 | - | 1N4148DICT-ND |
| 1 | - | D7 | - | 1N4001DICT-ND |
| - | - | Resistors | - | - |
| 2 | - | R1, 2 | - | 470KQBK |
| 4 | - | R3, 4, 13, 14 | - | 10KQBK |
| 1 | - | R5 | - | 100KQBK |
| - | - | Capacitors | - | Capacitors |
| 1 | - | C2 | - | P5177-ND |
| 2 | - | C3, 7 | - | P5174-ND |
| 1 | - | C4 | - | P5181-ND |
| - | - | Terminal Blocks | - | - |
| 1 | - | 2 Positions - 3.5mm | - | ED2635-ND |
| 3 | - | 3 Positions - 3.5mm | - | ED2636-ND |
The polarity control relay is switched OFF when Q1 is covered by the train and switched ON when Q2 is covered by the train.
The switch machines operate at together but in opposite directions. The turnouts move to the R position when the phototransistor nearest them is covered by a train.
One set of contacts from one switch machine is used so that the system remembers its last position when the power is applied to the circuit. Also, one set of contacts from each switch machine can be used for frog polarity control.
It would be less costly and less complicated to use a slow motion switch machines rather than twin coil machines. Slow motion machines have auxiliary contacts built into them and can move the turnout to the proper position when power is applied to the circuit.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
07 April, 2019