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So you’ve got this super-duper twisty single lane mountain climb thought up or maybe a narrow chair rail shelf that runs from yer TV recliner over to the beer fridge that would be just great for sending a slot car on a brewski run without having to get up (sorry, you’ll have to figure out the beer dispensing part on your own) and you’re thinkin’ to yourself “geeze, wunnit be great if I could have my car run out to the end of this h’yar single lane track and then turn around and come back??!” Now you could put a little loop at either end to turn the car around, but the power is going to be backwards to what’s needed for the return trip. Enter the automatic power switching circuit.
All the components are inexpensive and readily available online or at an electronics shop; and the circuitry is pretty straightforward (no programming, digital controllers, JK flip flops or even transistors or diodes). Just simple normal open (NO) or normal closed (NC) switches (either optical or mechanical type) for sensors and a 12 volt triple pole double throw (3PDT) relay.
A 3PDT relay is just three two-way switches (each has one input, two outputs) ganged together and all operated by one solenoid (electromagnet). The relay has 11 blade connections (pins) on one end. The inputs to be switched are connected to pins 7, 8 and 9. Applying 12 volts across pins 10 and 11 will activate a solenoid. When the solenoid is inactive, the inputs connect to the normal closed side of each switch output on pins 1, 2 and 3; when the solenoid is active, it pulls the relay switches closed connecting input to output pins 4, 5 and 6.
Here’s a simple single lane track with a loop at either end. The slot where each loop feeds back into the main track should have some sort of spring gate to keep the incoming car from heading up the wrong slot at the junction.
The sensor switches that tell the relay when to switch are marked S1 and S2 on the diagram; they are located at the start of the short section of dead strip (no rails) just before the loop merges back into the single track. The dead strip should be at least a couple of inches long to give the relay time to switch before the car gets power
Let’s take a look at how the electronics work:
The 12 volt power source (or whatever voltage you’re running your track at) comes in at the left of the diagram. The top inputs to the relay are set up as a latching circuit, meaning that when it is switched on (power applied to pin 10), it will stay on until it is reset. In this case S1, a normal open switch will switch the relay on, and S2, a normal closed switch, will reset the relay when S2 is opened.
In the initial state when power is turned on, S1 is open so no power goes to the solenoid input pin (10), meaning it is inactive and all the switches are in their Normal Closed position: input pin 7 connects to output 1, input 8 to 2 and input 9 to 3. For the variable voltage through the controller input (pin 8), the output (pin 2) connects to the right rail when traveling left to right. For the ground source input (pin 9), the output (pin 3) connects to the left rail when traveling in the left to right direction. In this initial state, the car will be traveling left to right on the main section of the track.
When the car goes around the right hand loop, it momentarily closes switch S1 this sends 12V to pin 10, which activates the solenoid to connect pin 7 (12V in) to pin 4 out. Because pin 4 output is looped back to pin 10, the relay latches in the active position until it is reset (described below). The variable voltage input (pin 8) is switched to pin 5, connected to the right rail when looking in the right to left direction. The ground connection input (pin 9), is switched to output pin 6 which goes to the left rail when looking in the right to left direction.
The car travels through the gate now going right to left on the single track.
When the car goes around the loop at the left end, it opens S2 (the normal closed switch) which breaks the latching current to pin 10 to reset the circuit back to its initial state, with the car again traveling left to right.
The astute observers out there will have noticed that the right hand rail when traveling left to right is the same thing as the left hand rail when going right to left – this means that we can jumper pins 2 and 6 together and pins 3 and 5 together (as indicated by the dashed red/gray lines in the diagram). Because these pins are ganged to their opposite switch position the ground on one can never* get connected to the power on the other.
* unless there is a catastrophic breakdown in the relay’s internal parts – very unlikely but not unknown to occur, hence the reason you ALWAYS have a fuse in the circuit.
more to come - stay tuned
Scott
All the components are inexpensive and readily available online or at an electronics shop; and the circuitry is pretty straightforward (no programming, digital controllers, JK flip flops or even transistors or diodes). Just simple normal open (NO) or normal closed (NC) switches (either optical or mechanical type) for sensors and a 12 volt triple pole double throw (3PDT) relay.
A 3PDT relay is just three two-way switches (each has one input, two outputs) ganged together and all operated by one solenoid (electromagnet). The relay has 11 blade connections (pins) on one end. The inputs to be switched are connected to pins 7, 8 and 9. Applying 12 volts across pins 10 and 11 will activate a solenoid. When the solenoid is inactive, the inputs connect to the normal closed side of each switch output on pins 1, 2 and 3; when the solenoid is active, it pulls the relay switches closed connecting input to output pins 4, 5 and 6.
Here’s a simple single lane track with a loop at either end. The slot where each loop feeds back into the main track should have some sort of spring gate to keep the incoming car from heading up the wrong slot at the junction.
The sensor switches that tell the relay when to switch are marked S1 and S2 on the diagram; they are located at the start of the short section of dead strip (no rails) just before the loop merges back into the single track. The dead strip should be at least a couple of inches long to give the relay time to switch before the car gets power
Let’s take a look at how the electronics work:
The 12 volt power source (or whatever voltage you’re running your track at) comes in at the left of the diagram. The top inputs to the relay are set up as a latching circuit, meaning that when it is switched on (power applied to pin 10), it will stay on until it is reset. In this case S1, a normal open switch will switch the relay on, and S2, a normal closed switch, will reset the relay when S2 is opened.
In the initial state when power is turned on, S1 is open so no power goes to the solenoid input pin (10), meaning it is inactive and all the switches are in their Normal Closed position: input pin 7 connects to output 1, input 8 to 2 and input 9 to 3. For the variable voltage through the controller input (pin 8), the output (pin 2) connects to the right rail when traveling left to right. For the ground source input (pin 9), the output (pin 3) connects to the left rail when traveling in the left to right direction. In this initial state, the car will be traveling left to right on the main section of the track.
When the car goes around the right hand loop, it momentarily closes switch S1 this sends 12V to pin 10, which activates the solenoid to connect pin 7 (12V in) to pin 4 out. Because pin 4 output is looped back to pin 10, the relay latches in the active position until it is reset (described below). The variable voltage input (pin 8) is switched to pin 5, connected to the right rail when looking in the right to left direction. The ground connection input (pin 9), is switched to output pin 6 which goes to the left rail when looking in the right to left direction.
The car travels through the gate now going right to left on the single track.
When the car goes around the loop at the left end, it opens S2 (the normal closed switch) which breaks the latching current to pin 10 to reset the circuit back to its initial state, with the car again traveling left to right.
The astute observers out there will have noticed that the right hand rail when traveling left to right is the same thing as the left hand rail when going right to left – this means that we can jumper pins 2 and 6 together and pins 3 and 5 together (as indicated by the dashed red/gray lines in the diagram). Because these pins are ganged to their opposite switch position the ground on one can never* get connected to the power on the other.
* unless there is a catastrophic breakdown in the relay’s internal parts – very unlikely but not unknown to occur, hence the reason you ALWAYS have a fuse in the circuit.
more to come - stay tuned
Scott
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