Electronics for a Model Roller Coaster Launch?

[coastergeekrtc]

I have been trying to figure out a way to launch my model roller coaster with a 14.4 V R/C car motor. To help you fully understand my situation, I made some pictures. The launch consists of two wheels that hold a cable. On the cable is a rod that hooks onto the train and launches it. I want to keep this rod in the launch run, the yellow track in one of the following pictures. The motor will be at the end of the launch. The end is the tallest point and the beginning is the lowest point of the yellow track. Here are the pictures to better explain.
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I would like to press a button that will turn on the motor at 14.4 V and launch the train towards the end of the launch track. This means that the rod on the cable is going from one end to the other of the launch run. I would like the motor to then go in reverse at a lower voltage to slow the rod on the cable to a stop. Once it is stopped the rod will continue backwards to its starting point and stop there. Now I can press that button again to launch the train another time.

I thought I could have two DC circuits, one at 14.4 V and another at a lower voltage (lets just say 6 V). I am using a train transformer to get the voltages where I want them. I wanted to use sensors that sense the train on the launch run and trigger a transistor to switch from the 14.4 V circuit to a reversing 6 V circuit. Then another sensor would sense the rod on the cable when it got to the starting point, shutting off all circuits and reseting the system so I can do it all over again. Is this possible, and what is the easiest way to sense the train? I am not an electronics wizard and need the easiest and cheapest system. I hope I gave enough information for you to help. Thanks in advance.

 

[ljcox]

I would put a LED on one side shining onto a LDR or photo transistor on the other side such that when the train reaches that point, the light is broken. Also, put another pair at the other end to detect when it has returned to the starting point.

This arrangement, together with a simple control system would do what you want.
We can help you further if necessary.

 

[coastergeekrtc]

Thank you for your help. I have been talking with a friend online about how to do this and I think I may have most of the system correct. Here is a schematic of what I think will happen, plus an explaination of what will happen when I try to make it work and some problems I have.
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To start I will turn the two power supplies on...easy enough. The DPDT Relay will be in the motor faorward position. To launch, I will press and hold a momentary switch. The train will launch toward the end of the launch run where a sensor (Side Note: I drew in a LDR and LEDs for now as the sensors but I may use some photosensors at RadioShack. Then I will not need the LEDs.) will sense the train and switch the relay into the motor reverse position. I now let go of the momentary switch. The rod on the cable will trigger the sensor at the start of the launch, switching the relay into the motor forward position. Now I can start all over by pressing the momentary button.

BUT, there is a problem now with the sensors. Lets map out a scenario. The launch is ready to go and the train is going through the roller coaster layout. It finishes and comes into the launch run, passing by the first sensor, switching the relay to the motor reverse position, and then the motor sends the launch rod backwards out of the launch run...not good. Now lets pretend that the train passed by the first sensor without triggering it, lets just say I didn't correctly aim it at the train. So now we can ignore the first sensor. The train launches, triggers the sensor, switching the relay to the motor reverse position, and the launch rod is pulled back to the start. But while on its way, it triggers the sensor at the end of the launch run, switching the relay to the motor forward position and everything stops (Unless I still have my finger on the momentary switch, and in that case the launch would start with the launch rod at the very end of the launch run. The launch rod would launch out of the launch run at a haig speed. This could break my roller coaster). So, the problem is that the sensors are always triggering the relay. I need to find a way to disconect them from the circuit at the points I want. Such as the sensor at the start of the launch should turn off once it senses the launch rod coming to its starting position. And when the train triggers the sensor at the end, it should connect the sensor at the beginning and then disconnect itself. And when the rod triggers the sensor at the beginning, it should connect the sensor at the end and then disconnect itself. I'm guessing relays would help in this situation,csuch as where the sensors trigger a relay that disconnects itself and connects the other sensor, but I can't figure out how (I'm tired and my brain is turned off).

Another question. I want the motor to reverse slowly. To do so I need to reduce the voltage in the reverse circuit to a certain voltage. However, I don't know what voltage will attain the correct speed. So I am asking you how do I figure out what I need to add to the reverse circuit to reduce the voltage from 14.4VDC to an unknown #VDC?

One more question. When the relay switches from the reverse position to the forward and vice versa, will the voltage reduction and momentary switch be harmed by the electricity? For example, the electricity will go through the foward circuit and the reverse circuit when the relay is in the forward position. Will the 14.4VDC harm the voltage reduciton parts? Should I add a diode on each wire entering the motor to direct the flow safely and stop it from entering an area it can damage? If so, which one should I use?

Thank you for your help in advance.

 

[coastergeekrtc]

Is anyone able to help?