For our Dream Player Crossing Gate application (see Dream Crossing Gates), I thought it would be handy to use a slow-motion switch machine to control the gate movement.
When thinking of slow-motion switch machines, the Circuitron Tortoise automatically comes to mind. Of course a Tortoise is not the only game in town, but since we had them already, and they are so popular, I thought it would be appropriate to use one for this example.
Trying to keep this a simple bench-top demonstration, I chose the refined and life-like posicle stick for my gate. Clearly this doesn't represent any useful value, but it does show the Tortoise motion better than just looking at the black plastic throw arm going up and down.
If you didn't already know, a Tortoise is a stall type motor as opposed to a 'snap' switch. The stall motor switch machine keeps voltage on the motor even after it has reached it's position. There are variations on this, but in essence this is how they work. In order to move the motor one direction, you apply power to the motor. To move it the other direction, you reverse the polarity of the power feeding the motor. Many DCC accessory decoders can directly drive a stall-type turnout motor. To do this they usually have a bipolar output, H-Bridge, relay, or other means of reversing the polarity. When operated manually, a DPDT toggle switch can be used quite easily.
Our only problem is that a Dream Player does not have a bipolar output, or any other means to revers the polatiry. Sure you could use an external H-Bridge driver, but wanting to keep this simple, I chose an external relay. That same relay could be used to control just about anything, so it seems quite appropriate. For our application, the relay is doing exactly the same that as a DPDT toggle switch would do, except we can change the position with the Dream Player.
I had some PCB mount DPDT relays laying around which have a 5VDC coil, so they would be perfect for the Dream Player. Since the coils are 5VDC I can use the Dream Player power supply to operate them and not require any more external parts. This particular relay is available from DigiKey as PB383-ND (or Mouser) and costs $1.68 each. The Dream Player output driver chip (ULN2803) can sink about 400MA per Control Output terminal, so our little 20MA relay will work quite nicely. The ULN2803 also has built-in clamping diodes, so there are no external diodes needed to keep the Dream Player from seeing the spikes that come out of a relay when you remove the power.
Here you can see that we have connected the relay coil to the Dream Player Control Output #2. So why #2? Well that happened to be the Audio Track in the Dream Player with the crossing gate bell sound. You could connect it to any output that was appropriate for your application. The external power supply for the Tortoise is for two reasons. First, the Dream Player only has 5VDC available on its outputs. Second, we really don't want the noise of the Tortoise motor getting into our audio, so using a seperate power source keeps everything isolated. Each Dream Player Control Output can SINK power, but not SOURCE it, so the relay is connected between the 5VDC output and any Control Output. The Clamp terminal is also connected to the 5VDC output to enable the protection diodes in the ULN2803 chip. We put the ULN2803 in a socket so it can be easily changed, if you have an accident with your Control Outputs.
Well now we have a Tortoise connected to our Dream Player. Of course you could control just about anything the same way as we have shown. One that is on my list of to-do's is controlling an AC powered light. Why? I want to have lightening flashes syncronised to a thunder storm. Stay tuned...
If you want to have a peek at the finished crossing gate demo, just click on this picture, and a video will play. Be prepared, it is a 1MB file in Windows Media format. If you want to download the Dream Player audio track used in this sample, check out our Dream Sounds link on the right side of this page.
Good luck, and as always, let me know what you think about this or any other project here on Bob's Bench.