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Hare & Wabbit Voltage Output and Push Button Operation

Posted by on Feb 26, 2015 in Tips DCC Components, Tony's Tips |

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DCC Guru Larry Maier tackles a Hare and Wabbit together:

I’m looking at interfacing a unit that will look like a single manual pushbutton switch to Hares or Wabbits, and have a couple of technical questions.

The unit I’m interfacing needs a small amount of power (about 3 ma at 5 volts or more would do) for its own purposes. This part of my unit is electrically isolated from the “output” of my unit, so the Hare will still just see a dry contact closure between pins 11 and 12. From the manual it appears the Hare has a positive voltage (probably 5 volts) available on pin 7, relative to a ground on pin 11 that I might be able to use for my source for this ? If so, are there any resistors in series with those two lines? If there are series resistors, what is their total value?

The Hare pin J1-12 is pulled up to +5V through 24K. J1-11 is ground. Note: DO NOT connect the grounds of different Hares (or Wabbits) together. Also note that the Hare/Wabbit is floating at the DCC potential, so no connections should be made to other circuitry except via an optical isolator or similar device.

I don’t see any way to accomplish something similar with the Wabbit, though. Even if I derived my power source from the DCC power (which wouldn’t be too hard), the Wabbit doesn’t appear to directly support single-pushbutton operation. Do you see any way to do this with the Wabbit?  If you added a CV setting that could select function of a single pushbutton on the Manual Throw input as a state toggle, instead of the usual two-pushbutton operation, it would allow users the option of using the Wabbit more like the Hare. But I don’t know if you’d be inclined to do that. Will the Jack Wabbit be any different than the Wabbit in these respects?

The Wabbit has the same single input feature as the Hare, except you must enable it. Use the Clear manual switch input and set CV71=1. The points will now change position any time there is a contact closure. Note the Wabbit has the same warnings about connecting to circuitry as the Hare with the exception that you can use a common ground or +5V connection for the two switch controls ON THE SAME WABBIT. While the Hare switch input is a pull down active, the Wabbit switch inputs are pull up active. The common goes to +5V through 750 ohms, while the Clear and Throw inputs are 24K to ground. A transistor output opto-coupler with the collector connected to common and the emitter connected to the Clear or Throw will work fine on these inputs. The Jack Wabbit will have the same feature except that either control input will reverse the points if CV71=1.

And finally, what is your output voltage to the Tortoise with the Hare or Wabbit? Does it vary depending on DCC system voltage?

The output voltage to the Tortoise is derived from the DCC, so it will change with the DCC voltage. NMRA recommended practice is to set the DCC to 14.25 volts. This will give 12V (approximately) at the Tortoise terminals, which is the rated Tortoise operating voltage.