Tortoise Control

[peteski]

While a discharged cap will look like a short at the instance of power up, the circuit is a symmetrical flip-flop. Both caps will be like dead short.  To me it looks like the initial power-up state will be random (or dependent on slight random variations in the tolerances of the components).

[bdennis]

Thanks for the info. I think I will store this one in the memory banks for later use..!

[LKOrailroad]

Great news. Thanks guys.

[LKOrailroad]

While a discharged cap will look like a short at the instance of power up, the circuit is a symmetrical flip-flop. Both caps will be like dead short.  To me it looks like the initial power-up state will be random (or dependent on slight random variations in the tolerances of the components).

That is the impression I got also. Railnerd has built it and says it power up resets just fine. I may breadboard this one myself and play with it for a while.

[peteski]

That is the impression I got also. Railnerd has built it and says it power up resets just fine. I may breadboard this one myself and play with it for a while.

Like I mentioned, the power-up state will be based on the slight tolerances of the components.  It might be consistent for each circuit you build but not predictable.

[railnerd]

Like I mentioned, the power-up state will be based on the slight tolerances of the components.  It might be consistent for each circuit you build but not predictable.

Guys— I didn't build the single-button "toggle" circuit, so I cannot confirm or deny that operation.  Should be possible to tweak those resistors to change the time constants— probably easier to breadboard this and mess with it, rather than firing up SPICE, though.

-Dave

[Hiroe]

I earlier had looked at this circuit. The missing part is the automatic default turnout positioning upon railroad power up. Otherwise, it is perfect and much simpler.

We've actually built about 60 or so copies of this design for use at the CVMRR club layout; and I can testify to the fact that they work extremely well, with a MTBF of about 6 years (based on the layout being powered-up about 1.3 times a week). I made the decision early-on to mount the timers in dip sockets; and evolution of the component layout has us currently building them with pairs of individual 555 timers, instead of the 556 dual timer.

The nice part about these circuits is that you can also use them with diode trees to drive all the necessary switches in a yard ladder, with one button for each given track. The default position for each driver board also gives us a good start every time we power up the club layout; as you never have to worry about someone leaving a switch set against the main when starting up the layout at the beginning of an ops session.

--Drew

[LKOrailroad]

Drew,
Appreciate the comment and relaying the practical experience. The issue for me with this particular circuit is the use of two push buttons. I want the Tortoise to toggle position with the press of a single push button. The 4.7uF cap in this circuit sets the power up default position. On the single push button version the 4.7uF cap is eliminated because it introduces instability into the circuit. As a result, the single push button version is iffy about its power up state. Some folks say it powers up in one state reliably while others say it doesn't. As soon as I can make a run to Radio $hack to get a 556 I am going to breadboard a test circuit.

[Hiroe]

The 4.7uf capacitor on the 'normal' input is necessary to ensure that the timer pair don't destroy themselves on power-up, as they will essentially 'race' each other to see who wins; with the 'losing' device usually getting killed in the process.

If you plan to breadboard this to test it, make sure you pick up a few spare timers. The problem with using 556-packaged timers is that when one fails, you have to replace both at the same time, by necessity of package design. Using pairs of discrete 555's, you can simply replace the damaged device.

[basementcalling]

Interesting discussion. Wish more of it was in English. 

[peteski]

Interesting discussion. Wish more of it was in English. 

It is in as plain English as you can have when discussing an electronic circuit design on the DCC/Electronic forum. 

[LKOrailroad]

Don't you just love it when you have an epiphany? Two of them back to back is just down right spectacular. That happened to me today.

Realization #1

Having a defined power up position of turnouts might not be such a good idea. What if a train, for whatever reason, is parked straddling a turnout? I may be setting myself up for a derailment as I turn on the railroad. Hmmm, maybe I had better not do that. Aborting the defined power up position of turnouts I set about rethinking the Tortoise control system.

Realization #2

Bistable dual coil latching relay. Tie one side of each coil to ground. Tie the other side of each coil to the contacts on one of the Tortoise's SPDT switches. Supply V+ to the push button. Connect the push button to the center of the Tortoise SPDT. When the Tortoise moves it will change the push button from being connected to one of the latching relay coils to the other. The Tortoise will do the coil assignment. Pressing the button triggers whichever coil is presently connected. The relay moves in only 4ms. No way the Tortoise contacts will open faster than the relay actuates.



Is that simple and foolproof or what? No ICs, no circuit boards, nothing more than a switch and a relay. I am ecstatic!

Digikey pt# PB1085-ND $1.96 ea.
Digikey pt# EG2031-ND $1.32 ea.
Total per Tortoise $3.28

I'm putting away the breadboard. This is a done deal.

[LKOrailroad]

Tortoise control schematic

[LKOrailroad]