Low cost "touch control" tortoise driver

Discussion in 'FAQs' started by BigJim, Feb 12, 2007.

  1. BigJim

    BigJim Member

    I hate mechanical switches. I hate spending money.

    I wanted an low cost method to control tortoise switch machines and to do "route control" to set multiple switches to predetermined routes while still being able to do individual switching.

    The solution for me is CMOS NAND gates and LM556 timer (used as drivers).

    CMOS NAND gates work great at 12 volts. The very high input resistance means it takes very little current to input a high or low logic value. Typical values are small fractions of a micro-amp. If you don't mind licking your finger every time you switch the "pull-up" resistors could be as low as 100K Ohm. Even with 10 Meg Ohm you may have to moisten your finger in dry areas.

    The 566 is not used as a timer but the bipolar output (200 ma source & sink) and the open collector output are purfect for controlling a tortoise and a relay at a low cost. There is some excellent application notes on using these versitle devices at
    http://home.cogeco.ca/~rpaisley4/CircuitIndex.html .

    Touching one input to ground will "flip" the flip-flop. I will stay that way until you touch the other input. Three screw heads (center one ground) would work great or hide some wires in a ground throw "look a like".

    For multiple route turnouts just tie one pair of the inputs for all the turnouts in the group together with one resistor for the set and one for the reset. If you need more than three routes for any one turnout feed one of the NAND gate inputs with the output of another quad input NAND gate to add three more possible routes. Unused inputs should be tied to +12 either with a resistor if you may use them in the future or directly.

    The LM556 also has two outputs that can be used to drive relays. Just be sure you add a diode to kill the reverse voltage generated when you turn the relay off.

    Depending on the number of circuits you put on each project board and where you buy the parts the cost of each control is about $1.25>$1.75 each.
  2. BigJim

    BigJim Member

    Quick notes:

    Since you only need to provide a path to ground for a very small amount of current (micro-amps) the set/reset/ground connections can be daisy-chained to multiple screw headl ocations or "touch points" around the layout with very small wire using the simple two input NAND gate circuit. You only need the quad NAND gates if you want a turnout to be part of multiple routes.

    I have read reviews about switch motor controls that bounce or oscillate. (http://www.tonystrains.com/technews/digitrax_ds44.htm) These use the LM324 to drive the tortoise. The LM324 has a current spec of 20ma typical but only 10ma minimum spec. This is very close or below the stall current of a tortoise. The 555/556 timers have 200ma minimum spec and will drive the tortoise at full speed and provide the full stall current to keep the points tight against the rail.
  3. BigJim

    BigJim Member

    P.S. If you add another pair of quad NAND gates to "Fan-out" the inputs you will need to invert the outputs before you feed it into the flip-flop NAND gate.
  4. BigJim

    BigJim Member

    Works like a champ

    Parts arrived and I made up a couple of the Tortoise control boards with the Quad input gates. 1 Meg ohm works great for the pull-up resistors and should eliminate any "false triggers" that you might get with 10 meg ohm. If your touch points are smooth it does require a very slightly moist finger. The 556s also have a open collector output (discharge) that I use to drive a 10 Amp relay for track polarity switching.

    I was able to put four complete controls on one 777 prototypeing board(http://futurlec.com/ProtoBoards.shtml). Total cost:

    Prototype board #777 $1.50
    4 x 556 dual timers $2.60
    4 x 4012 Quad NAND gate $.80
    24 1 Meg ohn resistors $.24
    8 14 pin IC sockets $.48

    Total $5.62

    Not bad - touch control three input (single turnout + ability to put the turnout in two "routes"), plus daisy chain any pair of "touch" points to any number of locations for less than $1.50 per Tortoise.
    P.S. hint - use the cut off resistorlegs forthe short jumpers. The rest is CAT 5 phone wire. Picture taken prior to adding all of the gate to card edge wires tobetter show the jumpers.

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