Help me turn a potentiometer into a on-off switch

Discussion in 'The Projects Forum' started by Gunny Webb, Feb 6, 2015.

  1. Gunny Webb

    Thread Starter New Member

    Feb 6, 2015
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    I have an old, 1963 vintage switch panel that is being repurposed for a steam punk project. It has a lever that is geared to a rotary potentiometer that has two sides. One swings from 9K to 10K and the other side swings from 10K to 9K. I need somehow to turn this into an on off switch. There is not enough space to change the potentiometer to another type of switch, so I guess I need some type of logic circuit to convert this difference into a trigger for a relay.
    I'm 55 and raised in the diode, relay and switch era. Understand the function of basic electronics and can read a schematic. But building something like a nixie clock is almost rocket science to me.
    Gunny
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Can you buy parts from Mouser or Digikey?

    What supply voltage do you have available? How about an old phone-charger wall-wart? What voltage is it?
     
  3. Gunny Webb

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    Feb 6, 2015
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    I buy stuff through my business from vendors like that.
     
  4. ISB123

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    May 21, 2014
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    Cant you get a rotary switch,they are pretty much the same size as pot.
     
  5. Gunny Webb

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    Feb 6, 2015
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    This panel has 7 switches stacked side by side. The pot has the letter AB like a manufacturer stamp and the part number A20869 on it. They have a about a 1/8 inch shaft in a worm gear and the pot is 7/16" round and about 3/8" long with a 7/8 inch long shaft. Thought about running several micro switch along the line of switches and that would work on the bottom row, but there's another row on top that is too close to them to make that feasable.
    Power is 12 DC.
     
  6. MikeML

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    Do you really need a relay?

    If the resistance change turned on a transistor, could you use that? How many Amps is the load?

    Which way do you want the switching: turn on a load where one end of the load is connected to ground (source)? Turn on a load where one end of the load is connected to +12V (sink)?
     
  7. Gunny Webb

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    Feb 6, 2015
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    Load is nominal, 1/2 amp if that. Just enough to power an LED and a small relay. Plan is to flip each switch and it turns on a LED, after all switches are on the current flowing through the series of switches turns on a relay.
    Did you ever see Dr Strangelove the movie? Recreating the panel in the bombadiers area where you flip a switch and a green light turns off and a red comes on, tripping all switches sends power to the next series of switches.
     
  8. MikeML

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    Would you mind answering all my questions, one by one?

    I'm confused. How many independent pots do you want to detect the resistance change of?

    What do the row of switches have to do with the pots?

    Each pot being moved from 9K to 10K should turn on a LED (but no relay)?

    You want to pull-in a relay only after the last pot is moved?
     
  9. Gunny Webb

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    Feb 6, 2015
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    One pot turns on one light.

    The switch levers are geared to the pot shaft, the lever moves vertically, the pot shaft turns.

    Yes and yes.

    Google Gables G2750, it's an old airplane audio panel. Looks like it has switches, but the lever has a rack gear on the rear that turns a brass gear that moves the pot. They did things in a much different fasion in the 60's.
     
  10. MikeML

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    How stable is the 12V supply? Regulated? 12V battery?

    Is it really a pot (with three connections) or only a rheostat (with only two)?

    If it is a pot, what is the end-to-end resistance? ~20K?
     
  11. Gunny Webb

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    Feb 6, 2015
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    Battery

    Three connectors. One increases and the other decreases when turned.

    I'll run out to the shop and check.
     
  12. Gunny Webb

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    Feb 6, 2015
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    Glad I double checked. Got odd readings with the meter so I used a diffrent one.

    Here's the readings, 1500 ohm on one and zero one the other.
    Turn the pot and it's zero and 1500.
    1500 total.

    So sorry for the bad info. Eronious meter now in trash. Chalk it up to a newb mistake. Again, sorry.
     
  13. MikeML

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    Ok, when measuring between the pot bottom end and wiper, the pot resistance changes from 0Ω to 1500Ω.
    When measuring between the pot top end and wiper, the pot resistance changes from 1500Ω to 0Ω.

    You would like it to trip when? as the resistance goes past about 90% of the travel, so about 1350Ω?
     
  14. Gunny Webb

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    Feb 6, 2015
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    You are correct, that would be perfect.
     
  15. ebeowulf17

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    Aug 12, 2014
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    There's likely a more elegant and/or compact solution, but I think the following would work:
    1) Create a 6V reference voltage with a pair of resistors.
    2) Use comparator chips (like the LM319) to activate an output (feeding LED and an input to an AND gate) for each pot when it passes the halfway mark.
    3) Feed all of those outputs into AND or NAND gates (maybe a HEF4068) so that when all the switches are on, your extra output is activated.

    Are the relays you mentioned controlling other circuits you've not yet mentioned, or are they just part of the plan for turning on an extra output after all the switches are active?
     
  16. ebeowulf17

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    Aug 12, 2014
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    On a side note, anyone here know why my posts/replies on this forum will sometimes change font sizes mid-paragraph when I haven't clicked, selected, or formatted anything differently? It's really odd.
     
  17. Gunny Webb

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    Feb 6, 2015
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    Think of it s 4 switches in a row. Each switch turns on one light. When all four are lit there is power available at the end that goes to the next set of lights.
    If you had two sets, the first set must be on before the second set has power.

    The LM319, such a simple but clever device. Never knew a thing like that existed. Already learned something. I like this forum.
     
  18. MikeML

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    Ok. Here is something that should work. You need One each of block A. It is just a 5V shunt voltage regulator. I tested the resistance detector B at battery voltages of 12V and 13V, and there was too much effect on the pot trip point, so regulation is needed.

    You need four to seven each of block B, depending on how many pots you want to utilize. This circuit trips when R1 (the Pot) is a bit less than R2, which is a standard 1% value. The TL431 is being used as a comparator where the voltage on r is compared to its internal 2.495V reference voltage. As the resistance of the pot increases, the TL431 cathode goes low, turning on LED D1, and causing K1 to drop low. Look at the simulation. As the pot resistance increases from 1.3K to 1.4K (x-axis), the Led turns on I(d1)=~14mA, and V(k1) falls.

    PotSense.gif

    Finally, you need one each of block C, which is an AND gate and relay driver. The relay pulls-in when the last Kn input signal goes low. This means you can move the pots in any order, but the relay clicks in only when the last one is moved so that Rpot>1.355K. See the simulation I(L1), the relay coil current. The relay is one with a 12Vdc coil. The catch diode is D3. Depending on what you are driving, you may be able to just connect it in place of the relay as long as one end of the load can be connected to Gnd. M1 should be a PFET with a Vt of ~-3V.
     
  19. Gunny Webb

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    Feb 6, 2015
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    Thank you for such a great explanation. Think I can handle that.
    This is like pre-school stuff for you, isn't it? New world for me.
    Thanks everyone!
     
    Last edited: Feb 8, 2015
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