lamp to LED switch makes relay fail

Discussion in 'The Projects Forum' started by FerventPiranha, May 1, 2012.

  1. FerventPiranha

    Thread Starter New Member

    Apr 30, 2012
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    First off, I'm pretty new to electronics, and no matter how I play with this circuit in Multisim I can't get it to work. A few months ago I got a little remote-controlled mouse cat toy, and I was bored, so I wired up some lamps to it for my cats to play with in the dark. I tapped into the battery for a constant hot, which feeds one set of lamps, then when the "go" button is pushed, it activates a SPDT (used as just a NC relay, all my local Radio Shack had that day) relay that swaps to the other set of lamps. The schematic is below. This is all fine and dandy, but I'm not comfortable with the cats potentially breaking the lamps.

    So I swapped to LED lights, and now it doesn't work as intended. At least in the sims, that is. I don't want to work on the toy until I know what I do will work. It seems with the LEDs in the places of the bulbs, the relay never flips over. With the "switch" (Go button) on, both sets of LEDs light up, instead of just the one set. The constant-on ones are truly constantly on, the relay doesn't cut them out. I'm pretty sure my cats don't really care, but I want to get my project working as intended.

    I've played with adding potentiometers in different parts of the circuit, trying to adjust the load, as well as a smattering of diodes, and nothing seems to work. I either get both sets on "Go" with the constants on otherwise, or just the "Go" lights on, and nothing on otherwise. I don't have the actual values of the LEDs I do have - they were just lying in a junk drawer from some other project long ago. The relay is part# 275-0248 from RadioShack. But, I'm still in the design stage, trying to figure out how to fix things.

    I feel like the fix is something simple that will make me hate myself, but I just can't figure it out. I'd appreciate any help or insight anybody can give. I've looked into transistors to see if that'd be easier, but I have no idea where to start with them. I'm sure my cats have forgotten about the mouse, but I'd like to get it working again, with added safety. Thank you in advance!

    Attached are the 2 very-similar schematics - cattoylamp works, cattoyled doesn't, much to my frustration. Ignore the sets of switches in cattoyled, I was trying to visualize connectors.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I doubt the way u have wired will work.
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    You can't put stuff in series with a relay coil, because the relay requires a certain current to switch. You probably got away with it with your bulbs because they are low resistance, but when you put LEDs there (and their current limiting resistors, don't forget those) there is no longer enough current to switch the relay.

    [​IMG]
     
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  4. FerventPiranha

    Thread Starter New Member

    Apr 30, 2012
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    It's just that simple, huh? I basically lucked out that it worked before? Glad all they had was SPDT relays I guess, will save me a trip. I knew it was something simple I overlooked, they's really nothing to the circuit. I appreciate the help.

    EDIT: in the sim the relay flips a bunch when the switch is opened, and settles down with the NC contacts completing the lighting. closing the switch does nothing as well. I'm thoroughly confused

    EDIT2: it works fine in Paul Falstad's online java circuit simulator, but it doesn't work as described above in Multisim. I don't get it
     
    Last edited: May 1, 2012
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Besides, a LED voltage ain't enuf to switch a relay, unless u have atleast say 5V as Vcc with a 3V relay coil voltage
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I've never used multisim so I can only speculate. From what I understand it is a higher level simulator, so it may be doing something complicated, like simulating switch contact bounce.
     
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  7. FerventPiranha

    Thread Starter New Member

    Apr 30, 2012
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    As long as it works in that other simulator, it should be fine then, right? I'll wire it up later today if that's the case. I appreciate all the help.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    should work fine. don't forget your current limiting resistors for your LEDs.
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Connect one set of LED's to the NC contacts, and the other set to the NO contacts. That's how it's done.
     
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  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    see post #3
     
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