Latching Circuit Game Show Buzzer Relays Buzzing

Discussion in 'The Projects Forum' started by seanmwalls, Apr 21, 2009.

  1. seanmwalls

    Thread Starter New Member

    Apr 9, 2009
    3
    0
    Hi, everyone!

    I'm new to the forum, and an amateur when it comes to circuit design, but I understand the basics. I would appreciate any help you can provide me with "mess on the floor" that my wife is trying to get me to finish!

    Here is the setup. I am trying to build a game show type setup with 4 stations. When a contestant pushes their button, I want the circuit to lockout the others from chiming in. I am currently using 4 DPDT relays that share a common connection that feeds their button using the NC connection on 1 side of each relay. When the contestant hits their button, I want the relay to latch for that contestant, breaking the NC connection for the others so they cannot chime in. I put the circuit in the CircuitLogix and it runs correctly but when I built the circuit, my relays do not latch. Instead they "buzz".

    I assume the problem is that the relay is bouncing or the fact that the NC contact is being broken before the NO latch feed to the relay can be made. Here is a sketch of my circuit: Game Show Buzzer

    Since I already have all of the components, I am trying to find a way to work with what I have, or make minor alterations (and low cost) to the design to make it work. I'm not sure how to proceed. Should I use a CR setup to sustain the secondary power to the relay once the button is pressed to allow it time make the new NO connection or is there another route I should be pursuing? I should mention that because this is for purely entertainment purposes, I am not concerned with occasional errors.

    I appreciate your help!
    Thanks,

    Sean
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    2,535
    Just a quick glance suggests you need some diodes to keep the voltage from the relays from backfeeding to the switches.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This is one of those things where simulations just don't quite match the real world.

    In the simulated relay, once the coil is energized, the NC contacts instantly break and the NO contacts instantly make, and there is no contact "bounce".

    In your real relay, as soon as the coil is energized, the NC contacts start sliding and "bouncing" reducing conductivity; as soon as the NC contacts break, the power to the coil is lost. There is not enough inertia remaining to cause the contacts to "fly" to make the NO contact; even if there was, the NO contacts will "bounce" for a period of time, reducing the available current.

    Bill Marsden's mention of the missing "flywheel" diodes across the coils may help the situation somewhat, as the coils will then have a current path when the contact is broken, which will delay the collapse of the coils' magnetic field.

    A 100uF or so cap from the buss powering the NO pushbuttons to ground would provide some power to the coil during the flight of the contacts.

    You also need to add diodes to prevent the NO pushbuttons from back-powering other relays if two or more buttons are pressed at the same time.
     
  4. seanmwalls

    Thread Starter New Member

    Apr 9, 2009
    3
    0
    Sgt Wookie and Bill,

    Thank you both for the help. I have modified the circuit to include the suggestions you both made. Is this the correct setup: Modified Design ?

    Thanks,
    Sean
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sean, that looks better. Note that 100uF was simply a SWAG, or Scientific Wild-Arsed Guess. ;) You will have to experiment to determine the "best compromise" value, since I don't know what relays you are actually using. YMWV (Your Mileage Will Vary).

    [eta]
    For the diodes, you can use the common 1N4001 through 1N4007 series; the actual number really doesn't matter for your application. Radio Shack sells a 25-pack for a couple of dollars (not cheap, but very convenient).

    Let us know how you made out.
     
  6. seanmwalls

    Thread Starter New Member

    Apr 9, 2009
    3
    0
    Worked like a charm! I was able to finish the project this weekend, and the living room floor is all cleaned up! The 100μf capacitors did the trick; your best guess worked!

    Again, many thanks! I appreciate your feedback and I can't wait to use it for our next team event at work!

    Thanks!
    -Sean
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    You might think about posting this in the Completed Projects Collection, this isn't the first project I've seen like this.

    Very nice.
     
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