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Latch explained in the form of relays

  1. Jul 17, 2004 #1
    Hey all,

    I was in class today, and in order to grasp concepts better, i try to relate them to things i can use them for.

    Latches are very very helpful for me, but I like to use 12v applications, so busting out the 74HC74 or 74HC112 (i'm still learning these numbers, so correct me if i'm wrong) isn't gonna work (or, could i use them by dropping the voltage before the IC gets power, maybe xener diode or the two transistor method to stabilize voltage?).

    Can anyone explain how i could wire a single relay for feedback? In other words, design a simple circuit that all i need to do is depress a pushbutton switch to energize the relay (4-pin/contact N-O), and then when i let go, the circuit will continue to run.

    Further, how would i go about stopping this circuit once it starts?

    My prof. and I drew the circuit on the board today, but i didn't write it on paper.
  2. jcsd
  3. Jul 17, 2004 #2
    Use a voltage source ( battery) connect to the relay coil ( maybe via a suitable resistor) via two simple press switches in series , one NO the other NC. connect the relay contact terminals across the NO switch. When you depress the NO switch the relay will close confirming the connection and stay on when switch is released, to break press the NC switch -- circuit returns to original state. Ray
    IF you use a resistor an alternative would be to use another NO switch placed across the relay coil -- the resistor would then limit the current flow from the battery as you turn the circuit off by bypassing the coil.
    Last edited: Jul 17, 2004
  4. Jul 18, 2004 #3
    well, there was a diagram up, but now it's gone... I'm not really following your description.

    i might have it, but i'm not sure...
    Last edited: Jul 18, 2004
  5. Jul 19, 2004 #4
    You need a normally closed and a normally opened. Put the relay in series with both buttons. So the relay and both buttons are in one series circuit. Now, place the contacts of the relay switch in parallel with the normally open. Make sure it's only in parallel with the normally opened. When you push the normally opened, it energizes the relay. The relay, now, powers itself, because it's switch shorts the normally opened. Push the normally closed to cut power to the whole circuit.
  6. Jul 19, 2004 #5
    No i gotcha, thanks for the clarification. I was putting the circuit after the coil in the relay.

    I figured it out after awhile that the switched circuit needs to be on the switched side of the relay.

    Thanks again.
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