I have a question regarding a 555 timer (TLC555CP) in bistable mode. Here is the schematic for the circuit in question. enter image description here

Now, this circuit works as I expect, when I hit the push button attached to the trigger the led goes on and stays on, and then when I hit the push button attached to the reset the led goes off and stays off. The problem that I have is when I disconnect the 555 timer from power.

If I were to open either S1 or S2, and then close it, the 555 timer does not act like it did before. In this situation, when I hit the trigger the led only goes on while the push button is down and goes off when it is up. Therefore the 555 timer does not act as I would expect. Eventually though, I can make it work again by either removing the capacitor, opening and closing S1 or S2, or a combination of both.

My question is, what is happening that I am not understanding in regards to why the 555 timer seems to lose its bistable functionality when it is cut off from power? Is there anything wrong with my circuit that would make this not happen? Thanks.

  • \$\begingroup\$ You are missing an important resistor in series with your LED to limit the forward current. \$\endgroup\$ – Michael Karas Dec 22 '15 at 4:48
  • \$\begingroup\$ @MichaelKaras, yeah, to make my question better I should have put that in, but that is not the problem. In my real life circuit I have the resistor. \$\endgroup\$ – Anthony Dito Dec 22 '15 at 4:49

The way that I fixed this is just connecting the Threshold pin (THR) to the ground. Leaving it floating caused problems even though I have seen it floating in other examples.


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