latchled
How can I have an LED turn on when a push button is pressed, turn off when the button is pressed again, and keep its state unchanged when the button is not pressed?
I don't want the LED to oscillate if the button is held down for a while, and I want immediate action (not having to hold the button down while waiting for a capacitor to charge).
Input selector in one of my amps worked that way (until I blew the chip up and had to recreate the selector since I could not get another chip).
I do not particularly like programming microcontrollers (why else would I make a dual frequency counter with a lot discrete logic chips?), so I always look for a way to avoid using one.
I made this with real buttons in mind (so I added debouncing). If you actually want to control this with logic output from some detector or chip (that does not need debouncing) you can get rid of R1-R4 and C2-C5. If your control signal is active low ("0" when the button is pressed) then leave the inverters. If your control signal is active high ("1" when the button is pressed) then you do not need to use U2C-U2F and just connect the control signals to the anodes of D1-D4.
How this works: When "1" appears on the anode of, say, D1, it also goes to the input pin of U1. The flip-flop is triggered by the riding edge of the clock. To allow for the input pin voltages to settle to their final values, a circuit made of U2A, R5, C1 and U2B delay the riding edge by a few ms. Actually, you could probably get rid of R5 and C1 too, the propagation delay of U2A and u2B would probably be enough, but I have no way of testing it without building it.
R7 and C6 reset the flop-flop to "0" (all LEDs off) state when the power is turned on, otherwise at each turn on it would get a random state.
I'm assuming your power supply is 5 volts, not 1 volt. As connected, the NPN transistor will attempt to short-circuit the power supply, which probably won't work. Try
simulate this circuit – Schematic created using CircuitLab
Best Answer
Assuming you don't want to do this with a simple microcontroller, I'd recommend you an easy logic gate circuit as from here:
Imagine Vout is low for a start. This will make the first gate (G1) output high. That makes the second gate (G2) output low, so this situation is stable.
Now, someone presses the button. That connects the high output of G1 to its input, so that the output becomes low. The low on the inputs of G2 makes the output of G2 high. We're in a stable situation again.
When someone presses the button again, the same happens, but in the opposite way: the low output of G1 is connected to its input, making the output high. That is connected to the input of G2, making the output of G2 low. We're back to the first situation.