When I build an SR flip flog on my breadboard I connect everything perfect, but when I connect the power supply, both LED's initially light up. The inputs SR(when S=0 and R=0) do not save the current input. So If I press the S button to make the input '1' it turns off the correct LED but when I release the switch the flip flop does not save the input and both LED's light up again. I am using just a basic nor gate connection with no clock input and I can not get it right for some reason. Please help!
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Interesting question and I think I know what is happening. You did not include the schematic, so I will assume that what you did is connect two NOR gates (output of one to input of other) and then put the LED on the output of each NOR gate. One NOR gate input is the S and the other NOR is the R. That is the classic way to make an SR flip-flop.
So why do both LEDs light up and why does one turn off when the S input is made a 1 and then they both light up after the S is back to a 0? Well, I now have to make another assumption and that is you put a current limiting resistor on the LED (between the LED and GND). If you didn't, well that still might work, as the NOR gate is limiting the amount of current the LED will see and unfortunately, you might be close to burning up the NOR gate if you hold the S for too long.
However, what is actually happening is that the current being used to power the LED is more than the NOR can deliver and the voltage on the output is dropping below the threshold voltage needed for the opposing NOR gate to see the level of 1 and what happens then is that the NOR gates switch to the opposite state. You have built an oscillator and both LEDs are lit because they are running at a fairly high frequency, so you eye can't see the turning on and off. If yo put a scope on the NOR output, you should see an oscillating waveform.
So, how to fix this? Increase the current limiting resistor by putting a series resistor in the circuit. If you calculate what the voltage on the output of the NOR is (disconnect both LEDs and then measure the voltage at no load) and then make the current through the LED be say 2ma, you can calculate the value of the series resistor. You can use a forward voltage drop of the LED to be 2v and it should be close enough.
If you don't know how to make this calculation, then you can experimentally change the series resistor from a high value starting at 1k and work down till you see only one light lit. I fyou have too high a resistor, both will go out.
So why do both LEDs light up and why does one turn off when the S input is made a 1 and then they both light up after the S is back to a 0? Well, I now have to make another assumption and that is you put a current limiting resistor on the LED (between the LED and GND). If you didn't, well that still might work, as the NOR gate is limiting the amount of current the LED will see and unfortunately, you might be close to burning up the NOR gate if you hold the S for too long.
However, what is actually happening is that the current being used to power the LED is more than the NOR can deliver and the voltage on the output is dropping below the threshold voltage needed for the opposing NOR gate to see the level of 1 and what happens then is that the NOR gates switch to the opposite state. You have built an oscillator and both LEDs are lit because they are running at a fairly high frequency, so you eye can't see the turning on and off. If yo put a scope on the NOR output, you should see an oscillating waveform.
So, how to fix this? Increase the current limiting resistor by putting a series resistor in the circuit. If you calculate what the voltage on the output of the NOR is (disconnect both LEDs and then measure the voltage at no load) and then make the current through the LED be say 2ma, you can calculate the value of the series resistor. You can use a forward voltage drop of the LED to be 2v and it should be close enough.
If you don't know how to make this calculation, then you can experimentally change the series resistor from a high value starting at 1k and work down till you see only one light lit. I fyou have too high a resistor, both will go out.
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Well, I have been doing electronics for more than 40 years, and you just remember some of these things. I don't know what you breadboard has for components, so that is why I suggested starting with a higher resistor. If I had know the logic family, I would have been able to tell you the resistor.
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Nice answer John! The second most common error that newbies make about LEDs is assuming that the forward volta
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