Interface LED between two logic chips

[panfilero]

Hello,

ok, this is my situation:

here are the players

U1: 4 to 16 Line Decoder/DeMux SN74HC154
U2: An 8-Input NAND gate SN74HC30
an LED
a switch

now, this is what I'm trying to do. I would like,

State 1: open switch
I don't care what the Decoder is doing, LED is on and NAND spits out a 0

State 2: closed switch
when my Decoder spits out a 1 my LED should turn off and my NAND gate spits out a 1

State 3: closed switch
when my Decoder spits out a 0 (low) my LED should light up and my NAND gate should spit out a 1 (high)

I've sketched this out, in the sketch I replace the NAND gate with an inverter for simplicity. My question is, I faith that the way I've sketched it State 1 is good, State 2 makes me feel funny... but I think it would work, state 3... might not be so great because I'd be relying on the voltage drop across the LED to pull down my NAND gate's input (inverter in the sketch)

can anyone comment on my thoughts, or tell me if there's a better way to wire this to accomplish the three states I'm trying to get.



much thanks!

(sorry about the dark pic)

 

[MikeMl]

It might work if the logic family runs on a supply voltage of ~15V; it will not work on any 5V or 3.3V logic family. The forward drop of the LED is too high (~2 to ~3V). When the upstream logic input is a low, the voltage seen by the downstream logic input is higher by Vf of the LED. For low voltage logic families, this puts the input right at or above the logic threshold.

 

[panfilero]

thanks Mike, I agree and for these chips their VIL is 1.3V, so I'm really at the cusp... so my state 3 is bad, what about state 2, do you see any problem with the way I have that hooked up?

 

[MikeMl]

thanks Mike, I agree and for these chips their VIL is 1.3V, so I'm really at the cusp... so my state 3 is bad, what about state 2, do you see any problem with the way I have that hooked up?

The logic gate input is always a 1; the LED never lights up???

 

[panfilero]

The logic gate input is always a 1; the LED never lights up???

Yup that's the way I see it. The diode will be reverse biased because of the two 5V voltages on either side of it and by being reverse biased it does not turn on and isolates the left side of the circuit from the right.

Do you think this is weird?

 

[MikeMl]

Yup that's the way I see it. The diode will be reverse biased because of the two 5V voltages on either side of it and by being reverse biased it does not turn on and isolates the left side of the circuit from the right.

Do you think this is weird?

The diode is doing exactly what you are telling it to do. I think it is weird to build a circuit that is not useful...

 

[panfilero]

The diode is doing exactly what you are telling it to do. I think it is weird to build a circuit that is not useful...

ummmm... if you are saying that the diode is off when I want it to be off, then good, mission accomplished. My question "do you think this is weird" is in regards to how I accomplish state 2, which is:


State 2: closed switch
when my Decoder spits out a 1 my LED should turn off and my NAND gate spits out a 1

I also agree it would be weird to build a circuit that is not useful