I notice that all of the parts internal to your seven segment decoder have 74xx part numbers. The other thing I notice is that there is a 220 Ω in series with each of the decoder outputs.

Your 7-seg display may not have current limiting resistors as part of it. Put a 220 Ω resistor in series with each input. I don't know how your simulator reacts if a logic gate attempts to drive a diode with no resistor.

Some simulators automatically insert D/A and A/D blocks at the junction of digital and analog components. It's possible that in your two-input circuit this isn't happening.

You might also try replacing your logic gates with the same 74xxN parts that the decoder uses.

 

You show that you are using 7406 inverters. These inverters have open collector outputs. This may cause problems in both the simulation and the real world. The part you want to use instead of the 7406 is the 7404.

 

You show that you are using 7406 inverters. These inverters have open collector outputs. This may cause problems in both the simulation and the real world. The part you want to use instead of the 7406 is the 7404.

Good observation. Since these are in the decoder that the sim shows is working, the mechanism would be that it is relying on the natural pull-up behavior of unconnected TTL inputs. Not something to rely on, in either sim or the real world.

 

Yeeeeesssss it works with the 7404 instead of 7406 !!! I don't know yet what is an open collector output but I will search on the net.

Thanks thanks thanks!

 

Yeeeeesssss it works with the 7404 instead of 7406 !!! I don't know yet what is an open collector output but I will search on the net.

Thanks thanks thanks!

Obviously I'm missing something here. I don't see that your two-input circuit specifies either a 7404 or a 7406 -- it's just got generic logic parts. The only place that I saw the 7406N was in the four-input decoder circuit that you said was working. So where did you make the change?

Open collector means that the output can pull the line LO but it can't pull it HI. Something else, such as a pullup resistor, must do that.

 

Obviously I'm missing something here. I don't see that your two-input circuit specifies either a 7404 or a 7406 -- it's just got generic logic parts. The only place that I saw the 7406N was in the four-input decoder circuit that you said was working. So where did you make the change?

Open collector means that the output can pull the line LO but it can't pull it HI. Something else, such as a pullup resistor, must do that.

Yes you don't see that it specifies 7406 because I desabled the display of the "value" or name of the component (my bad...). I changed the 7406 in the two-input circuit for 7404 and it works.

But the 7406 are still in the four-input circuit and it works... Hum...

 

Yes you don't see that it specifies 7406 because I desabled the display of the "value" or name of the component (my bad...). I changed the 7406 in the two-input circuit for 7404 and it works.

But the 7406 are still in the four-input circuit and it works... Hum...

This now makes sense.

In your two-input circuit you are trying to drive some of the display inputs directly with the output of an open-collector inverter but a common cathode display requires an active HI signal to drive the display.

In your four-input circuit all the inverters are only driving other gates and TTL gate inputs generally act as if they are HI when unconnected.

You still need to be sure that you have adequate current limiting resistors on each display input and that you aren't exceeding the maximum gate and package output current, particularly if you plan to build it.

 

In your four-input circuit all the inverters are only driving other gates and TTL gate inputs generally act as if they are HI when unconnected.

Wow, that's true.

You still need to be sure that you have adequate current limiting resistors on each display input and that you aren't exceeding the maximum gate and package output current, particularly if you plan to build it.

Ok !