I'm developing the following circuit as a prototype for an automation project. I'm using 3 inputs from switches, which all have their own pull down resistor, just as the circuit bellow.

However, when mouting the circuit in a protoboard, only Va and Vb show the desired 5V voltage. Vc has a voltage of 2.2V, that is affecting the IC's output voltage for S2 and S1.
I've already checked all connections and they are all correct.

 

Hi raphael,
Welcome to AAC.
What are the resistance values of the pull down resistors?
E

Update:
I see that they are 1K, that is too high for a TTL pull down

 

So as is common for these problems, you didn't look at the data sheet.
It shows that for the maximum logic low voltage of 0.4V, the current flowing out of the input is 1.6mA (below).
Thus the maximum resistor to ground to achieve that is 0.4V / 1.6mA = 250Ω.
The 1k resistor you used is 4 times too high.



If you want to use a higher resistor value, you can use a normally closed switch to ground and a 10kΩ pullup resistor to 5V, since the high input current is very low (40µA max).

 

Hi raphael,
Welcome to AAC.
What are the resistance values of the pull down resistors?
E

They are all 1K resistors.

 

So as is common for these problems, you didn't look at the data sheet.
It shows that for the maximum logic low voltage of 0.4V, the current flowing out of the input is 1.6mA (below).
Thus the maximum resistor to ground to achieve that is 0.4V / 1.6mA = 250Ω.
The 1k resistor you used is 4 times too high.

View attachment 330983

If you want to use a lower resistor value, you can use a normally closed switch to ground and a 10kΩ pullup resistor to 5V, since the high input current is very low (40µA max).

Thanks, I'll try to replace those resistors with 250Ω ones. But shouldn't the other resistors not work as well? Because Va and Vb are correct, even with 1K resistors.

 

hi r,
Look at this AAC link, it is a common problem
E
https://forum.allaboutcircuits.com/threads/7408-not-working-as-expected.128872/

 

This is one of the TTL "quirks" that goes away with high impedance CMOS inputs. No more worries about inputs sourcing current. This is also why pull downs are rare in TTL circuits. Pullups and inverters are more common or the use of Negative logic where LOW is equivalent to TRUE. The best thing you can do is consign these parts to the dustbin of history.

 

Thanks, I'll try to replace those resistors with 250Ω ones. But shouldn't the other resistors not work as well? Because Va and Vb are correct, even with 1K resistors.

Do you not understand Ohm's law?
The low Va and Vb voltages are not correct when the switches are open.
The resistors need to sink 1.6mA at 0.4V.

 

In all the TTL circuits that I have ever designed there was NEVER a single pull down resistor. IMHO you should eliminate them by appropriate choices of logic level meanings.

 

You have two inputs connected to each pull-down resistor, so 250 Ohms is definitely as high as you should go!

 

In all the TTL circuits that I have ever designed there was NEVER a single pull down resistor. IMHO you should eliminate them by appropriate choices of logic level meanings.

In all the TTL circuits that I have ever designed there was NEVER a single pull down resistor. IMHO you should eliminate them by appropriate choices of logic level meanings.

So in this case, I should use a pullup with 10KΩ resistors and change the circuits logic?

 

7400 TTL inputs will source 1.6 mA.
If you want to pull the inputs below 0.4 V, you will need a pulldown resistor of R,

R = 0.4 V / 1.6 mA = 250 Ω or lower.

Try 220 Ω.

 

So in this case, I should use a pullup with 10KΩ resistors and change the circuits logic?

Typical input current is 40 μA.
Typical value for 7400 series pull up resistor is 4.7 kΩ.
This will give a voltage drop of 40 μA x 4.7 kΩ which is about 0.2 V.
This gives a logic HI of 4.8 V.

 

Guys, he's said

However, when mouting the circuit in a protoboard, only Va and Vb show the desired 5V voltage. Vc has a voltage of 2.2V, that is affecting the IC's output voltage for S2 and S1.
I've already checked all connections and they are all correct.

He's talking about (or at least seems like he's talking about) the voltage on Va and Vb when it is supposed to be HI (i.e., when the switches are closed), not when the switches are open and the output should be being pulled LO.

If his 5 V supply can't pull Va and Vb up to close to 5 V, then the problem is not that the resistor values are too big (which they still are for the purposes of being acceptable TTL pulldowns), but rather that his supply can't deliver the needed current to feed them (and so making them smaller will only make the problem worse). It might be a wiring issue, or it might truly be a supply issue (what supply is he using and how much current is it supposed to be able to provide?).

@raphaelprados : You have three unused inverters. Bring your signals into them with pullups and let them invert your signals to give the Va, Vb, and Vc signals in your schematic. Nothing else need change (except now your switches will be connected to 0 V instead of 5 V).

 

I'm developing the following circuit as a prototype for an automation project. I'm using 3 inputs from switches, which all have their own pull down resistor, just as the circuit bellow.
View attachment 330980
However, when mouting the circuit in a protoboard, only Va and Vb show the desired 5V voltage. Vc has a voltage of 2.2V, that is affecting the IC's output voltage for S2 and S1.
I've already checked all connections and they are all correct.

Amongst other responses here, be aware that Vc has a resistor between it and ground, which affects level of ground.

 

No one yet asked if the measured voltages were made with the switches open or closed. The answer could make a very big difference!