# Logic gates, pull up/down resistor

Discussion in 'Electronics Homework Help' started by Luke Vassallo, Mar 9, 2015.

1. ### Luke Vassallo

33
4
Dec 10, 2014
I am encountering a logic problem with my or gates. when all inputs have 0v, the output has around 0.8-0.9v which is lighting one of the segments of my display.
I have tried a pul down resistor (from the output pin of the ic to ground) but this didn't solve the problem. I have tried a 4.7k(the value the lecture suggested) resistor and after that I tried a 220 ohms resistor (current will choose the path of lowest resistance). but still the voltage at the output remained the same.

any tips ?

2. ### Gryd3

4,098
875
Jun 25, 2014
Show us a schematic to help further.

Pull-Up / Pull-Down are only required on the input side of logic depending on A) The Logic IC being used, and B) The circuit that is connected to it. Current limiting resistors are usually (but not always) required if a Logic or other IC is directly driving an LED.

3. ### Luke Vassallo

33
4
Dec 10, 2014
That is what I was thinking since the input logic voltages need to be high or low (i.e not undefined). This is the schematic, so far I have only built the logic for segments A and B, and I am troubleshooting them before I move on to the other segments.

the input voltages are correct at around 4.97v high and 0v low, none undefined.

4. ### Luke Vassallo

33
4
Dec 10, 2014
schematic in case the one above does not display

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5. ### Gryd3

4,098
875
Jun 25, 2014
Strange, I'm not sure at the moment why that would be the case...
Personally, I would put a 220Ω in series with each segment, but I would also test the logic IC's independently to make sure the output of each behaves as expected.

5,164
1,081
Dec 18, 2013
What series of logic are you using?

7. ### Luke Vassallo

33
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Dec 10, 2014
I checked every single input to make sure that the logic is correct. with regards to ic I am using everything cmos and a not about the XOR gate the chip number is HEF4070BP by Philips. I will put the 220ohm resistors with the led segment because currently there are none and see what happens

5,164
1,081
Dec 18, 2013
What Seven Seg display are you using?

9. ### Luke Vassallo

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Dec 10, 2014
common cathode. I think I may have found the problem the XOR gate chip is giving undefined voltages at around 2.5v but I put a pull up resistor and it works correctly then I change the binary input number and gives undefined values again.

10. ### Gryd3

4,098
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Jun 25, 2014
Perhaps I am misreading, but it looks as though the XOR should have a defined output of at most 0.05V for a 'LOW'.
It looks as though the remaining inputs are tied to VDD, GND, or to another input as the Data Sheet suggest. It looks like that in the schematic, but I can't see the built circuit.

Seems like Adam maybe has a hunch?

5,164
1,081
Dec 18, 2013
Can you post the rest of the circuit that we can't see?
Thanks

12. ### Luke Vassallo

33
4
Dec 10, 2014
there are also some images of the build. I was wondering could I have damaged the chip since it is outputting a 0.8v at the ouput even with just vdd and gnd connected to it

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13. ### Gryd3

4,098
875
Jun 25, 2014
The inputs should never be left floating.
For the test, tie ALL of the inputs to to Ground or Vdd.
A floating input is undefined... ICs don't treat undefined as an unknown... they simply produce unknown or unpredictable outputs!

5,164
1,081
Dec 18, 2013
It looks like you have added pull down resistors which are not on your schematic. Is this correct?

5,164
1,081
Dec 18, 2013
You don't have any decoupling on the supply lines. Why have you got one IC miles away from the others. if you have any gates that are not used they inputs must be terminated.

16. ### Luke Vassallo

33
4
Dec 10, 2014
in the schematic, I didn't place any resistors, but when I started to build the circuit, I added 4.7k pull downs yes

5,164
1,081
Dec 18, 2013
Is the IC in the middle grounded?

18. ### Luke Vassallo

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Dec 10, 2014
what do you mean by terminated ? short them to ground ? .. \decoupling I have tried, what value would you suggest ?

19. ### Luke Vassallo

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Dec 10, 2014
yes yes it is.