## Logic Gate Problem!!!!!!!!!!!!!

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stewie36
Posts: 2
Joined: Fri Nov 18, 2005 1:01 am
Location: College Station
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### Logic Gate Problem!!!!!!!!!!!!!

Hey, I am somewhat just beginning in electronics and I was assigned a project that uses capacitors and logic gates. When I built 4 RC timing circuits (In which a LOW signal and a HIGH signal connect at an OR gate, but the HIGH signal first goes through a resistor and a capacitor (not in series, of course) before it connects to the OR gate, the result was that a few seconds would pass (as expected) but then all of them would turn on a once (I used different resistors in each timing circuit).

````````````````````````````````````(74LS32)
(5V)-0-*---------------0--------------[OR] --->
```````|```````````````````````````````|
``[Inverter](74LS04)```````````````````|
```````|```````````````````````````````|
```````1```````````````````````````````|
```````|```````````````````````````````|
```````-- [ Resistor (1 – 1000 K) -----*
```````````````````````````````````````|
``````````````````````````````` [Capacitor](10 uF]
```````````````````````````````````````|
`````````````````````````````````````{GND}

After trying to solve the issue, I found out that the OR logic gate was producing 1.5 V (with current) at the inputs of the logic gate. In other words, the logic gate is starting at HIGH for the inputs instead of LOW. I think this would also explain why all of the timing circuits hit HIGH at the same time (the OR gate was charging the capacitor on all of the circuits, instead of the HIGH signal that runs through the resistor). Now the logic itself still works on the chip (If I ground both HIGH signals coming out of the OR inputs, then I will get zero as an output, as expected (The OR gate output is HIGH initially if nothing else is connected to it (with the exception of Vcc and GND, of course). I know I am sort of new to electronics, but isn’t the logic gate designed to start at LOW (or at least lower than 1.5 V with a 5 V supply)? I think I am doing something wrong, since it happens to all of the other chips I replace it with (not just OR gates either). Thanks in advance!

Gorgon
Posts: 325
Joined: Wed May 04, 2005 1:01 am
Location: Norway
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### Re: Logic Gate Problem!!!!!!!!!!!!!

Hi,
Since you are using 74LS chips I suppose you kow that this is a proper TTL circuits, low power but still TTL. The TTL logic need active drivers for both low inputs and high inputs. (less for high)

To pull a -ls input low you need 0.36 mA. So, if your resistor is >13.5k you have trouble keeping the input low('0'). If you leave a -ls input open it will adjust itself to about 1.5V and act as a high, '1' input. So, to do as you want, you need to change to a 74HCxx chip, this will act as a high impedance input and you can use your resistor capacitor connection.

TOK
Gorgon the Caretaker - Character in a childrens TV-show from 1968.

stewie36
Posts: 2
Joined: Fri Nov 18, 2005 1:01 am
Location: College Station
Contact:

### Re: Logic Gate Problem!!!!!!!!!!!!!

Thank You! I'd probably gone for days trying to figure this out. Thanks again.

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