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Registered Member #1822
Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300
The 74191 is a TTL chip that is a 4 Bit binary counter that increases(or decreases) the count based on a clock signal. I think that it is picking up RF and counting something. When I put a 3" wire on the clock input and touch it then LED's on the output all flash. When I stop touching the wire then the LED's display a random binary digit. I thought that the solution was very obvious, put a cap across the pin and ground but, its not helping. Any ideas? I dont think that anyone would need a schematic.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
I think you are picking up the 60Hz around you as you are acting as an antenna. I would just say to not touch the input unless you wanted to either make a random number or use the 60Hz signal for something. If you wanted to use the 60Hz signal, it would probably be best no to capacitively couple the signal and just use a transformer and a zener combination (maybe a Schmitt trigger NOT gate to prevent other noise from affecting the output count).
Registered Member #1822
Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300
I think you are correct. Partially. It is more likely 30Hz since I can see the flicker. Thanks for your help! Did you change your signiture just for me?
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The TTL chips are not so sensitive as the CMOS versions but you should never have any floating inputs unless specified by the datasheets. So connect a resistor from the input to ground or to the supply voltage. This goes for all inputs, if you use them or not. You can drop the resistor and connect directly for inputs you are not going to use.
Even if you have a switch that always connect the input to 5V or GND it will break before make and for a short instant the input will be floating and you can get hundreds of noise pulses unless you have the resistor there to give a default input. When that is sorted you are subject to bouncing in the switch and you need to add a capacitor so you have a RC filter (or make a more complex circuit) on your input to filter out any high frequency noise.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
10k is the 'standard', but anything from 100r to 100k will usually to the trick
To echo was Bjorn said, you have to make sure to add a pull up or down, to all inputs used or not. For unused inputs you can skip the resistor and just connect them directly to Vcc or Vdd. The reason for this is that the oscillations you observed can in rare cases damage the chip by causing it to oscilate faster than it is designed for. This is more of an issue with analog chips, but its good practice for any component you use.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
General: As many others have said, assume that floating inputs have undefined (and not necessarily constant) logic levels. Unless you know otherwise from the datasheet, or familiarity with the logic family.
Specific: If your logic is "plain old TTL", like the 74191, it has very unsymmetric input current requirements. To pull an input HIGH, a resistor of 75K to +5V is strong enough. But to reliably pull it LOW with resistor to GND, use a value less than 500 ohms. (must sink 1.6 mA and get below 800 mV.)
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74191 Picking up RF
