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help with SR NOR latch

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hakerbitz

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Just bought a box of quad 2-input nor gates. specifically ( M74HC02B1 )/ Tried to make a simple SR latch with enable input to set or reset the state of 2 led's on Q and Q'.
I used 2 chips to simulate AND gates out of NOR gates and the sr latch on the third chip. The logic seems to work when i enable input but looses its value when enable is grounded or floating. Currently i am using 3 3-way switches to input either +5V or GND to the S,R,Enable gates, and the led's are connected directly to the outputs then ground. I think i may need to incorporate resistors but am unsure where to put them or what resistance value should be. Any help is appreciated.
 

audioguru

Well-Known Member
Most Helpful Member
You should download the datasheets for the ICs to see what is the max supply voltage (6V) and max allowed output current (25mA). of course the LEDs need series resistors to limit the current to 25mA or less. Use Ohm's Law to calculate the value.

Post your schematic for us to see if the LEDs are holding the output voltage too low so that it is too low to feed another logic input.

The 74HCxxx is Cmos so inputs should never float.
 

kchriste

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Forum Supporter
Post a schematic. You probably need to debounce the switches, though the SR inputs wouldn't really need it because they latch the flip flop anyway. Debounce the enable line and use pulldown resistors on the switches. 1K to 10K should be fine.
 
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ericgibbs

Well-Known Member
Most Helpful Member
Just bought a box of quad 2-input nor gates. specifically ( M74HC02B1 )/ Tried to make a simple SR latch with enable input to set or reset the state of 2 led's on Q and Q'.
I used 2 chips to simulate AND gates out of NOR gates and the sr latch on the third chip. The logic seems to work when i enable input but looses its value when enable is grounded or floating. Currently i am using 3 3-way switches to input either +5V or GND to the S,R,Enable gates, and the led's are connected directly to the outputs then ground. I think i may need to incorporate resistors but am unsure where to put them or what resistance value should be. Any help is appreciated.
hi,
Dont leave inputs 'floating', connect via a resistor to +V or 0V, depending on what logic level you want the input to have when not connected to a driving source.
Typical resistor values for HC devices are 10K thru 47K.

Look at this image to see how 74HC02 are connected as a latch.
The pulse generator and U3 are for Simulation only, the U3 ic is configured as an invertor.

Do you follow OK.?
 

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