The question...To buffer or not?

E

eTech

Well-Known Member

• May 9, 2012

• #1

I have a circuit that contains panel mounted pushbuttons, PB1-3.
See attached.

U101A,B is a 74HCT174 Dual D Flip Flop. NOR's are 74HCT02.

There is about 5-10 inches of wire from each leg of each pushbutton
to the circuits on each PCB (2 PCBs').

The question:
Should I use a hex buffer as shown (dashed symbol) to buffer the signal? 74HCT07?
Or maybe I actually need additional buffers to each PB?

Suggestions?

thanks

 

Last edited: May 9, 2012

alec_t

Well-Known Member

Most Helpful Member

• May 9, 2012

• #2

I wouldn't have thought a buffer was necessary. I would be more concerned with overcoming switch bounce.

 

E

eTech

Well-Known Member

• May 9, 2012

• #3

Hi Alec...

Do you think the debouncing shown in the circuit is adequate?
I was thinkin maybe adding buffers might help with debouncing,
but I wasn't sure if I needed more debouncing..

 

alec_t

Well-Known Member

Most Helpful Member

• May 9, 2012

• #4

Switch bounce may result in a train of pulses lasting several millisecs in total. I don't see anything in your circuit with a time constant long enough to cover that. I see 1k/.01n combos on the IC inputs, but the time constant there is only 10 microsecs. Also, slugging the clock input of the latches like that may be a problem as the latches need rapidly rising and falling clock edges.

 

crutschow

Well-Known Member

Most Helpful Member

• May 9, 2012

• #5

Using a Schmitt trigger buffer, such as the CD4584, with an input RC time-constant of at least 50ms, should provide a good debounce function while still having a fast rise-time output.

 

C

canadaelk

Active Member

• May 9, 2012

• #6

Those caps ar 10nF not .01nF
I would put a small value resitor in line with PB3 to protect the output of IC101a(?) E

 

alec_t

Well-Known Member

Most Helpful Member

• May 9, 2012

• #7

Those caps ar 10nF not .01nF

Click to expand...

My bad. Should have typed .01u.
You can connect the '174 as per the attached to provide debounce.

 

E

eTech

Well-Known Member

• May 9, 2012

• #8

Hi canad...
So the debounce looks ok as is right? Except for adding a
small resistor, say... 1K ohm?
Thanks

 

E

eTech

Well-Known Member

• May 9, 2012

• #9

Hi Alec
Im not clear...How does your circuit help debounce?

 

alec_t

Well-Known Member

Most Helpful Member

• May 10, 2012

• #10

How does your circuit help debounce?

Click to expand...

When the button is pressed the clock input of the latch goes high/low/high/low/.....umpteen times because of contact-bounce. The first high toggles the latch. If we assume D was, say, low at the time then Q goes low and not-Q goes high. Because of R2/C1, D takes about 50mS to change from low to high. During that 50mS any further clock pulses will all try to set Q low, i.e. they can't flip the Q state rapidly low/high/low/high.... Interestingly, Crutschow also refers to a 50mS time-constant for debouncing.

 

E

eTech

Well-Known Member

• May 11, 2012

• #11

Hi Alec

Cool!...I like that!!

Thanks!!