I am a console game developer by profession, and has no formal education on electronics / electrical engineering. In gaming lingo, a 'noob' if you will, at it. And yet I have a new found passion for fiddling with this wonderful craft.
At the moment, I am trying to learn and have fun while make a 'scrolling LED' toy for my daughter. My idea for it is quite simple, I have a shift register (SN74HC595), and 8 LEDs taking input from it. There will be two buttons. When button A is pressed, it 'inserts' a 1 into the register, and when B is pressed, it pushes a 0 into the register. With this you can 'program your LED pattern'. And then with the flick of another toggle, the shift register will get clocked (using a simple RC astable multi vibrator) (looping QH' to QA). Making the pattern to scroll. Simple enough.
But, my problem comes at the part which does the 'inserting' of data into the register. Let's say, to insert a 1 to the register when button A is pressed and released, I first have to pull the 'SER' (data) line of the register high, then after a brief period of time (setup time, 25ns for the register's datasheet) pull the SRCLK (Shift register clock) high. And that's it. However..
My switch buttons are debounced. I learned about debouncing from this wonderful article : Debouncing. I didn't had any parts that would help me with hysterisis so I had to just ignore it. But using the methods specified there, I was able to calculate the exact values of my resistors and cap, to keep the voltage just right while my switch clank and knock with itself. The switch bounces for 10ms ( got myself a cheap O-scope, china one. But does the job ), so am keeping it steady for about 20ms. My calculations and the results of simulating it using Multisim.com matched. So my button debouncing works as expected.
Except, when I try to use that 'signal'. I have to do two things with this signal. First, route it to SER line of the register. And then after a brief delay (25ns - a few ms?) route it to SRCLK line to clock it. In other words, first pull SER high, then pull SRCLK high. And to introduce this 'delay' I turned to the only delay mechanism I know - RC network. So once I get the debounced output, I charge a cap through a resistor, and that charged output is what I use to drive SRCLK.
The moment this new capacitor and resistor gets added to my 'debounce circuit',my calculations for the parts value doesn't hold true anymore because the new 'delay' capacitor and resistor is now in parallel or serial (I don't even know anymore) with my good and previously well behaving 'debounce' capacitor.
All this predicament makes me wonder if even using an RC delay timer is even the right thing to do. How do I connect input from a debounced switch to a node that already has a capacitor, without messing up all my calculations.,or to rephrase it, How would you all implement a simple delay of a signal without using all fancy micro controller etc..?
Here is the schematic of the denounce and delay. Note that this is only addressing the single button (not both A and B). SW2 in the first image is just for debugging.