74194 Shift Register

[rsfairman]

Who has experience with the '194 shift register? I'm using the 74HC194N (Mouser part # 771-74HC194N), and the datasheet
can be found at http://www.nxp.com/documents/data_sheet ... 94_CNV.pdf.

I'm trying to build a circuit with 4 LEDs and one button. Initially, LED #1 is to be on while the other 3 LEDs are off. Each time the button is pushed, the LED that's turned on changes. The pattern should change as follows.

1000
0100
0010
0001
1000
etc.

The circuit I am using is basically similar to the one at http://www.eleccircuit.com/wp-content/u ... -74194.GIF (which is intended to drive a stepper motor). Essentially the same circuit appears on p. 820 of "Practical Electronics for Inventors".

Just replace everything in these diagrams that is related to the 555 with a push button to supply the pulses -- i.e., the pulses into pin 11 come from a push button -- and replace each of the motor coils with an LED and resistor.

I'm getting flaky behavior. The LEDs blink somewhat randomly or they get locked up into a particular state. I suspect that I need a capacitor somewhere to reduce noise, and I've tried putting one (or more) in a variety of places, but no luck. I also tried some other circuits with this IC to shift bits in other ways, and I still get odd behavior. Sometimes it works for a pulse or two, but then it goes haywire.

I have very little experience, but this project ought to be as simple as they come. My goal in use this circuit in conjunction with a stepper motor, but if I can't get a few LEDs to blink, then the show's over.

Thanks,
Randall

[sghioto]

Need to "debounce" the push button switch.

Steve G,

[rsfairman]

Yes, de-bouncing is a good idea. I'll see what I can whip up for that with my Arduino. That's using a golden hammer for this problem, but it's handy and easy.

[dacflyer]

sounds like you could do the same thing with a 4017 chip.. count to N push button connected to the CLK ...Hmmmm???

[rsfairman]

It works! I made a de-bounced circuit with an Arduino board (OK, de-bounce *program* would be more accurate), and it works perfectly.

I knew about switch bounce in the abstract, but, never having done anything like this, I didn't know how much of a problem it could be. I just assumed that you'd get some occasional double-steps; instead the circuit was totally gonzo. Before being allowed to try certain circuits, we should be required to get a couple of de-bounce schematics tattooed somewhere on our bodies.

I might have spent a couple of very frustrated days scratching my head over this, but now I know. Thanks.

[dacflyer]

there is a 555 circuit that is a debounce switch. its inthe 555 cookbook from radio shack,, someone here might have it to post.. i have no way to scan anything here.

[jimmy101]

Or, debounce with just a series resistor followed by a cap to ground, it's a low pass filter. A 555 will take what two resistors and a cap and the 555 and ...

Figure something like a 10MFD cap and a 10K resistor, RC~0.1S (~10Hz).