momentary signal to a shift register - how?

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
FredS
Posts: 4
Joined: Wed Feb 20, 2002 1:01 am
Contact:

momentary signal to a shift register - how?

Post by FredS » Wed Feb 20, 2002 10:26 am

I'm building an LED back/forth sequencer based on 74LS194 bi-directional shift. I have a problem because the sequential outputs (A,B,C,D) stay high until reset, but I need them to go A high/low, B high/low, C high/low, D high/low, then reset. I've found that I can do this if I send a high signal from my switch to the chip for the first clock cycle, then take the signal low.<p>What is the best way to take a high signal from my switch, to the chip, then take the signal low without turning the switch off? I am using a slide switch & I don't want to go to a pushbutton.<p>TIA

Ron H
Posts: 360
Joined: Tue Dec 18, 2001 1:01 am
Location: Boise, ID
Contact:

Re: momentary signal to a shift register - how?

Post by Ron H » Wed Feb 20, 2002 11:23 am

You need a monostable multivibrator, also known as a one-shot. You can make one from a 555 timer IC, or there are various TTL one-shots available, such as 74121, 74123, 74221, etc. Look at http://www.fairchildsemi.com/an/AN/AN-366.pdf and at http://www.uoguelph.ca/~antoon/gadgets/555.htm for more info.<p>Ron H

russlk
Posts: 563
Joined: Thu Dec 06, 2001 1:01 am
Location: New Hampshire
Contact:

Re: momentary signal to a shift register - how?

Post by russlk » Wed Feb 20, 2002 2:45 pm

An R-C could work but the pulse thru the R-C must occur a few microseconds (nano seconds?)before the clock and be high during the clock rising edge

FredS
Posts: 4
Joined: Wed Feb 20, 2002 1:01 am
Contact:

Re: momentary signal to a shift register - how?

Post by FredS » Wed Feb 20, 2002 10:38 pm

Thanks guys, but...<p>After my original post, I found the 555 info on Tony's site as suggested by Ron. I've tried making a one-shot by using a 555 in monostable mode as shown there, but I can't get it to work consistently. Once it starts, I'm feeding a signal back to the 555 to reset it instead of using RC for a fixed time - that works fine. But it needs a pulse to trigger the start. That's what I'm looking for in the first place! I want to start it with either a steady 1 or 0, not a pulse.<p>I'm going to try to find a dedicated one-shot (74LS123) and see how that works. <p>Does anyone know where I can find an online schematic showing a circuit similar to what I need?

russlk
Posts: 563
Joined: Thu Dec 06, 2001 1:01 am
Location: New Hampshire
Contact:

Re: momentary signal to a shift register - how?

Post by russlk » Thu Feb 21, 2002 4:00 pm

You cannot do anything with a steady 1 or 0. It has to change state or there is no information. The rising edge of the switch signal can start the 555 timer. If that does not do what you want, there is no solution to your problem.

rckbrwn
Posts: 14
Joined: Mon Dec 17, 2001 1:01 am
Contact:

Re: momentary signal to a shift register - how?

Post by rckbrwn » Thu Feb 21, 2002 5:25 pm

ac couple the transition from the switch to the 555.<p>Rick

User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Re: momentary signal to a shift register - how?

Post by Chris Smith » Thu Feb 21, 2002 7:46 pm

I believe your trouble lies in the area of whats called a "bounceless" switch.

FredS
Posts: 4
Joined: Wed Feb 20, 2002 1:01 am
Contact:

Re: momentary signal to a shift register - how?

Post by FredS » Fri Feb 22, 2002 10:36 am

I understand that I need some change to trigger the 555, but it doesn't seem to work correctly with a change from low to high with the switch. It works sometimes, but not always. To work consistently, it seems to want 2 changes – low-to-high, then high-to-low (or high-to-low then low-to-high).<p>I’ll try finding something to eliminate the bounce that may be induced by the switch – maybe that’s it.<p>Thanks all

User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Re: momentary signal to a shift register - how?

Post by Chris Smith » Fri Feb 22, 2002 11:40 am

Did you get the email Fred?

toejam
Posts: 253
Joined: Wed Dec 05, 2001 1:01 am
Location: n.c.
Contact:

Re: momentary signal to a shift register - how?

Post by toejam » Sat Feb 23, 2002 5:57 am

bouncy switch betch'a
tj

Post Reply

Who is online

Users browsing this forum: No registered users and 48 guests