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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

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

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

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?

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.

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

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

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

Did you get the email Fred?

bouncy switch betch'a
tj