# Repeating circuit: 1, 2, 3, 1, 2, 3...

Discussion in 'Electronic Basics' started by Don Gilmore, Dec 8, 2012.

1. ### Don GilmoreGuest

Guys:

I'm an ME working on a small project where I'd like to do something ridiculously simple, yet I keep coming up with complex solutions. There is probably a simple way to do this that I am missing.

What I'd like to do is cycle between three outputs--let's say three discrete LED's--in a continuous circle using a square wave clock input. If the LED's are numbered 1, 2 and 3, I want them to cycle 123123123123... The inputwould be the square wave from a function generator and I'd like to controlthe speed of the LED's by the frequency.

I keep coming up with complicated ideas like using a counter that outputs to a decoder. Surely there is an easier way?

Don
Kansas City

2. ### Don GilmoreGuest

A 4017 decoded decade counter, connected to reset at three. If the

I thought about a 4017, but didn't know that I could reset it at three. How is this done? Would I connect output 3 to the reset line? It is important that each output gets equal time...no pauses or skipped pulses.

Don

3. ### petrus bitbyterGuest

Connect Q4 to MR and the counter resets immediately after the 3rd pulse. So
you will have a short pause in the ns area between pulse 3 and puls 1. You
may need to lengthen it somewhat if the Q4 does not fully reset the counter.
A diode and an RC will do that trick.

An alternative is a synchronous serial in parallel out shift register. "NOR"
the first and the second output and feed back to the serial input.

petrus bitbyter

4. ### whit3rdGuest

I'm not sure why a function generator is useful, but if you have one,
set it to a ramp waveform (sawtooth) and use a bargraph display
chip (LM3914 or somesuch). One chip and the LEDs, and you're done.

Decoded counters (CD4017 in CMOS, SN74142 in TTL) are also available in a single
chip, you just need to worry about reset setup and hold times when you truncate
their sequence.

5. ### ehsjrGuest

And it appears you're under thinking it. The op wants to control
the leds with an external generator:
"The input would be the square wave from a function generator and
I'd like to control the speed of the LED's by the frequency."

I don't see where you or David has addressed that with the picaxe
idea in any of the posts in this thread.

Ed