On Sat, 17 Sep 2005 13:56:04 -0400, "Bart"
wrote:

Hi all,
I want to make 4 LED's light up in the following sequence (looping):
O O _ _
_ O O _
_ _ O O
O _ _ O
O O _ _
_ O O _
_ _ O O
O _ _ O

the above shows it has gone through two sequences, I'll want it to be
continuous and with variable frequency (a potentiometer?)

I've read up on 555 oscillators, logic gates (nand, nor, etc.), flip-flops,
op-amps..........
I don't know where to start. I DO have a nine volt battery and some
LEDs.
Any guidance is greatly appreciated,
Bart

---

OK, here's something that'll work using a CMOS 555 and a couple of
dual "D" type flip-flops:


7555 Vcc
+-------+ |
+--|TH R|O-+
| |___ |
+-O|DIS OUT|--+
| +-------+ |
| | Vcc Vcc
+----[POT]---+ +-----+ | |
| 1M | +----|D | | [100K]
| | | | _| | |
[1µF] +------| S|O------+----+
| | | | _| | | |
| | | +--|Q R|O--+ | [0.1µF]
| | | | +-----+ | |
| | | | HC74/2 | |
+-[LED]--[R]-----+ | |
| | | | | |
| | | | +-----+ | |
| | | +--|D | | |
| | | | _| | |
| +------| S|O------+
| | | | _| | |
| | | +--|Q R|O--+ |
| | | | +-----+ | |
| | | | HC74/2 | |
+-[LED]--[R]-----+ | |
| | | | | |
| | | | +-----+ | |
| | | +--|D | | |
| | | | _| | |
| +------| S|O--+ |
| | | | _| | |
| | | +--|Q R|O------+
| | | | +-----+ | |
| | | | HC74/2 | |
+-[LED]--[R]-----+ | |
| | | | | |
| | | | +-----+ | |
GND | | +--|D | | |
| | | _| | |
+-|----| S|O--+ |
| | _| |
+----|Q R|O------+
+-----+
HC74/2


The values of your LED dropping resistors will be:

Vcc - Vled
R = ------------
Iled

For your 9V supply,


Vcc - Vled 9V - 2V
R = ------------ = --------- = 350 ohms
Iled 0.02A


360 ohms is a standard 5% value, so the power they each need to
dissipate will be


(Vcc - Vled)² (9V - 2V)²
P = --------------- = ------------ ~ 0.136 watts
R 360R


so a standard 5% 360 ohm 1/4 watt carbon film resistor will be fine.

--
John Fields
Professional Circuit Designer