LEDs on outputs of 4051 not blinking

[stolenfat]

Hi all. Mind helping me trouble shoot my issue? If it turns into a topic that's relevant to the sub then I wont delete it, but if my issue gets solved pretty quickly i may just purge the topic.

Anyways, so I built a little 4051 pattern generator based on this schematic:


http://milkcrate.com.au/_other/sea-moss/08_custom.gif

I nixed the 4040 and the 40106 cause I already have those patchable on my lunetta.


First i I just added the led's inline with the pot's center lug and when i turned it on, I instantly saw the error of my ways. With the pots wired to V and ground and the lights only got bright at the higher turns of the pots which makes sense as it just controls the voltage that runs to the led.

So then i had to postulate how to get the lights to blink at each step so I know which gate im tweaking and for a little visual feedback. I tried adding a NPN (3904) and connected the collector to the LED and V the emitter towards ground and 4051 multiplexer Q outs to the base (center pin). It seemed to work, mostly, but I soon discovered I was low on transistors so I couldn't do one for each step with out ordering more parts.

So i figured why not just link another 4051 up. So i tied another chips A, B, and C logic inputs together to the first one with some wire. Then I just straight up tied the Q outs to the leds and placed them on the my surface in the corresponding location to match the pots of the other 4051. to my surprise it didnt work. So i took it to the bread board did the same thing and tried to trouble shoot. I couldnt get the LEDs to fire off the Q outs even when not tied to the 4051. I swear in the past I was able to get this to work simply.

Is there something missing? I checked my anode /cathode orientation, tested the LEDs are good, multimetered the chip to see the voltages pulses on the Q outs but it wont drive the LEDs. Whats up?

Also my enable was grounded and that weird Vee pin was grounded too. Oh and my first 4051 works absolutely love, exactly what I wanted (just need these lights to blink!)

Any help would be appreciated!

[PHOBoS]

[stolenfat]

I uh,,, left it hanging because i was only interested in getting an led blink for each Q step. If all i needed to do was ground pin 3 i am going to slap my self.

[PHOBoS]

The 4051 is a multiplexer (mux) which is basically a switch. Depending on the data on the ABC inputs pin 3 (X) will
be connected to one of the in-/outputs (X0..X7). So if you don't connect it to anything the LEDs will never light up.
There are several ways how you could connect it up, first of all the orientation of the LEDs:
X connected to V+, the anodes of the LEDs connected to X0..X7, cathodes connected to GND.
X connected to GND, the cathodes of the LEDs connected to X0..X7, anodes connected to V+

You can't just connect the LEDs directly though or the current will be too high so you need a resistor in series.
Since there is only 1 LED on at a time you only need 1 resistor, although you could add resistors to all LEDs, which
you might have done already. What I usually do is connect a resistor between pin X and V+, connect the anodes
of the LEDs to X0..X7 and connect all the cathodes directly to GND. But you could also connect X directly to V+
connect all the cathodes of the LEDs together and put a resistor between them and GND.

[PHOBoS]

This might make it easier to understand.

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[trav]

Or you could use a 4028. But multiplexors are so much more flexible :)

[PHOBoS]

funny you mention the 4028. I've often wondered why there wasn't a simple 3-bit binary to decimal decoder chip as
there are 4-bit chips available (4514, 4515). So I looked it up again yesterday to see if there might be some obsolete
chip I didn't know about and that's when I noticed the 4028 (which isn't obsolete) . I actually have it in my collection
of CMOS chips but don't remember ever using it. So maybe it will turn up in one of my future designs. 4051 has always
worked for me though and it is indeed more versatile.

[stolenfat]

Phobos thank you so much for the detailed information. I had to take some time off of building due to work and other obligations (not to mention a subtle bought of uncreative laziness) and I was able to use your responses to figure out my issue just now. Big thank you.

Turns out, due to the way I had my leds arranged, I needed to run pin 3 through a small resistor to v+. Connecting directly to v+ was causing me some issues but adding a 1k resistor (+1 ks on each led) seemed to balance things out in a working manner as I had hoped.

Tip of the hat sir.

[PHOBoS]

that's great to hear, I'm glad I could be of help.