Hi all,
I want to have the 8 outputs of a shift register "choosing" different resistors for the frequency of a 555 timer over time. The register would be wired as a LFSR.

Thanks,
Max

 

Did you mean Last In, First Out?

That'll be kind of hard to do, as even though shift registers are available with 3-state outputs, those are an "all or nothing" proposition.

Better to use a mux/demux IC connected to a counter.

 

On a similar note, here's a 4017 5-stage Johnson counter and a 4093 NAND gate for a clock. The 4017 "chooses" a new delay value for the 4093 clock, every time the 4093 outputs a new clock. The diodes enable just 1 resistor at a time to charge the timing cap.

[eta]
Oops, used a 40106 hex Schmitt-trigger inverter instead of a 4093 quad Schmitt-trigger NAND gate.
You can also use the 4093 NAND as four individual timers; with just a single resistor and cap needed per gate.

 

LFSR means Linear Feedback Shift Register. They are used for generating pseudorandom sequences. Maximal length for an 8 bit LFSR is 255 states before repeating.
In order to get 255 different frequencies from the 555, an 8 bit digital to analog current converter would be required.

 

Here is a place to start. There are other ways to do the D/A conversion, some of which might be simpler. This is just what I first thought of.

 

Thanks guys!
i guys i was thinking u could somehow have a set resistor for each output ( 7 if using 1 shift register wired as LFSR i think). And the shift register could pick a resistor at random/////
I mean, the real question is how do you switch frequency to the 555 without turning a knob (automate the frequency change...)
Tnx for any insights, and I'll check out the DAC schema.
-M

 

With 8 bits, you can have 256 (255 with an LFSR) different combinations of outputs, which means you can have up to 256 (255) different frequencies. to do this with 8 resistors, they have to be weighted in binary fashion, i.e, R, 2R, 4R, 8R,..., 128R. This is one type of D/A converter. You could also use an R-2R network (Google), which only requires 2 resistor values, but roughly twice as many.

 

I think I forgot to thank u for the schematic.
Thanks buddy!
-M
Just to make sure, I don't need bi=polar power for this? Most of the DACs i've been looking at online require bipolar power supply. me no like....

 

I think I forgot to thank u for the schematic.
Thanks buddy!
-M
Just to make sure, I don't need bi=polar power for this? Most of the DACs i've been looking at online require bipolar power supply. me no like....

The circuit I posted only requires one supply. Keep in mind that I haven't tested or simulated it.

 

I actually found a way to do this that works well. The LFSR data goes to output router (0,0,0...1,1,1,etc) of demultiplexer (chip CD4051). The common input is pin 7 of 555 and then I have 8 or more outputs with different resistors going to pin 8. Works!
Best,
M

 

If you find the time, I would like to see the schematic for this. Its got me a-thinkin'

 

sure, i can draw one up but I suck at it at in software/

 

However you do it is fine with me. (Just dont use pink paper with a brown marker)

 

um, yeah.
I didn't put any components (resistors, caps) cause it would be too messy, and its up to you what values anyway. shoot me a mess if u have any questions.
Best,
M
**** eagle haha

 

I get the picture. thanks. I was basically interested in the wiring diagram, which you gave. Thanks.

 

Pin 8 on a 555 is Vcc. How does that work?

 

input of 4051. pin 7 of 555
outputs of 4051. various resistors to pin 8 of 555
LFSR/4051 circuit changes resistance between pin 7-8 of 555.

 

input of 4051. pin 7 of 555
outputs of 4051. various resistors to pin 8 of 555
LFSR/4051 circuit changes resistance between pin 7-8 of 555.

OK, I get it.