RG's effects switcher - CD4049 needs larger cap?

[composition4]

Hi all,

I'm building a latching relay based effects switcher (exclusive one-of-n effect loops), and have based the LED indicator for which bank/preset is selected on RG's "Programmable FX Switcher" at http://geofex.com/Article_Folders/fxswitchr/fxswitchr.htm - I'm using +9v to run it all.

I have built it pretty much as the picture http://geofex.com/Article_Folders/fxswitchr/fxselex7.gif shows, based on the 74c373 octal latch.  It works fine driving the latch just switching the 74c373's inputs directly to ground/+9v.

When I connect the CD4049 to it, if I  momentarily short one channel's 0.01uF cap to ground it flashes the LED on the 74c373 but it doesn't stay on. Measuring the voltage on the CD4049's output when I do this, it goes from 0 to +9, and it stays at +9.  I think that the output needs to go to +9v then return to 0 - because -LE on the 74c373 needs to drop low again for the Qs to latch to whatever the Ds were.  So that's probably a poor explanation but I am only just grasping the concept of CMOS "logic" I just ruled out that last bit, the voltage doesn't stay at +9 regardless of what cap I use... don't know why i thought it did.  Anyway...

In any case, I experimented and if I use anything above a 4.7uF cap in place of the 0.01uF caps it works fine - when the cap is momentarily shorted to ground, the corresponding CD4049's output goes from 0v to +9v then quickly back to 0v.  And the 74c373 latches on nicely to the correct LED, and disables the others.

So a couple of questions: First, if I go ahead and build my switcher with 4.7uF or 10uF caps instead of 0.01uF caps, is this likely to give me any problems?

And second: is RG's schematic wrong, or am I missing something? I'm guessing the latter... but would like to hear your thoughts on this.

Thanks
Jonathan

[oskar]

Hi!

In any case I think you need a pulldown resistor on -LE. Something like 10k to ground from pin 11.

oskar

[composition4]

Thanks so much oskar: 10k didn't do the trick, but 100k+ did, now it works with the original 0.01uF caps!  Was this pulldown needed so that -LE drops back low after going high?

Also another point to note, no combination seems to work with a buffered 4049 (MC14049BCP), but with a regular unbuffered CD4049UBE it works... not sure why this would be either...

Again thanks for your help.  If you or anyone could explain why 1. The smaller caps didn't work but the larger ones did when I didn't have a pulldown on -LE, and 2. Why all caps work with a pulldown on -LE, and 3. Why my buffered 4049s don't work but the unbuffered ones do

then I'd love to hear from you. I appreciate your solution though, worked almost right away!  Gotta go to bed now, will check back after work tomorrow
THanks
Jonathan

[oskar]

Thanks so much oskar: 10k didn't do the trick, but 100k+ did, now it works with the original 0.01uF caps!  Was this pulldown needed so that -LE drops back low after going high?

Yes.

Check out the datasheet for the 74c373
http://www.fairchildsemi.com/ds/MM/MM74C373.pdf

The truthtable on page 2 tells us that the circuit remembers what is on the inputs when -LE goes low. Therefore -LE must go low first!.
There is another circuit that is similar, the 74c374 which pushes the values on the inputs to the outputs/ memory, when the equivalent of -LE, the clock input goes high. The 74c374 only triggers on the positive going flank ( transition from low to high ).
To me it looks as if this circuit would work better.

The reason I think the buffered 4049 didn't work is because they work to fast. When they go low there is still a voltage lingering on -LE while the other inputs take a dive so when -LE finaly goes low the latch will hold logical zeros. 0V.

[R.G.]

I could not have said it better myself Oskar. Good insight!

This makes me wonder if I had some unknown leakages on my prototype that made it work without a pulldown on LE.

[composition4]

Thanks oskar, good explanation.

I only had one unbuffered CD4049, so I ordered some more - I'm using all 8 inputs of the octal latch hence I need 8 inverters.  So I thought I'd try it with an unbuffered CD4069 (I have heaps of those).

I increased the caps on the inverter to 0.1uF and have a 220k pulldown resistor on -LE of the 74c373 and it works like a charm, so I think I'll stick with  the 4069s unless anyone tells me this is a bad idea?

I'm planning on running my effects switcher off a 12V sealed lead acid battery ( http://www.jaycar.com.au/productView.asp?ID=SB2482&CATID=18&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=250 ) running through a 5v LM7805 regulator for the relays but keeping 12v driving the 74c373 +  CD4069s.

The relays I'm using are panasonic NAIS DS4E-SL2-DC5V.  They are 4PDT two coil latching relays - looks like they're normally US$11.49 at Mouser, but I got 60 (sixty) of them brand new for a total of $25USD.  That's 42 cents each!  I was pretty pleased with that deal, and they work great.

Anyway sorry this is a bit off topic, I'm just a little excited at planning and putting together my effects loop switcher, and think the design I've come up with works well, in conjunction with using RG's design to control the LED section

I'll post a video once I complete it, could take a month or two at this time of year!

Jonathan

[R.G.]

The relays I'm using are panasonic NAIS DS4E-SL2-DC5V.  They are 4PDT two coil latching relays - looks like they're normally US$11.49 at Mouser, but I got 60 (sixty) of them brand new for a total of $25USD.  That's 42 cents each!  I was pretty pleased with that deal, and they work great.

Two coil latching? You have taken into account that these require special drives, right? Pulsing one coil to turn on, pulsing the other coil to turn off.

I suspect you probably have, just making sure you noticed this point.

[composition4]

Yup sure have RG. I chose latching because I want to run it off the sealed lead acid battery I showed before. Actually I'll probably run it from a 1300mah SLA rather than the 2200mah SLA, because it's half the size and I'm assuming would still last a long long time powering just a handful of LEDs and switching a few latching relays.

My plan is as follows:

One relay per effects loop - energising coil one switches the loop in, coil two to bypass it.
One end of each coil goes to +5vdc.  The other ends of the two coils go to either side of a spdt switch - position one is loop on, position 2 is loop off.  The common pole of this switch connects to the channel's momentary footswitch, which when pressed provides a path for the whole thing to ground.  Which energises the correct coil depending on the position of the spdt "loop on or off" switch.

I've got 8 channels, each with 6 of these spdt switches going to the 6 relays of the different effects loops. There's a diode on each pole of the SPDT switches (yep that's 96 diodes, i know...) to ensure they don't interact with the settings on other channels.  Oh, and there's a LED which lights up on each effects loop so it shows what loops are switched in when you change channels.

The octal latch part of it just controls the LEDs to light up to show which channel is active.  The SPST momentary footswitch for each channel has one end connected to ground, the other end connected to both short the CD4069's cap to ground, and also to ground the previously mentioned effect loop relays.

Hope that makes sense!

If anyone sees an issue with that setup (assuming anyone can make sense of it without a diagram!) please feel free to let me know!!

Thanks,
Jonathan

[R.G.]

Oh, cool. I feel a little silly at being functionally fixated.

Your RELAYs do the latching, and the CDxxxx is only for display. Neat, straightforward and simple. 

[composition4]

I'm sure you'll be hearing from me again asking for help a number of times before it's completed

Thanks
Jonathan