The CD4007 soft switch in DOD pedals: power up on or off?

[Fancy Lime]

Hi there,

I have that switch on the breadboard at the moment. Works fine. I was wondering if that one is supposed to come up consistently and predictably, either ON or OFF upon power up. It seems to always come up ON for me with the power supply I am using but I have noticed in the past that CD4069 based switches (see geofex) came up consistently and reproducibly ON with one power supply but just as consistently OFF with another supply. My suspicion is that there is no predictability there and the different behavior with different supplies is due to slight differences in ramp up behavior or something of the kind. At least I cannot see from the schematic why it should come up one way or the other. Is there a way to make that predictable, like there is with the Boss switch (different size caps, see geofex again)?

Thanks and cheers,
Andy

[Vivek]

Can this be the basis of a Random Number Generator ?

[Vivek]

For a switching system I was designing, I used resistor and cap to force default power on condition (reset)with time constant of 100ms, to give enough time for things to settle down.

[anotherjim]

It's hard to read a DOD schematic 'cause I think they show the 4007 as a numbered DIL box. However, in general, if the latched state depends on the voltage level stored on a capacitor, then it may start with 0v. But if the power goes off and comes back before a stored high level can discharge it may act on the high.
If the cap is ground referenced, then a reverse diode to +supply will discharge the cap whenever the supply goes off.
OTOH, if the power goes off momentarily by accident, there might be a chance the pedal will carry on in the required state when the power comes back if you leave the cap alone and let it remember... except...
...CMOS inverters and the 4007 have in and out protection diodes built-in, so if the cap is always connected to any of them it always has the required discharge diode!
In a GEO scheme, the cap is connected via a 1M resistor so it can only discharge as fast as the resistor and its capacitance allow, so it could still power up charged if there isn't enough time to discharge. It might benefit from an added diode?

[Fancy Lime]

It's hard to read a DOD schematic 'cause I think they show the 4007 as a numbered DIL box. However, in general, if the latched state depends on the voltage level stored on a capacitor, then it may start with 0v. But if the power goes off and comes back before a stored high level can discharge it may act on the high.
If the cap is ground referenced, then a reverse diode to +supply will discharge the cap whenever the supply goes off.
OTOH, if the power goes off momentarily by accident, there might be a chance the pedal will carry on in the required state when the power comes back if you leave the cap alone and let it remember... except...
...CMOS inverters and the 4007 have in and out protection diodes built-in, so if the cap is always connected to any of them it always has the required discharge diode!
In a GEO scheme, the cap is connected via a 1M resistor so it can only discharge as fast as the resistor and its capacitance allow, so it could still power up charged if there isn't enough time to discharge. It might benefit from an added diode?

Yes, most of the DOD schematics are a bit of a mess that way. I worked off this schematic instead:
https://hammer.ampage.org/files/dodswitch.gif

The diode idea is interesting, I'll have to try that. However, the cap will only hold it's charge for so long before parasitic currents eat it all up. But it just gave me the idea that if we circumvent the power smoothing cap that sits before the whole switching circuit, we may be able to fill the holding cap of the switch before the 4007 gets powered up and solve the problem that way. Needs testing as well....

Thanks,
Andy

[Rob Strand]

I have that switch on the breadboard at the moment. Works fine. I was wondering if that one is supposed to come up consistently and predictably, either ON or OFF upon power up. It seems to always come up ON for me with the power supply I am using but I have noticed in the past that CD4069 based switches (see geofex) came up consistently and reproducibly ON with one power supply but just as consistently OFF with another supply. My suspicion is that there is no predictability there and the different behavior with different supplies is due to slight differences in ramp up behavior or something of the kind. At least I cannot see from the schematic why it should come up one way or the other. Is there a way to make that predictable, like there is with the Boss switch (different size caps, see geofex again)?

Most circuits power-up randomly and individual cases often power-up in a particular state because of tolerances.  IIRC, some of the early Boss pedals had asymmetry somewhere which urged them to power-up in a particular state.

There's a few sources of asymmetry in that circuit.  The loading of the gate by the LED transistor base resistors, the asymmetry between the P and N devices, tiny difference in each gate due to the fact the 4007 gate isn't 100% symmetrical, the cap powers-up at 0V.    If anything you would expect the base resistor to hold the circuit in the off-state.    If you raised the power rails slowly you might see outputs stay at zero or rise to a small voltage.   Then at some point the gate starts to act as an inverter and those start-up voltages become solid "logic" voltages.   The thing is, ignoring the base resistors, there's nothing which makes one of the two gates want to drive up or down more than the other, even with the P and N asymmetry.    The only thing left is the cap.  If the single P switch stays open when the power rails rise then the cap has no effect.  With the 1M gate pull-down resistor it's hard to see how the P switch can conduct at all.  Supposing it did then the discharged cap would cause the LED to be off.   So, using your example circuit the few weak effects would do the opposite of what you are seeing! (at least superficially)

That said, did you actually build the circuit as drawn in your example?   if you look at the DOD Chorus FX60 it has the LED connected to the "other" gate.   That would push the cap asymmetry the other way (not the base resistors).

On old CMOS circuits if you wanted things to power-up in a certain state you had to  add a reset circuit.  The reset circuit forced the circuit to start-up in a particular state.    The reset circuit usually had a R and C to do the start-up pulse and also had the diode across the R to make sure you always got a power-up reset.  An example is on this circuit, between D7 and D8,

https://delptronics.com/images/misc/Boss%20DR-110%20Cymbal%20&%20Noise%20Circuits.png

[anotherjim]

The cap C3 is expected to be discharged on power-up. It will discharge via R13 and the 4007 protection diodes. A residual charge of about 0.6v will remain for longer than you might expect but it won't change the timings by much.
A fast discharge path for an extra diode from C3 needs to find 0v via the +9v rail & I think the 4007 power pin14 is as good as any.