Hello.
I made a Diezel VH4 a few years ago.
Since I lack related knowledge and did not plan to use unnecessary functions, so I was looking for a simple switching circuit that could only perform the function of switching channels instead of a complex MIDI circuit.
With the help of Ampgarage members, I was can get a very simple circuit that using one 40175 IC, and I am using it well.
It is on the second page of the thread below.
https://ampgarage.com/forum/viewtopic.php?f=6&t=36333
However, the unsatisfactory thing about this switching circuit is that it does not reset.
When the amp is turned on, this switching circuit does not select any channel.
Sometimes, a random channel is selected, but more than 90% of the time, no channel is selected.
In fact, I do not consider it a special problem because I can operate the desired channel by touching any channel switch, but I feel like it is unfinished product. I'm thinking it would be nice to have a certain channel on when I power it up, even without doing anything. Is there any way to add this functionality to this circuit? If possible, it would be nice if channel 1 or 3 were selected.
How to add a reset function to a latching circuit?
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psychepool
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How to add a reset function to a latching circuit?
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Re: How to add a reset function to a latching circuit?
It might be possible with some judicious MML - Mickey Mouse Logic. The reset pin is available - pin 1 - and currently pulled high. A capacitor to ground from pin 1 might well cause it to reset to all 0s. I only say "might" because this depends on the input circuit of that particular pin, but chances are good with most CMOS.
It gets trickier to get a preset for one channel. Two possibilities are to change chips for one with a preset as well as a clear. Another is to add an inverter chip to do a two-stage reset then set the desired channel on. That gets to more complex MML.
It gets trickier to get a preset for one channel. Two possibilities are to change chips for one with a preset as well as a clear. Another is to add an inverter chip to do a two-stage reset then set the desired channel on. That gets to more complex MML.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain
Re: How to add a reset function to a latching circuit?
Is the intent to have 1 of 4 only (also called, 1-hot) or do you wish to allow multiple channels selected at once?
Re: How to add a reset function to a latching circuit?
Psychopool, I'm in a similar boat as you. I'm a tube amp tinkerer, not an EE, just guy with a lot of ambition. 
Anyway, I'm working on a 4 channel preamp and have the same issue as you've described. For my purposes, I want the POR to default to channel 1.
I've come up with a solution using an RC Differentiator. I've breadboarded it and it does indeed work, BUT, I'd like feedback from those who know what they're doing to critique it for errors and/or improvements.
I'm using a polarized cap (it's all I have on hand to get the pulse time constant to a short burst, about 20ms). In series is a 1k resistor, which forms a voltage divider with the 10k resistor (the pulldown resistors from D1-4 pins in your switching schematic). This outputs 4.55V, which is high enough to trigger a high state with the 40175. Without that resistor, when the amp is powered off and you pressed CH1 (or w/e channel you have set to default) you'd short the capacitor and discharge it. I figured that's a bad idea so I added the 1k.
Again, I'm just a novice with digital stuff, so maybe this solution is total junk. Anyone with insight please advise.
ETA: After a bit more testing, I upped the cap to 10uF. 1uF was a bit too short a pulse and was unreliable.
Anyway, I'm working on a 4 channel preamp and have the same issue as you've described. For my purposes, I want the POR to default to channel 1.
I've come up with a solution using an RC Differentiator. I've breadboarded it and it does indeed work, BUT, I'd like feedback from those who know what they're doing to critique it for errors and/or improvements.
I'm using a polarized cap (it's all I have on hand to get the pulse time constant to a short burst, about 20ms). In series is a 1k resistor, which forms a voltage divider with the 10k resistor (the pulldown resistors from D1-4 pins in your switching schematic). This outputs 4.55V, which is high enough to trigger a high state with the 40175. Without that resistor, when the amp is powered off and you pressed CH1 (or w/e channel you have set to default) you'd short the capacitor and discharge it. I figured that's a bad idea so I added the 1k.
Again, I'm just a novice with digital stuff, so maybe this solution is total junk. Anyone with insight please advise.
ETA: After a bit more testing, I upped the cap to 10uF. 1uF was a bit too short a pulse and was unreliable.
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Re: How to add a reset function to a latching circuit?
If you go to the post linked in the OP's original question, there is a diagram by Randall Aiken of a 3-channel switch, which incorporates a reset function that always selects channel 1 on power up, and it implements "1-hot" switching, meaning only a single channel can be active at a time. That's probably what is desired in this switching arrangement.
The cool thing is that the circuit is easily extended to 4 channels, using the existing components. The AND gate chip actually has 4 gates in it, one being grounded and unused, and the latch is also a 4-channel latch, with the fourth simply being grounded and unimplemented in Randall's drawing.
To get to 4 channels, just add a switch and the other few parts and implement the additional unused gates and latch in the two chips.
Not sure how to link the original schematic Aiken did but this might work.
https://ampgarage.com/forum/download/fi ... &mode=view
The cool thing is that the circuit is easily extended to 4 channels, using the existing components. The AND gate chip actually has 4 gates in it, one being grounded and unused, and the latch is also a 4-channel latch, with the fourth simply being grounded and unimplemented in Randall's drawing.
To get to 4 channels, just add a switch and the other few parts and implement the additional unused gates and latch in the two chips.
Not sure how to link the original schematic Aiken did but this might work.
https://ampgarage.com/forum/download/fi ... &mode=view