SPDT Capacitor Switch Activate LED?

spargo · July 13, 2010, 04:35:07 AM

I have an SPDT switch that switches between two different capacitors that both go to ground. I would like it so when the switch is flipped one of the directions an LED is turned on, and the LED is off in the other switch position. Is this possible? I have no DPDT switches at the moment.

deadastronaut · July 13, 2010, 04:44:40 AM

use a 3dpt...

search the schematics and layouts /switching threads..im sure you'll find something to help you there ok.

rob.

earthtonesaudio · July 13, 2010, 08:33:37 AM

I have two suggestions that each uses a SPDT.

Common to both are: SPDT pole is tied to ground. One throw goes to the LED's cathode, and the anode connects to the positive supply through a current limiting resistor.

-in version 1, you have one cap-to-ground always connected, and the other cap-to-ground goes to the other switch throw instead of ground. This means your two cap values are the one cap by itself, and the parallel combination of the two. When the switch is selecting both caps, the LED is off. If this is for a lowpass filter, the LED is off at the lower of the two filter frequencies.

-version two is slightly different: this time you have two caps in series going to ground, and the switch throw goes to their junction. This shorts the bottom cap and so your switch options are the series combination of the two caps, or just one cap. When the switch is selecting both caps, the LED is on. If this is for a lowpass filter, the LED is off at the lower of the two filter frequencies, just like above.

As always, the caps should have pulldown resistors if popping is a problem.

spargo · July 13, 2010, 06:16:28 PM

Thanks earthtonesaudio, that is helpful. Is there a way to have the LED on when selecting both caps for version 1, while they are in parallel?

I'm switching between a .01uF cap and a .22uF cap. For the latter option I am ok with having the two in parallel since the difference between that and the .22uF alone is small. I'd like the LED to be on when the .22uF cap, or when both caps in parallel are selected. Is this possible? Thanks again!

R.G. · July 13, 2010, 07:30:33 PM

Use an SP?? switch and use active switching to do what you want with signals and LEDs.

spargo · July 13, 2010, 08:15:02 PM

What exactly do you mean by using active switching?

R.G. · July 13, 2010, 09:04:03 PM

Quote from: spargo on July 13, 2010, 08:15:02 PM
What exactly do you mean by using active switching?

Something other than a mechanical switch. Mechanical switches are good, but expensive for what they do, multiply expensive for more than simple single pole switches, and being mechanical, unreliable compared to electronics.

Transistors can be switches. There are ICs that are switches. JFETs can do switching. I actually suspect that there are several orders of magnitude more electronic switches on the planet than there are mechanical switches ever built. Each new pentium i7 contains half a *billion* MOS transistors used as switches.

The thing about hacking effects is you gotta go learn, hard and fast. It's a mental work more than a physical one.

spargo · July 13, 2010, 09:16:59 PM

But I'm building this effect from the ground up, and I want a mechanical switch. Isn't there a good way to do it as described earlier, but just to get the LED to light when the switch is in the other direction instead of as described?

PRR · July 13, 2010, 10:28:36 PM

> have the LED on when selecting both caps

R.G. · July 13, 2010, 11:52:49 PM

I'm psychic, Paul. The next post will be "how come I get this pop when I flip the switch?"

@spargo: put a 1M resistor between the end of the "opened" capacitor and ground and use the switch to "short" the resistor.

@ the audience: shunt logic solves all kinds of things. It's one of those things you need in the tool box. Paul's been around. Watch him and do.

spargo · July 14, 2010, 01:32:15 AM

I *think* I got it working correctly. Without a resistor there is no pop when I switch, but there is a lot of hum when the LED is on. How would I fix this? Also, I think it sounds slightly different than switching without the LED.

R.G. · July 14, 2010, 01:36:39 AM

Quote from: spargo on July 14, 2010, 01:32:15 AM
I *think* I got it working correctly. Without a resistor there is no pop when I switch, but there is a lot of hum when the LED is on. How would I fix this? Also, I think it sounds slightly different than switching without the LED.

It depends entirely on what the rest of the circuit is. Hum with the LED on is probably power supply ripple caused by the increased current drain.

But it could be layout problems or low frequency motorboating too. Can't tell with the info at hand.

spargo · July 14, 2010, 01:56:51 AM

By the way, thanks for all the help guys.

It's a bluesbreaker circuit. I'm switching the .01uF C3. I would like to select between the default (.01uF) and an alternative (.22uF - this in parallel with the original cap is fine). When the .22uF is selected I want an LED to come on via the SPDT switch. I am currently using a white LED. Checking again, there is a pop now when I switch and the hum is still present. Where exactly does the popping resistor go? I think I put it in the wrong spot so it still popped but gave me a lot more gain.

The rest of the circuit is built according to the GGG schematic, power supply filtering section included. Is the power supply filtering on the schematic sufficient? Sometimes I still notice a bit of a pop when switching the effect on and off with true bypass.

spargo · July 14, 2010, 04:06:01 PM

Is this the correct way to do it according to the drawing? I'm having some trouble understanding it. And according to this schematic, where does the pulldown resistor go to prevent popping?

earthtonesaudio · July 15, 2010, 09:02:06 AM

You'd add a large value (1M ish) resistor across lugs 1 and 2 of the switch, or more specifically, between the lower leg of C11 and ground. The resistor keeps draining the charge from the bottom plate of C11, so it can't built up any appreciable voltage, which is what you hear as a pop when you flick the switch.

Gurner · July 15, 2010, 09:27:43 AM

I'm not hip to your groove about the intended application, but I'd hazard a guess that your ground and input connections need swapping. And then you'll also need to flip your LED around 180° so the cathode then connects to ground.

earthtonesaudio · July 15, 2010, 11:17:59 AM

Quote from: Gurner on July 15, 2010, 09:27:43 AM

I'm not hip to your groove about the intended application, but I'd hazard a guess that your ground and input connections need swapping. And then you'll also need to flip your diode around 180° so the cathode then connects to ground.

Good catch! I agree with this.

spargo · July 15, 2010, 01:14:25 PM

Thanks guys. Yeah, I figured that out.

But putting a 1M resistor from that one capacitor to ground doesn't seem to help. Do I need a bigger value?

PRR · July 15, 2010, 03:10:18 PM

> I'm psychic, Paul. The next post will be "how come I get this pop when I flip the switch?"

It is too humid to draw unessential resistors; anyway it pops once then not again for minutes or hours depending on cap leakage. And the POP may be slight (or bad!), depending on circuit impedances and AD/DC ratios.

> Without a resistor there is no pop when I switch....
> but there is a lot of hum

Yes, hum is unexpected. And it isn't "increased current drain": drain is 3.7mA with LED on, 4.5mA with LED off. (Yes, a real waste; but spargo declines to use a more-pole switch or get into heavy active-switch design.) Ah, "white LED".... the ON current is even less.

OK, a later post shows LED current dumping into the "?" point. That's an ambiguity in my hasty sketch. Ground was not labeled. Thanks, Gurner!

With Ground and "?" in the right place, use 100K from switch end of C0.22 to ground.

Totally popless switching is impossible, and low-pop switching generally leads to bipolar supplies and slow (not mechanical) switches. Just play louder.

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SPDT Capacitor Switch Activate LED?