TAPLFO PWM output + transistor PWM buffer issue - can't go above 5V?

[midwayfair]

I'm working on a PCB that involves the TAPLFO. Datasheet is here:

http://www.electricdruid.net/datasheets/TAPLFO2Datasheet.pdf

The tremolo schematic on page 7 uses a transistor to buffer the LED from the PWM output.

The LEDs are referenced to 5V.

One thing I tried to do on the breadboard at one point was to reference the LED to +9V for one reason or another, thinking, "well, the base won't see the 9V, so I should be safe." Nope. Dead chip.

Dave Rolo's twin peaks (http://i1277.photobucket.com/albums/y492/drolobucket/TWINPEAKStremoloschematics_zps4641a0b2.png), interestingly, uses a 6V regulator to get very slightly more voltage out of the LED driver.

I don't understand why either device needs to have the voltage knocked down, even though I understand that getting 9V within sneezing distance of a digital chip is instant death. Shouldn't the only voltage on the base of the transistor be the 2.5V swing from the PWM output? Where's it "finding" enough voltage to fry itself? Similarly, even if the op amp were running on 9V, the pin would sit at 4.5V, which is still less than 9V.

It also can't just be pure supply voltage. After all, the more complicated full filter circuit in the datasheet runs on +15/-15.

I'd take a chance that I did something wrong the first time, but I'm not made of TAPLFO chips and that was one of my most expensive mistakes on a breadboard. So if anyone has insight into the why of this -- or an assurance that it's safe to do it -- I'd like to hear it.

It doesn't actually matter on the layout I'm making (the 9V and 5V traces are both within reasonable distance from where they'd need to connect), but I'm curious.

[slacker]

There's no reason using 9 Volts for the LED supply shouldn't work, there must have been something wrong if you killed the PIC.
Using transistors connected like in the page 7 schematic to control much higher Voltages with a micro controller is done all the time. Examples from the stompbox world would be some of the relay switching projects.
Even if you somehow got 9 Volts on the base of the transistor, the 10k resistor between it and the PIC's pin would limit the current enough that the pin wouldn't be damaged.

The little trem circuit I knocked up to test my LFO runs the LED off 9 Volts.

[midwayfair]

There's no reason using 9 Volts for the LED supply shouldn't work, there must have been something wrong if you killed the PIC.
Using transistors connected like in the page 7 schematic to control much higher Voltages with a micro controller is done all the time. Examples from the stompbox world would be some of the relay switching projects.
Even if you somehow got 9 Volts on the base of the transistor, the 10k resistor between it and the PIC's pin would limit the current enough that the pin wouldn't be damaged.

The little trem circuit I knocked up to test my LFO runs the LED off 9 Volts.

Thanks for the help. I'll give it another shot on the breadboard ... I must have done something else wrong that killed the PIC. Maybe a stray wire touched something it shouldn't have and I got 9V directly onto the PIC.

I guess I should also ask: Will feeding it to +9V actually result in a larger swing for the LED set up like in the datasheet, or is that impossible without some gain?

[slacker]

Running the LED off 9 Volts probably won't make any difference. Most LEDs are at maximum brightness with about 20-30mA of current, shove too much more through them and you let out the magic smoke. Depending on the forward voltage of the LED a current limiting resistor of between about 100-200 Ohms should get you maximum brightness from 5 Volts, so using 9 Volts won't make them any brighter. If there's some reason you can't run them at maximum brightness at 5 Volts then using 9 Volts could help.

If you need more range you might need to make changes to the circuit of whatever the LEDs are driving.

[midwayfair]

Running the LED off 9 Volts probably won't make any difference. Most LEDs are at maximum brightness with about 20-30mA of current, shove too much more through them and you let out the magic smoke. Depending on the forward voltage of the LED a current limiting resistor of between about 100-200 Ohms should get you maximum brightness from 5 Volts, so using 9 Volts won't make them any brighter. If there's some reason you can't run them at maximum brightness at 5 Volts then using 9 Volts could help.
Depending on what you want more range for you might need to tweak whatever the LEDs are driving.

Thanks. I had a crosspost going on Madbean, and Dave Rolo suggested the same thing (that 9V didn't change things at least how the setup is currently working). I'll post this there, it's really good info.

LEDs are driving a VTL5C1. It's pretty much the biggest range I can possibly get! The range of the vactrol isn't really the problem but rather that I was hedging my bets to make up for having a second indicator LED in series and the possibility of increasing the resistance (whether from the PWM or the CLR) to avoid ticking.