Invert LED blink in TAPLFO3

[lars-musik]

Dear Forum,

here I am back with another stupid question. I was asked by a musician who plays the mandolin in a bluegrass band to build a simple, tap–triggered optical metronome for him (no extras, just a tap switch and a LED) as he sets the rhythm with his instrument. "No problem" - I thought, I'd just take a TAPLFO, set it on square wave, fix it on the 1x multiplier and we are good to go.
Now he's basically happy BUT he'd much much prefer he LED to blink in off-beat to the tempo he tapped.

NOW FINALLY my question: Can I somehow "invert" the LED, so that it is dark on time and lights up on the offbeat? My first thought: I'd modify the PIC code. But as I cannot code that won't work. Second thought: A voltage inverter might do the trick. But.... No idea.
Better to ask you guys as you always come up with a smart thing.

[antonis]

Can I somehow "invert" the LED, so that it is dark on time and lights up on the offbeat?

Try a 2N3906 (p-n-p) for TR2..
(without written permission of Electric Druid - sorry Tom.. )

P.S.
I haven't the slightest idea about TAPFLO 3 internal circuitry so it might not work..
(but, as far as pin5 "off" state exhibits low enough DC resistance, it should work..)

[PRR]

[lars-musik]

Thanks you two! Two simpkle solutions, I couldn't have hoped for more.

[ElectricDruid]

+1 what they said.

If you're just blinking an LED, you don't even need the transistor. The PIC's output will produce up to 20mA max, so you can run an LED pretty bright without frying it (10mA is loads for most modern LEDs).

Then you can put the LED either from +5V to the pin, or from the pin to ground, to get the LED either in-time or off-beat.

Also if I was just blinking an LED, I'd probably use the StompLFO to minimise the overkill, but then again, maybe the Multiplier feature on the TapLFO is nice for this application.

[lars-musik]

Thanks Tom!
I think I'll stick to the transistor and test the two versions above. I have a small PCB for the TAPLFO3 but not for the STOMPLFO, so I'll stick to it, too. The guy I built this for is pretty minimalist, he doesn't even want the multiplier. But the metronome consists of two enclosures, one 1590BB with a Tap footswitch and a "LED mute" footswitch (housing the TAPLFO and a battery) and a breakout 1590a with the LED and a pot for brightness, both connected via a standard guitar cable. I imagine that, depending on the length of the cable, the transistor might benefit the circuit.

[antonis]

Also, a voltage comparator (using any general purpose op-amp in case of appropriate IC lack..) should make things easier..

[lars-musik]

I am a bit slow at the moment.
Am I not getting a permantly lit LED in this setting? That's a LED from +5 with a CLR to ground, is it not?

[antonis]

Am I not getting a permantly lit ED in this setting? That's a LED form +5 with a CLR to ground, is it not?

Of course it does..

It only lits when TR2 is OFF.. When it's ON, LED is almost shunted to GND (0V plus TR2 V_CEsat)

[ElectricDruid]

+1 what Antonis said. TR2 shorts the LED out when it is switched on.