How would you duplicate an LFO dc pulse?

[acehobojoe]

Say you want to use that to power Multiple LEDs and LDRs, how would you do that? Would it be difficult since an LED is a current device?

I'm just wondering about it. I'm not the most knowledgeable on this.

[R.G.]

I don't know how other people would do it, but I'd use multiple-output current mirrors.

A closely-allied question is how to do matching out-of-phase LFO signals with LEDs - put them in the collector circuit of a differential amplifier, which is another form of "current mirror" although that's not the major use of it.

[midwayfair]

How many LEDs are we talking here? You can also do a couple LEDs in series and they'll blink the same. I've done up to three without major problems using the Lune LFO but any more than about 4V of a drop and it becomes pretty lousy.

You can also just put multiple LDRs around the same LFO. I mean, that's what most designs do.

[acehobojoe]

I was actually thinking of trying to stick tom's TAPLFO chip in a univibe or something similar. Pretty sure the Univibe uses around 4 LDR's to work.

[R.G.]

MWF is right; putting LEDs in series makes the current through each one identically the same. Using a multi-tail current mirror makes them only close. And power supply limits how many you can string together.

The difference in the applications would be how much voltage you have to play with. Red LEDs are 1.2-1.4V each. 9V batteries are considered "good" until they get down to 7V, so 7V minus the 5.6V of even Red LEDs only leaves you 1.4V for your driver circuit; that's pretty tight.

If you were wanting to drive the LED with a voltage through a resistor, the resistor voltage really needs to be as big as the LED's voltage for any kind of predictability.

However, if you have the 15-18V or more of a Univibe, you can use four 2V green LEDs and not sweat the power supply. In fact, with a larger power supply, you WANT them in series so the drivers don't have to eat the extra power.

So as improbable as it sounds, maybe a better answer would have been "How big is your power supply voltage?"

The other question is "how big is your LFO signal?". I haven't looked at the TAPLFO, but I suspect that it's a uC, and they nearly all run on 5V or less. You'll probably have to do some amplification/conversion to get the LFO into a form to drive LEDs.

[midwayfair]

I was actually thinking of trying to stick tom's TAPLFO chip in a univibe or something similar. Pretty sure the Univibe uses around 4 LDR's to work.

Yes, it's four LDRs. Why do you need multiple LEDs?

Did you look in the datasheet? It has an example of a schematic for an LED for the audio side and a rate indicator. I'm not sure what the problem is. I'm not saying that it didn't result in some useful posts, but I just mean it seems odd that it came up in your situation. It even has an example that converts it to a +-12V pulse.

[acehobojoe]

ok, technically it could be accomplished with just one LED and 4 LDR's together, That is an option as long as they are separated properly on the board and isolated well.

I believe the application sheet of the TAPLFO has current mirroring or something similar with that 2n3904 transistor it uses.

or it works by basically varying the ground of the LEDs, which is esentially the same.

So I may need a larger supply if I need to use 4 different optos, but might not if I am able to use just one LED and 4 LDR's like used in the original.