Say I have a 12vdc circuit and I need to pull 1.6vdc from it somehow... how?

[guitylerham]

I am building a CX-3 Leslie simulator from a layout from the interwebs. To my knowledge, the effect in its natural state is in the high speed setting. To activate the slow speed mode, I need to feed a pin on the pcb a +1.6vdc via a stompswitch. I want to use an LED indicator. The instructions are to "use a 1K5 resistor in series with the indicator led and switch the earth accordingly - then take the the junction of the led and the 1K5 for the small voltage source - +1.6v... also, the feeder cable should include a 10k resistor in series with it, to avoid dimming the slow indicator led." Does my drawing look valid?

[Gurner]

I'm not familiar with your leslie simulator, but from what you decribe, your 1k6 resistor *needs* to be on the other side of the LED (between 12V & the led anode)....you switch the cathode of the ground to earth to have it light up.

When it is lit, assuming a red led, there'll be about 2V at the junction (whereas your present method would have about 10V at the junction)

[guitylerham]

Ah yes, does this seem correct now? Man, I need to research LED's in general. I didn't realize different colors had different properties. Would you just use Ohm's Law to figure this sort of thing out? I've got a bit to get under my belt before I can intuit what needs to come from where. Thanks, Gurner! The CX-3 was a Korg organ simulator and it had a Leslie sim inside. A guy took that circuit out and drafted up a standalone version. I've pretty excited to hear it myself.

[guitylerham]

Bumpity bump!

[PRR]

> Bumpity bump!

[guitylerham]

I appreciate your ultra clear schematic PRR! Wouldn't the 12v flow through the 1k6, the 10k, and to the "Slow" pin continuously regardless of what the switch does? It seems like the only thing the switch is doing here is turning on the LED while leaving the effect constantly in Slow mode.

ORRRR

Is it true that electricity follows the path of least resistance! So that when the switch is broken, current is forced to flow through the resistors to the Slow pin and illuminate the LED but when the switch is connected, the current completely flows past it all and goes to ground? If that is true, than your circuit is way more awesome than mine!

And the 10k is just what the designer instructed to use. I'm still figuring out how to set up the formula correctly. After passing through the 1k6 resistor, the current is divided between the 10k and LED that are in parallel. If we take Ohm's Law... aaaand I'm lost @_@

Oh also, rereading the comments by the designer, he stated, "The slow speed is enabled by applying a small positive voltage to the marked pin on the board ( I used a 1K5 resistor in series with my three indicator leds and switched the earths accordingly - I took the the junction of the leds and the 1K5 for my small voltage source - +1.6v."

So apparently +1.6v isn't the exact target, just a small voltage is desired (don't know how small is small though). I'm assuming the designer used 1 LED for the main effect indicator, 1 LED for "Slow," and 1 LED for "Fast." It shouldn't make any difference if I just use one LED here right? I plan to wire up the main effect indicator LED just like normal with the other bypass stompswitch.

The fun and the curse of researching online is that you never get the whole picture! You always need to spend due effort in piecing it all together. Thanks for your help so far, guys.

[R.G.]

I did a quick count of ways to do this and stopped counting when I got up to 2.83 billion. 

If you have a link to the instructions, it might help ferret out some of it.

I believe what the designer said in "I used a 1.5K resistor in series... and switched the earths accordingly" meant that he had three LEDs in parallel, current limited in some way not shown, and put a 1.5K resistor in series on the ground side of one or more of the LEDs. Without more info, exactly how that hookup was done is very foggy, and that's probably part of your issue, there. Something else is limiting the current to the LEDs to about 1mA, and the 1.5K "eats" the same 1ma as the slow LED, producing about 1.5V when current flows through it.

If the original "designer" didn't give you more than the info you've presented, it's going to be really tough to do better than "easter-egging" parts and arrangements.

Got a link?

[guitylerham]

I see, RG. The design came from a guy named Bajaman from "another" prominent forum that I've concluded is not popular here. Is it ok to post the link to the thread here? Really the only two bits of information pertaining to this aspect are what I quoted earlier. I've tried messaging several people who have completed the circuit but no one has gotten back so I'm in the dark!

"The slow speed is enabled by applying a small positive voltage to the marked pin on the board ( I used a 1K5 resistor in series with my three indicator leds and switched the earths accordingly - I took the the junction of the leds and the 1K5 for my small voltage source - +1.6v)"

"To activate the slow setting I suggested running a feed from the junction of the 1k5 current limiting resistor and the led status lights. I forgot to mention that the feeder cable should also include a 10k resistor in series with it, to avoid dimming the slow indicator led."

That's all she (he) wrote! Here's a snap of the circuit as well as the schematic however. Perhaps it contains another
component that you speak of.

The farthest bottom right-hand corner show the point "C" titled "Slow/Fast."


All transistors used are 2N5089 instead of the 2SC945 listed in the layout.

[R.G.]

That clears up a lot.

That point is a transistor base input. Do NOT feed it a voltage. Instead, feed it several volts through resistor. The exact number of volts and the resistance is not all that important as long as there is enough but not too much. Switching that point to 12V through anything between 2.2K and 10K should do it.

All that does is turn on a transistor, and to do that you have to feed a current into it. If the current is not limited, the transistor base can be damaged. It may be that the transistor base *measures* 1.6V when fed from some other limited source, but trying to force it to 1.6V is a bad idea. If you need an LED indicator, you can put the LED in series with the limiting resistor to 12V.

[guitylerham]

Ahah! That does make more sense. Thank you so much, RG. I'm forever grateful to you and all the other very knowledgeable contributors on this board who chime in with info on my projects.

Here is the updated circuit. This should be correct, right?

[PRR]

Go with my circuit.

In your circuit, there is no reason to have both 1K6 and 10K, works same-as 11K6.

12V - 1.6Vled - 0.6Vq = 9.8V in 11K6 resistor. This is 0.8mA current.

0.8mA in LED is not very bright.

0.8mA in transistor is maybe more than it needs. (Though R.G. is suggesting 12V thru 2K or 6mA, which is fine also.)

My way....

ON, the 1K6 flows about 6.5mA toward LED. LED drops 1.6V. 10K diverts some of this to the 0.6V transistor input, leaving 1V across 10K resistor. That's 0.1mA, which is enuff to turn-on a transistor with only about 2mA of load current. LED gets 6.5mA-0.1mA= 6.4mA, plenty bright.

OFF, with LED shorted, the full 12V appears across 1K6, 7.5mA current flowing. LED current is zero (shorted by switch) and likewise transistor input gets zero volts and current.

An objection is that the OFF position wastes 12V at 7.5mA or 0.09 Watts of power. IMHO, so what?

A mild benefit is that the demand on the supply switches from 6.5mA to 7.5mA, instead of 6.5mA to zero mA. A 1mA glitch instead of a 6.5mA glitch. Probably insignificant.... I'm just in the habit of not letting lamp-clicks get into the power chain where they may contaminate the audio.

[guitylerham]

Awesome, PRR. Much thanks to you! I'll go ahead and use your design. It looks like it'll be perfect. Thank you all so much, I was way off. I should be getting parts soon and will post here when I get it working. Thanks again!