Reverse Millenium ByPass ?

[tubelectron]

Hi All,

This request must have been answered but I did not find the path to the reply... Sorry !

I need to build a Millenium ByPass that is "Reverse" working.

Here's the schematic of a "Normal" working Millenium ByPass applied to a tube HV PSU overdrive pedal, that I built with success :



When the effect is ON (engaged), the gate of the 2N5457 is unconnected : it sees and infinite resistance, and that makes the leds to be ON (glowing)

When the effect is OFF (disengaged), the gate of the 2N5457 is connected to the wiper of the FX output pot : it sees a resistance to GND of a value between 0 to 500K (no matter), and that makes the leds to be OFF (shutdown)

In fact, any resistor from the gate to GND < or = to 1M provokes the shutdown of the led. This is the "Normal" operation of the Millenium ByPass.

Again thanks to Mr Keen for that genious and reliable device... But :


What I need is the opposite : a Millenium ByPass that would make the led ON (glowing) when the gate is connected to GND through a closed switch, or at least at a very low resistance, typically less than 10 ohms... And would make the led OFF with any higher resistance value connected (say  : more than 10 ohms)... 


Any of you has already experimented that kind of "Reverse" Millenium ByPass ? Any idea or schematic ?

Thanks & A+!

[earthtonesaudio]

Inverter

[boogietone]

Someone correct me but would not an n-channel enhancement mode MOSFET in place of the jfet work?

[tubelectron]

boogietone,

I would be unable to correct you, but I have found a solution with a simple comparator driving the led.
Now I need to experiment it to see if it works for my Boogie MKIIA FSW design...

A+!

[R.G.]

The Millenium 1 detects a low resistance as "LED off". When that happens, it puts current through its LED. If you think about it, it really makes "current off" when the resistance is low. You're asking for "current off" when the resistance is high.

You can do this a couple of ways. All of them involve, as ES said, inverters. Frankly, the simplest thing is to go do the Millenium C with a CMOS inverter chip. That gives you all possible inversions because not all the inverters are used. Another way is to use the Millenium 1, and then two bipolar transistors. The Millenium 1 is a logical follower; you need to use it to run an inverter so that when the Mill 1 is on, the output current is off. That takes two transistors because one is a "translator" to change the current of the Mill 1 to a form the second transistor can use.

A comparator chip works.

An N-channel MOSFET in place of the JFET has problems because of the Vgs needing to be several voltage positive to make it turn on, as opposed to the JFETs one to several voltage negative to turn off. The N-MOSFET is best used in the Millenium 2 circuit, which is an inverter not a follower. This lets you hook a resistor where the LED is on the Mill 2, then run an NPN with the LED in its collector, emitter to ground and base to the MOSFET drain. When the MOSFET is off, the resistor at the MOSFET drain supplies current to the NPN base, and the LED lights. (notice that you still have to limit the LED current with a resistor in series with it). When the MOSFET comes on, it pulls the current from the drain resistor to ground, preventing the NPN from getting base current, so the NPN goes off , and so does the LED.

[earthtonesaudio]

Or just put the LED in parallel with the FET instead of in series... that might work well enough.

[slacker]

I was thinking that as well, seems like it should work, but I couldn't quite get my round it enough to be sure.

[R.G.]

Yep, might work. Depends on the JFET and how big a difference in brightness you want between on and off.

[tubelectron]

Good !

Thanks to all for technical advices +++.

I think I will go for a comparator directly driving a low power/hi brightness led since there is probably less components involved, and moreover I need inverting and non inverting functions as well. A single polarity supply quad comparatorLM339 (or even some 741s or 1458s !) should do well...

A+!