Help with DPDT LED switching

[Yuan Han]

I'm using "Led Trick" (http://analogalchemy.com/pedals/ledtrick.gif) on a PNP Rangemaster.

i connected the 9V in the schemo to the positive 9V in the circuit (i.e. the ground of the circuit) and the ground to the actual batterty ground (ie.e the -9V of the circuit).

Now, when the circuit is bypassed, it is bright, when it is activated, it becomes dimmer.

I was using 2N3904 (thought it was blown), but changed to 2N5089, but well, same results !

Did I do something drastically wrong ?

mmm Or should I place the LED at the Collector instead of the Emitter ?

Thanks
Han

[Mike Burgundy]

The Millennium2 has better contrast and less consumption, I reckon the LEDtrick circuit will always be a little on.
That said - negative ground pedals usually involve changing between NPN and PNP (or N-channel and P-channel) devices.
What the LEDtrick should do is:
pedal bypass:
3904 base bleeds to almost 0V through the effects output resistor. That 330k wants to pull it up to 9V though, so the voltage on the base (and the amount of residual light) depends on the ratio 330k/output resistor effect.
This is the low state.
pedal on: 3904 base not connected to effect. This allows the base to be pulled to a full 9V, the transistor conducts and the LED lights.

Now, with the PNP pedal, in bypass you're pulling the base to ground which is actually +9V as far as the 3904 is concerned! It'll probably be brighter because it's charging to 9V through two parallel resistors instead of just one (the 330k AND the output resistor).
Flipping the output resistor from ground to the negative supply rail should work, but will introduce DC to the output which you DO NOT want.
You're going to need another transistor, and flip the diode. Make sure the effect actually has an output resistor after the blocking cap.

[Rodgre]

I think this circuit isn't going to work with a PNP (positive ground) circuit.

R.G., isn't there a PNP version of the Millenium Bypass circuit?

Roger

[Mike Burgundy]

isn't there a PNP version of the Millenium Bypass circuit?

There is:
http://www.geofex.com/Article_Folders/Millenium/millen.htm
only use a P-channel MOSFET and reverse the diodes.

[Yuan Han]

Ah, thanks for the explainations.

I guess... i'll dump using DPDT, and use a 3PDT for bypass.
Don't have any P-channel FET ard!

I tried N-channel mosfet (millenium bypass2) with +9 to battery 9v and 0v to battery 0v but it was the same as the current situation.

Han

[Mike Burgundy]

Ah, thanks for the explainations.

I guess... i'll dump using DPDT, and use a 3PDT for bypass.
Don't have any P-channel FET ard!

I tried N-channel mosfet (millenium bypass2) with +9 to battery 9v and 0v to battery 0v but it was the same as the current situation.

Han

Correct - you need a P-channel MOSFET (such as, uh, NDT454?TP0606?  - do a search in the archives, I'm sure there's advice there)for Mil2, or a PNP transistor (such as 2n3906) for the LEDTrick when using them with positive-ground systems.

[R.G.]

You can use CMOS logic chips instead of a Pchannel MOSFET in the Millenium 2. The circuit is the same - diode leakage to get an incredibly small bias current and click voltage. That is shorted out by the output stage when the DPDT connects it. The output of a CMOS gate is a drive voltage in either direction, so you just connect the LED to the gate output through a resistor, and the LED direction dictates which way it's on.

I can draw schemos, but it's a trivial variation of the basic Millenium 2.

You can also use a BS250P p-channel MOSFET.