double gang pot: cut/boost + cut LED and boost LED...

[gregwbush]

Hello all
I'm finishing off a parametric EQ build
http://www.diystompboxes.com/smfforum/index.php?topic=95483.0
and since i've got a double gang 10k pot and the circuit only calls for a normal 10k pot i had this idea where i'd try connecting up a couple of LED's that change brightness according to the amount of boost or cut.



The above is what i've mocked up so far and has the following observed behavior(s)

At center, The total resistance between supply and LED1 is 5k + 1k = 6k
             The total resistance between supply and LED2 is 5k + 1k = 6k

Therefor at center both LED's are on (dim, but on)

When i turn the pot; one LED get's dimmer while the other get's brighter
At full rotation in either direction; one LED has ~1k current limiting (bright), and the other ~11k (quite dark)



Now, here come the questions!
How can i add some complexity to this very basic idea to get the following outcome:

Neither LED lit when pot is centered
Boost LED lights up and get's brighter toward full CW (while Cut LED stays off)
Cut LED lights up and get's brighter toward full CCW (while Boost LED stays off)

I know i'm asking too much of a circuit with a part count of 4!

Any ideas?
Thoughts?
Transistors, diodes, IC's, Logic??

Cheers,
gregwbush

[slacker]

No good at explaining stuff so here's a picture.



For the one on the left both resistors should be the same value to give 4.5 volts, depending on your LEDs you might be able to use the existing 4.5 volt voltage divider in your pedal. When the pot is in the middle both ends of the LED are at 4.5 volts so there's no voltage across them and they're both off. As you turn the pot away from the middle the voltage across one of the LEDs increases until it's enough to turn it on and it then gets brighter, the other one is reverse biased so it stays off. There will be a dead spot in the middle portion the pots range where both LEDs stay off.

The second version has no dead spot, when the pot's in the middle both LEDs are off and as soon as you turn the pot one of them will start to light up.  Start with say 1k for the resistor and tweak to taste.  

[gregwbush]

Brilliant!

Thanks slacker

I'll have a play around later

[gregwbush]

Alrighty...

I didn't muck around with the simpler design, just went straight for the "better" one and made this layout in DIYLC



I've built it and (although i don't necessarily know how it works) it works!

The 2 on board resistors are 12k (had 'em laying around) creating a voltage divider to get 4.5v at the op-amp's non inverting input, as shown in slacker's schematic

The IC is a single op-amp, ua741cp, i had laying around

Resistor between pot and LED's is 1k

Pot, LED's and 1k resistor are currently on breadboard

Here is a very CRUDE vid to show that it works!

The big pot (24mm) is 10k linear, just for testing.. i shall be using a 9mm double gang 10k linear, with one gang wired as above and the other wired to the cut/boost thingy on my parametric EQ!

This forum rocks

Cheers,
gregwbush