follow-up:
1) tested last night the expression input of the PS-3. found out that shorting the Ring and Sleeve or Ring and Tip did nothing, but connecting the Tip and Sleeve produces the desired octave effect as per knob settings. sabi sa manual, you can use the FS-5u for the expression input; ibig sabihin a mono cable is is fine which basically shunts the Ring to the sleeve, and using only the tip as the trigger. so by using an FS-5u, you are basically connecting the 3 terminals together to create the pitch shift effect. gotta look at the schem later as i dont feel this is safe for the unit.
2) i decided to just use a stereo cable and hooked up a pot to vary the resistance between the Sleeve and Tip, leaving the Ring not connected. found out that a resistance of about 200-300K works well.
3) may Roland EV-5 ako i got from a trade which i modified dati to have a detachable stereo cable. however, the biggest bummer was that it used an ordinary (think normal) pot which had a 7:00 to 5:00 sweep while Roland uses a special pot for the EV-5 which a narrower sweep. kumbaga, a pot with a 9:00 to 3:00 lang na sweep ang naka-install for the EV-5 (and those similar FV-50 pedals). initially i installed a 100K pot wired as a voltage divider (bridged by a 10K fixed resistor to drop the resistance due to the narrow sweep of the pot), but felt parang may kulang, lalo na sa sweep ng rotation pag-apak sa treadle.
this is when i idea came to me:
pwede akong gumamit ng 1Meg pot wired as a variable resistor since ang kailangan ko lang naman eh 200K-300K na sa tantya ko e papatak sa 7-12:00 na limited sweep ng pot. bam! swabe, mas maganda yung sweep ng treadle. i'll compare this sa totoong EV-5 in the future.
as for the optical expression plug, an LDR with a dark resistance of 1M+ is fine, but connect the leads to the sleeve and tip lang of the stereo plug. IIRC, devi - for her PEEPlug - wires the LDR with the Ring and Tip connected, and the other leg of the LDR dun sa sleeve connected. tried her method initially, pero bitin.