This is a library of perfboard and single-sided PCB effect layouts for guitar and bass. I'm not an electrical engineer by any stretch of the imagination, just a DIY'er who likes drawing layouts. It is meant for the hobbyist (so commercial use of any of these layout is not allowed without permission) and as a way to give back to the online DIY community.

Thursday, November 29, 2018

Baja Optical Limiter

Here's one of Bajaman's non-amp emulating circuits, the Optical Limiter. It's based on the Demeter Compulator, but is not a compressor but a limiter. What's the difference?

The ratio is the main difference between a compressor and a limiter. A compressor has a low ratio that turns down SOME of the volume when it goes above the threshold. A limiter has a huge ratio that turns down ALL of the volume that goes above the threshold. (from Musician on a Mission)

Here's the schematic for reference. It's designed as a buffered bypass pedal, with the input jack going to the In pad on the board, and a third of a a 3PDT footswitch selecting between the buffered out and effect out. The off board LED pulses with the signal but probably isn't going to be great as an on/off indicator. Just use another third of the 3PDT to make/break the connection between the LED- pad and the cathode of the LED. And then use the final third of the footswitch to wire an on/off indicator LED as you normally would. (At least I think that's what Bajaman had in mind...) Also, the LDR should have 5k light resistance, 500k dark resistance (a GL5516 should work). Oh, and I added an extra pad so GSD pinout JFETs (J201, 2N5457, etc.) can be used in place of the 2SK117.


  1. I read in a thread that for the ldr. It needs to have 90 ohm resistance. So, put it in light and check resistance. If it’s to high or low it won’t work as intended.

  2. An optocoupler can be used in place of the LED/LDR, no? If so, I'm wondering if anyone has experience using a 6-pin in place of a 4-pin. This article indicates the pinouts:

  3. Great layout!

    A little request: caline mark4 please!!!

  4. Pretty sure you can use an optocoupler. Not sure about the 6 pin.

  5. Verified the PCB version. No modifications or changes. Works as a limiter quite well. Used a Chinese supplied GL5528 LDR and 5mm diffused Yellow LED. The LDR was found to have under 100R resistance in direct sunlight.

  6. Would a TLP185 optocoupler work? Tayda has them :