This pedal is a clone of the Ibanez Tube Screamer, based around the PCB from Tonepad.

The TS is a relatively simple build, so being me I had to make it way more difficult by adding a daughterboard with different optional clipping diode pairs.  The character of the Tube Screamer is largely defined by the use of a diode pair in the feedback loop, which results in soft clipping that is further sculpted by the frequency shaping elsewhere in the circuit.

The classic TS uses a pair of identical Si diodes (in parallel with opposite polarities) to achieve symmetric clipping, while the older Boss OD-1 used a single Is diode in parallel with a pair of Si diodes, in opposing polarity, to achieve asymmetric clipping.

I added a rotary switch that replaces the fixed diode pair with the following seven options:

1. Si / Si (the stock TS configuration)
2. Si / 2Si (the Boss OD-1 configuration)
3. LED / LED (louder and more open with higher headroom)
4. Ge / Ge (soft, dark, and compressed)
5. Ge / Si (Si clipping on one half of the waveform, Ge clipping on the other)
6. Open circuit (clean boost although it will clip when the waveform hits the rails)
7. Short circuit (acts like a unity gain buffer;  shorts the tone pot)

The differences are pretty subtle in some cases - I've attached scope traces showing an input 1kHz sine at 300 mVpp (yellow) and the clipped output waveform (green).  Note that the vertical scaling on the output waveform is changed in some cases to capture the full range.

It sounds like a Tube Screamer and the diode settings are subtle but fun - I'm still exploring them.

Yet another Green Overdrive Pedal - with "diode madness" feature