I have a bit of a fascination with dub sirens. This one, the NJD, is particularly iconic. A schematic is floating around, but it's not the most pleasant to look at.
I redrew the schematic to make the different parts more clear. You have an LFO that's a pretty standard two-transistor astable, and a main oscillator that's very similar. In between you have some LFO shaping and an LED driver, along with two buttons.
Dials at the top select different combinations of resistors that change the oscillator rates, along with switching modulation routing.
The LFO is not very interesting. It only outputs a squarewave. It can be reset via switch S1 and D3. Its output goes through D4 and R22 to change C5. This gives us a slightly more interesting, uneven triangle shape. The discharge rate is set by R23 and R24. Depending on the position of S3, R24 might be switched out via R23 being grounded. C5 can also be manually charged slowly via R25 and S2 or quickly via D4 and S1.
S4 lets you select between combinations of the square LFO and the shaped LFO as modulators for the main oscillator. It can also force the osc to a fixed pitch via D1, D2, R8, and R9.
The main oscillator is similarly square, though the pulse width will change depending on how the two halves of it are modulated relative to each other. The harsh highs of the square wave are rounded off by the cascaded lowpass stages before the final output at R30.
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