According to your parts overlay, the collector of the BC547 is connected to the ground plane, and the emitter is connected to the 555 trigger pin. (You need to flip the component overlay for that, or you'll end up fitting the transistor the wrong way around.)
Considering the off-board connections, I think I'd probably use more than just the single 0.1uF decoupling capacitor that's on pin 8 of the 555, too. It wouldn't hurt to add something a little more substantial across the power pins of the Hall-effect sensor, perhaps 1uF to 10uF. Not absolutely necessary, but I'd do it, to avoid potential problems. It would smoothe things out a bit over the whole board.
I'd put an 0.1uF cap directly across the TMP36 power pins too, as recommended on the datasheet.
You can't really have too much decoupling, (within reason), but it's very easy to have too little.