Convert Metronome circuit to Visual Metronome (LED blinker)
This circuit has several problems, including the problem of rapidly draining the battery. At a minimum you need to swap the 2N3906 emitter and collector. As shown it might happen work, but if so then it will still be very inefficient. Also you need to put a resistor on the collector of the 2N3904 so power isn't shorted to ground when the transistors are on. Regarding the LED, you will need a series resistor if the LED's typical forward bias voltage is notably below the 3V (so it doesn't drain the battery unnecessarily) and a parallel resistor so that the output voltage is pulled low when the transistors are off, to provide proper feedback through the cap. I strongly suggest using a timer IC.