In the newest Make magazine, they show how to connect LEDs in line to speakers to have the LEDs light up when the speaker is playing music (pulse with the music). They say you may want to hook up the LEDs to a 9 volt battery in order for the LEDs to come on at lower volumes. They accomplish this via a transistor. I don't get how this works.
The connections are as follows:
Base goes to + lead from Amp
Emitter goes to Negative lead of Amp and 9V battery
Collector goes to cathode of LED
Positive lead of battery goes to anode of LED (through resistor)
Somehow, when voltage is applied to the + lead of the Amp (music is playing), this turns the LED on?
I would have thought the LEDs would always be on, the connection goes + battery, LED, collector of transistor, emitter of transistor, - of battery. I thought that if a positive voltage was applied to the collector, with the emitter at ground, this would be a closed circuit?