# Transistor With No Voltage to Base

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:

NPN transistor:

Base goes to + lead from Amp

Emitter goes to Negative lead of Amp and 9V battery

Collector goes to cathode of LED

Battery:

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?

• Please add a schematic next time. You don't need fancy software, a hand-drawn schematic is still better than a description. Sep 7 '13 at 11:17

The transistor may act as a switch or a variable resistor. If no voltage is applied to the base (more precisely: no current flowing into the base) then the switch is open. As base current is applied it gets amplified by the transistor into an N times larger collector current. The "N" is an important transistor parameter, called $H_{FE}$, and it defines the current amplification factor. For general purpose transistors this is often around 100.