Audio Engineering

136 Chapter 4

example is seen in Figure 4.13. Like switches, jack plugs and sockets are more complex
than they might at fi rst seem.

4.1.5.4 Light-Emitting Diodes

The LED is today’s solid-state marvel, the equivalent of the fi lament indicator lamp
of years gone by. When I started in hobby electronics, especially in the building of
amplifi ers, I always had to use fi lament indicator lamps as power on/off indicators.
They took up a lot more space than LEDs, but, more critically, the current they drew
was enormous. Fortunately, with the advent of the integrated circuit era came also the
solid-state electronics age, with the LED soon becoming the universal indicator device.
Small, light, extremely robust, and drawing an economical amount of current, the LED
is a natural for panel indicators. In absolute terms, the current drawn is not insignifi cant,
however, but as the rest of the electronics technology speeds ahead to devices that use
much less power, the indicator remains locked (at least for the time being) with the
LED. Fundamentally, if the LED is to be used as a relatively long-range viewing device,
current has to be supplied to produce the visible light energy. Typically, current through
the device is limited with a resistor to just a few milliamps for acceptable viewing.
LEDs come in a limited range of colors—red, green, yellow—but red is by far the most
common and useful color. They come in different shapes (cylindrical and rectangular)
and sizes, from pin-head tiny to jumbo sized, the most commonly used size being
something like the size of a TV remote button. There are some special LEDs with very

Normally closed socket

1/8′′ jack socket

Plug Out: Pins B and C are shorted

Plug In: Pin A grounded, Pin C to

signal

Signal

Ground

Jack plug

Jack plug

External

speaker

Internal

speaker

Ground

Amp output

Normally closed Radio

jack socket

A

B

C

Side view

Jack socket

Figure 4.13 : An example of a normally closed jack socket.