Make Electronics

What Next? 251

Experiment 29: Filtering Frequencies

0.15uF 0.15uF

0.22uF

10K

33K

100uF

NP

8 ohms

100ft hookup 10uF NP

wire on spool

100uF

100uF

NP

100uF

100uF

1

2

3

4 13

14

15

16

5

6

7

8 9

10

11

12

9V

DC

100uF

TEA2025B

Input from

headphone jack

on audio device

Figure 5-41. The audio amplifier chip should be wired with capacitors around it as shown,
“NP” denoting the ones that are not polarized. The acronym“BP,” meaning bipolar, is also
often used to mean the same thing. The output from pins 2 and 15 of the chip can be
passed through a coil or a 10 μF capacitor to demonstrate audio filtering.

The input shown in the schematic can receive a signal from a typical media
player, such as a portable MP3 player, CD player, or cassette player. To connect
its headphone jack to the breadboard, you can use an adapter that converts
it to a pair of RCA-type audio jacks, and then stick a wire into one of them as
shown in Figure 5-42. The wire will connect to the 33K resistor on the bread-
board circuit. The chromed neck of the RCA jack (which is sometimes gold-
plated, or at least gold-colored) must be connected with the negative side of
your power supply on the breadboard; otherwise, you won’t hear anything.
You can ignore the second output on the adapter, because we’re working in
mono, here, not stereo.

Figure 5-42. To sample the output from the

headphone socket of a music player, you

can use this adapter and push the stripped

end of a piece of hookup wire into one of

the sockets. Then use alligator clips on a

jumper wire to connect the audio to your

breadboarded circuit. Don’t forget to use

an additional jumper wire to connect the

outside of the socket to the negative side

of the power supply on the breadboard.

Because we’re only using one speaker, the

amplifier is connected to only one of the

stereo outputs from your player. The other

is ignored.