We can use a small 4 or 8 ohm speaker as an earphone, using for this purpose an amplifier circuit and impedance adapter. This circuit is what we show in this article.
With this circuit we can use a speaker of 2.5 or 5 cm as a high impedance earphone, around 10 k. The circuit uses few components and is powered by 4 common batteries, which will have great durability, given the low consumption.
We can put the speaker in a box or even adapt it to an old phone which is out of use or has its capsule damaged; In Figure 1, we have the complete circuit of the adapter.
The smaller components of the assembly can be welded on a terminal strip, as shown in Figure 2.
The small transformer is the type of output found on old transistorized radios. For the input can be used a cable with a plug, as in the common headphones, or two wires with claws.
Material list
Q1 - BC548 - NPN general purpose transistor
T1 - Output transformer - see text
FTE- 2.5cm to 5cm speaker - 4 or 8 ohms
B1 - 2 or 4 small batteries
R1 - 22k ohms x 1/8 W - resistor - red, red, orange
R2 - 15k x 1/8 W- resistor - brown, green, orange
R3 - 330 ohms x 1/8 W- resistor - orange, orange, brown
C1, C3-10 uF x 6 V or more - electrolytic capacitors
C2 -1n2 - ceramic capacitor
Miscellaneous: battery holder, terminal bridge, mounting box, wires, welding, etc.
