A surface mount project . .
Other surface mount
This project is called "mini" because its size is small and the output
It uses surface mount technology and this will be a challenge for most
Before starting this project, refer to our article on
Surface Mount. It
covers identification, handling and soldering of these tiny components.
You must have a fine tipped soldering iron that is temperature
controlled if you intend to solder this type of component.
If you have an ordinary soldering iron, its temperature can be
controlled by connecting it to a lamp-dimmer control or reduced slightly by adding a power
diode (1N4004) in series with one of the leads.
This can be done inside the plug or by adding a "cord switch" and
placing the diode inside the switch.
You will also need fine solder (included in the kit) as this will
improve your soldering 100%.
You cannot solder any of the transistors and diodes without a fine tip.
You must be able to solder these devices quickly so they are not
A fine tip will allow you to get close to the pin and make the solder
run under the pin.
The Audio Amplifier
HOW THE CIRCUIT WORKS
The output is push-pull and consumes
less than 3mA (with no signal) but drives the earpiece to a very loud level when
audio is detected.
The whole circuit is DC coupled and this makes it extremely difficult to set up.
Basically you don't know where to start with the biasing. The two most critical
components are 8k2 between the emitter of the first transistor and 0v rail and
the 470R resistor.
The 8k2 across the 47u sets the emitter voltage on the BC 547 and this turns it
on. The collector is directly connected to the base of a BC 557, called the
driver transistor. Both these transistors are now turned on and the output of
the BC 557 causes current to flow through the 1k and 470R resistors so that the
voltage developed across each resistor turns on the two output transistors. The
end result is mid-rail voltage on the join of the two emitters.
When setting up the circuit, the first thing you aim for is mid-rail voltage on
With a circuit such as this, the most important factor is stability. It is very
easy to create unwanted instability called "motor-boating" or
"self-oscillation" due to a signal (waveform) on the power rail being detected
by the front end and getting amplified to a point where the resulting amplitude
completely over-rides the audio you are tying to detect.
The 8k2 feedback resistor provides major negative feedback while the 330p
prevents high-frequency oscillations occurring.
IF IT DOESN'T WORK
If the circuit doesn't work, you have a
challenge on your hands.
The 4 transistors are DC coupled and you cannot isolate any of them to carry out
SM Parts Identification:
|BC 547 (with leads)
BC 847 (surface mount)
1A to 1M (approx) look for "1"
|BC 557 (with leads)
BC 857 (surface mount)
3A to 4C (approx) look for "4"
|1N 4148 (with leads)
BAS 16 (surface mount)
A6 look for "A6"
1 - 333k|
1 - 1m fine solder
1 - Audio Amplifier PC
Other components can be
bought from your local electronics store.
Kits for Audio Amplifier can be obtained from Talking Electronics: