A surface mount project . . .

Other surface mount projects:
Phone Ring
Infinity Bug

P1   P2   P3

This project is called "mini" because its size is small and the output is small.
It uses surface mount technology and this will be a challenge for most constructors.
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 overheated. A fine tip will allow you to get close to the pin and make the solder run under the pin.




The Audio Amplifier circuit

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 the emitters.
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 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 individual testing.




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"


Audio Amplifier (mini)

1 - 333k
1 - 1m fine solder

1 - Audio Amplifier PC board
Other components can be bought from your local electronics store.

Kits for Audio Amplifier can be obtained from Talking Electronics: