Theory Of Operation; Schematic Diagram - Elenco Electronics AK-100/SP-1A Assembly And Instruction Manual

THEORY OF OPERATION

The solder practice kit consists of a circuit oscillating
at one hertz (one cycle per second). The oscillator
consists of two transistors Q1 and Q2, and resistors,
R1 - R11 and capacitors C1 and C2.
configuration is known as a multivibrator circuit.
The solder practice kit produces the sound of the
European siren. It consists of two oscillators, a one
hertz (one cycle per second) and a 1500Hz. The one
hertz oscillator consists of two transistors Q1 and
Q2, and resistors R1, R2, R6 and R7 capacitors C1
and C2. This configuration is known as a
multivibrator circuit.
When voltage is first applied to this multivibrator
circuit, one transistor (possibly Q1) will conduct
faster, causing transistor Q2 to stay off. Q1 will
continue to conduct until it saturates. At this point,
Q2 will start to conduct, causing Q1 to rapidly cutoff.
This process continues alternately causing Q1 or Q2
to conduct. The output will be a square wave. The

SCHEMATIC DIAGRAM

frequency is determined by the time constants of
resistor R6 and capacitor C1, also R4 and C2. Two
LED diodes are placed in the collectors of the
This transistors. The LED's will light when current is
passing through them. Resistors R2, R1 and R7
determine the current passing through the LED's.
Integrated circuit IC1 is the heart of the second
oscillator. A 555 timer IC is used in the circuit. This
IC contains many transistors and resistors on a
silicon chip and thus eliminates many external parts.
The frequency of this oscillator is determined by
resistors R5, R9 and capacitor C4. Capacitor C3
couples the output of operations of IC1 via resistor
R8. This changes the operations of IC1 during one
half cycle of the multivibrator causing the frequency
to change from 1500Hz to 2200Hz. This results in a
speaker output that varies constantly in pitch. The
multivibrator circuit not only causes the LED to flash,
but also varies the pitch at the speaker output.
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