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BS1 CIRCUIT DESCRIPTION
In the BS1, as in all electronic devices, specific power and voltage
requirements exist. Although appearing quite simple at a glance, the power
supply of this unit provides three separate outputs (+12V unregulated, +5V, and
+3V regulated.) The +12V comes from an external source such as battery or
wall adapter and is filtered or "smoothed" by capacitor C21. D4 is a reverse
polarity protection diode which acts as a switch for the applied voltage. If proper
polarity is observed, D4 is forward biased and simply turns on like a switch.
However, if for some reason the applied voltage is reversed, D4 becomes
reverse biased and turns off, thus protecting all components. VR1 is a voltage
regulator which takes the raw +12V and provides a very clean +5 volts output.
D1, D2, and D3 together drop about 2 volts leaving a nice clean +3 volts
required by U2.
Most of the "magic" inside the BS1 lies within the ISD4004 IC (U2). This unique
chip manages all the recording, audio storage, and playback functions requiring
only address and control information from the microcontroller. An internal 8kHz
sampling frequency provides high quality voice reproduction. A four-wire serial
peripheral interface provides the means of data transfer into and out of the chip.
If the ISD4004 IC were the "heart" of the BS1, then the 68HC705K1
microcontroller (U4) would be the "brains" of the outfit. The micro takes inputs
from the pushbutton switches and through the use of an internal program,
processes this information in order to light the LEDs and send data to control
the voice chip. R15 and C18 with U4 set up an oscillator used for timing
purposes.
U1, the LM358 is a dual opamp by trade. One side (U1:A) is used as an active
low-pass filter with supporting components R9, C4 and R10, C10. A low-pass
filter is used to prevent any undesired high frequency noise from U2 and U4
from passing on to the audio amplifiers, degrading the audio output. U1:B is
used as an audio amplifier to increase the small signal of U2 to a line level
sufficient for driving a high power amplifier or to feed into a transmitter for
broadcasting. The LM386 is self contained audio amplifier which is used here to
amplify our audio up to about half a watt, which will drive the average sized
speaker to comfortable listening levels.
The on-board jumper J2 selects either the microphone or external line level
inputs for recording.
BS-1 • 5
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