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occur, the manual will lead you through step by step in the troubleshooting
guide until you find the problem and are able to correct it.
CIRCUIT DESCRIPTION
Most of the difficult work in this circuit is performed by the ISD1020 voice
recorder IC chip (U4) and the MC68HC705 series microprocessor (U3).
These two ICs "talk" (yes, pun intended!) to each other to access the different
addresses, modes, when to start playback and record on U4. Not only does
the microprocessor control this, but it also runs the two displays, scans the
switches, and provides for a user friendly interface - not bad for a little 16 pin
chip.
The pins PA5, PA6, and PA7 of the microprocessor do a number of tasks.
They not only select the address of where playback and record is to be done,
but at the same time drive U1 to display the information of what address and
mode is selected. PB0 selects which display is turned on. The two displays
are not actually on at the same time, but are switched fast enough by the
microprocessor so that they appear to the eye to be on continuously. PA0
through PA2 scan the switches for mode, play, and address. PA3 chip
selects the voice recorder, and through the pulse shaping circuit of Q7, resets
the ISD1020 IC chip.
The SP output of the ISD1020 is taken through volume control, POT1 and
then fed into the audio speaker driver U2 for amplification. Amplifier U2
provides 2 watts of audio output, capable of driving a speaker to excellent
volume. A simple keying detection scheme consisting of Q8, Q9, and
surrounding components allows sensing of an external microphone's push-to
talk switch. This circuitry simulates the pressing of the play button when there
is a resistive loading effect caused by the keying of the mike. The advantage
of this system is that it allows you to use your existing radio microphone to
activate playback and record.
PB1 on the microprocessor is the line that controls the keying of a connected
transceiver. When the voice recorder is activating your transceiver, decimal
points on the display are illuminated to let you know. Transistors Q1 and Q2
are connected as a Darlington pair to key the radio, and R7 is optional for
either a resistive loading type of keying, or a simple pull down scheme. VR1
is a three terminal voltage regulator to provide the +5 volts required by the IC
circuitry and the microprocessor.
CB-1 • 5
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