DECEMBER 26, 2021

“Building a Digital Voice Recorder – Part 1” 

Due to not receiving the chassis for this project as of December 25, I will present it as a Two-part project.

Part 1 – This week I will cover the electronics and costs of this endeavor.

Digital voice recorders for ham radio use have been around for quite some time both as a software application to run on your PC or as a standalone unit.

Standalone units can get pretty expensive and do fill a need though.

If you aren’t the home brew building type then they are great to use for that long weekend of operating a contest and saving your voice.

#1 MFJ Voice recorder ($269.95 Multi message unit)

However if you want to spend a little quality time with your soldering iron, and a whole lot less money (about $10.00 less your cabinet) then the ISD1820 Voice Recorder module may be the way to go.

#2 ISD1820 Voice Recorder Module (Qty 2 – $7.49 by HiLetGo on Amazon) delivery was the next day.

This module has been around for years and is very reliable and reproduces a good clean audio signal.

This module board is based on ISD1820PY, which is a single-chip single-message record/playback device.

Recordings are stored into on-chip non-volatile memory, providing zero-power message storage. With the embedded Flash memory employed, data retention up to 100 years and typical 100,000 erase/record cycles can be reached. Time for recording is 8-20 seconds. A lot of the interfacing to the ISD1820 integrated circuit has been already been done for you on the printed circuit board, leaving a few connections that will give you an adjustable audio level control and a transmitter PTT function via a transistor output or a small relay if you so desire a dry set of contacts for the PTT to the radio.

Features
– Push-button interface, playback can be edge or level activated
– Automatic power-down mode
– On-chip 8Ω speaker driver
– Signal 3-5.4V Power Supply
– Can be controlled both manually or by MCU
– Sample rate and duration changeble by replacing a single resistor
– Record up to 20 seconds of audio
– Dimensions: 37 x 54 mm

If you want to change record duration, an external resistor is necessary to select the record duration and sampling frequency, which can range from 8 – 20 seconds (4-12kHz sampling frequency). The Voice Record Module has a default 200k resistor through P2 by shorting jumper. So the default record duration is 20s.

#3 There are 3 keys on the board: REC, PLAYE and PLAYL

1) REC
The REC input is an active-HIGH record signal. The device records whenever REC is
HIGH. This pin must remain HIGH for the duration of the recording. REC takes precedence
over either playback (PLAYL or PLAYE) signal. If REC is pulled HIGH during a playback
cycle, the playback immediately ceases and recording begins. A record cycle is completed
when REC is pulled LOW. An End-of-Message (EOM) marker is internally recorded,
enabling a subsequent playback cycle to terminate appropriately. The device automatically
powers down to standby mode when REC goes LOW. This pin has an internal pull-down
device. Holding this pin HIGH will increase standby current consumption.

2) PLAY
There are two modes to play the voice in the voice chip: edge activated mode controlled by
PLAYE pin and level activated mode controlled by PLAYL pin.

(1) Playback, Edge-activated: When a HIGH-going transition is detected on this input pin,
a playback cycle begins. Playback continues until an End-of-Message (EOM) marker is
encountered or the end of the memory space is reached. Upon completion of the
playback cycle, the device automatically powers down into standby mode. Taking
PLAYE LOW during a playback cycle will not terminate the current cycle. This pin has
an internal pull-down device. Holding this pin HIGH will increase standby current
consumption.

(2) Playback, Level-activated: When this input pin level transits from LOW to HIGH, a
playback cycle is initiated. Playback continues until PLAYL is pulled LOW or an End-of-
Message (EOM) marker is detected, or the end of the memory space is reached.

The device automatically powers down to standby mode upon completion of the playback
cycle. This pin has an internal pull-down device. Holding this pin HIGH will increase
standby current consumption.
On the board there are two switches for Feed Through function and REPEAT.
(1)Feed Through: This mode allows use of the speaker drivers for external signals. The
signal between the MIC and MIC_REF pins will pass through the AGC, the filter and
the speaker drivers to the speaker outputs SP+ and SP-.

The input FT controls the feed through mode. To operate this mode, the control pins REC, PLAYE and PLAYL are held LOW at Vss. The pin FT is held HIGH to Vcc. For normal operation of record, play and power down, the FT pin is held at Vss. The FT pin has a weak pull-down to Vss

(2)REPEAT. If this switch is on, the current voice clip will be played back repeatedly.

#4 Schematic

By looking at the schematic, its a pretty straight forward little module lending it self to experimentation for any type of need of a repeatable message. (Note-doorbell)

In part 2, I will go into how we can stack multiple units to give us a multi-message unit similar to the high dollar brands without the high dollar price tag. Using a LM386N audio amp as a audio mixer from several ISD1820 modules w/push button switches.

Fabrication of the switches and circuit boards into the chassis (if it comes this week) and connecting to your radio will be covered.

See everyone next week…

DE WB6AMT EARL

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