The solid state voice recorder module published in the May 2005 issue of
SILICON CHIP proved to be very popular.
It has been used in all sorts of applications where messages or sounds needed to
be recorded and played back reliably under either manual or micro control.
But it had limitations – one of which was that it could only be
used to record and play back one long message or a number of short messages in
sequential ‘tape recorder’ fashion.
This was despite the fact that the recorder chip we used was
capable of recording and playing back up to eight messages in ‘random access’
mode. The module needed a fair bit of ‘hacking’ to make the chip work in this
Another limitation was that the playback sound quality of the
module was fairly noisy and each message played back was accompanied by an
irritating ‘click’ at the start and finish. With the benefit of hindsight this
was due to the way we had chosen to couple the output audio from only one side
of the recorder chip’s push-pull output.
Fig.1: above is the block diagram of the HK828 voice recorder chip. While the recording process relies on audio sampling, the audio is not stored digitally but using an analog sample-and-hold system. The analog samples are stored in the cells of a 256K flash EEPROM. Each analog storage cell can store any of 256 different voltage levels, making it equivalent to an 8-bit digital recording.
Hindsight also revealed a third limitation: the 2005 module had
been designed to operate from a 6V battery, whereas many people wanted to use it
from a nominal 12V DC source.
It was with these limitations in mind that we decided to
develop the new and improved sound recorder module described here.
It’s based on the same HK828 chip used in the 2005 module but
with the rest of the circuit designed to allow more flexibility in terms of
message storage and to provide much cleaner and click-free playback audio.
Finally, the new circuit can run from any source of DC between 9V and 14V.
The HK828 chip has the ability to store single or multiple
messages with a total length of between 40 and 60 seconds, depending on the
sampling rate and the voice quality you want.
In this new recorder module the chip is again teamed up with a
low-cost electret microphone to allow easy message recording, plus an LM358 dual
op amp IC which allows the recorded messages to be played back as a line level
audio signal available for feeding an external amplifier and speaker.
A suitable small amplifier for use with the module would be
"The Champ", as described in the February 1994 issue of SILICON CHIP. This is
available at low cost ($5.95) from Jaycar Electronics as KC-5152.
We’ve given the new module a set of ‘jumper links’ so it can be
easily configured to record and play back messages in any of four modes: either
two, four or eight messages in random access mode or one or more messages in
sequential access ‘tape mode’. Another link allows the HK-828 chip’s message
start ‘beeps’ to be enabled or disabled, as you wish.
All message selection, record and play functions are controlled
externally, by connections to a row of screw terminals along the side of the
module. All functions are enabled by switches or logic signals. This makes it
easy to record or play back messages using a set of pushbuttons and a switch or
under the control of a PC, microcontroller or security system if you prefer.
By the way, since the HK828 voice recorder chip is only
available from Jaycar Electronics in Australia and New Zealand, kits for the new
recorder module will only be available from Jaycar and its