Datasheet

Record Playback Essentials
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How the Record Playback Module Works
The main component in the circuit is the ISD1820, which is a Record Playback IC. This is the main boxed section in
the block diagram above. During the record phase, the chip amplifies the signal from the microphone and digitizes
this allowing the recording to be stored in memory. This memory is non-volatile, which means that the information is
retained even when the power is removed. During playback the data is taken out of the memory, converted back
from a digital signal into an analogue signal which is then amplified before it is output to the speaker.
The timing control section of the chip uses a resistor / capacitor network to set how fast the data is stored or
retrieved from the on board memory. The timing resistor (R5) along with an internal capacitor sets the record /
playback time to 10 seconds. This can be adjusted from 8 seconds to 16 seconds, however the longer the record time
the worse the quality of the audio as the sample rate is reduced.
The device control block checks the state of both the play switch and the record switch and either plays back the
current message or records a new message.
The device control block also turns the LED on to show that recording is in progress or that playback has finished.
When neither record nor playback is in progress the device control block puts the whole unit into sleep where it
takes virtually no current, thus allowing the battery to remain connected when the device is not in use.
The switches SW1 & SW2 are connected to the positive supply and the IC. There are internal pull down resistors for
both inputs inside the IC. A 1nF capacitor (C1) is present on the record line to remove any switch bounce that could
cause a brief re-record to take place as the switch is released at the end of recording. The status LED requires a
current limit resistor (R1) and there is a 100nF capacitor (C4) connected across the power supply to make sure it is
smooth. All the remaining resistors and capacitors are used to power the microphone and filter the audio from it.
Amp Amp
Device control
A2D D2A
Non-volatile
Memory
Timing control
Play switch Record switch
ISD1820
Microphone
Speaker
Status LED
Timing resistor