Lab Notes: 26/07/18

ISD1820 Voice Recorder Module

Recently I returned to the ISD1820 Voice Recorder Module schematic I’ve been in the process of designing. I decided to test the circuit on a breadboard, results were mixed. I found it was quite straightforward to add an L78L05 voltage regulator to the design as shown below:

Screen Shot 2018-07-26 at 15.26.42

The L78L05 datasheet suggests placing capacitors between both the input and output pins and ground, presumably for decoupling the power rails and subsequently reducing high-frequency noise that might be harmful to ICs. A more detailed explanation of capacitor applications can be found at the Sparkfun website.

Screen Shot 2018-07-26 at 15.28.18

Image source: https://www.mouser.co.uk/datasheet/2/389/l78-974043.pdf

Unfortunately, the rest of my circuit proved problematic. I had attempted to implement a PN2907A PNP transistor to automatically switch between mic and line inputs when a mono jack is connected to the audio input socket. This failed in a rather pleasing manner! All that I was able to record was high frequency distorted noise that wouldn’t be out of place in some of the techno tracks I listened to when I was younger. Still, I was hoping for a nice clean signal for this particular project, which I certainly wasn’t achieving. I decided to strip the circuit back and review and test each part piece by piece. A review of how PNP transistors work proved useful. I discovered that diagrams representing the configuration of the collector, base and emitter can sometimes be inconsistent in their layout. In the examples below you can see the emitter being shown as the top pin on the first diagram and the bottom pin on the second. Perhaps a minor point, although, it still led me to some confusion.

PNP-transistor-biasing

Image source: http://www.learningaboutelectronics.com/Articles/Difference-between-a-NPN-and-a-PNP-transistor

Screen Shot 2018-07-26 at 09.35.32

Image source: https://cdn-shop.adafruit.com/product-files/3598/PN2907A-D.PDF

Further difficulties were encountered in recording audio with a simplified version of the circuit. In fact, the circuit I tested was the one I originally followed successfully from the Bastl site. The audio was very distorted when being played back. The ISD1820 I was using originally came from a kit purchased from a Maker Faire. As such it came on a small PCB, complete with buttons for each function (i.e. Play and Record) and a speaker. The Voice Recording Module was working absolutely fine on this PCB. I began to wonder the benefits of designing a new board from scratch, particularly as the PCB provided includes pins for all of the parameters I hoped to control.

With deadlines looming, I’ve decided to simplify the overall design. I’m keen for the finished device to be used in a workshop by people interested in music but who may not necessarily have a technical background. It would be fantastic if the workshop participants were able to be actively involved in the design of the devices they are using. For this reason, I’ve decided to concentrate on capacitive touch control of the Voice Recorder Module parameters, this way there is potential for users to design their own interface using either copper tape or conductive paint.

My intention when starting this project was for all sensors to be controlled and processed by ATTiny85 microcontrollers. I’ve had some success with this, with different sensors and controls. It should be fairly straightforward to implement a touch switch using an ATTiny85. The main area that might be a challenge is that of calibration to ensure that false triggers aren’t encountered.

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