January 9, 2022

“Digital Recorder Build – Part 2”

Continuing with Part-2 of the digital Voice Recorder build, we will go over the schematic of the overall layout, including the Interface board that will bring the connections we need to connect it to a radio transmitters microphone audio input and PTT.

#1

Originally I was going to build this into a chassis box with the front panel push buttons, etc.

However after quizing a couple of hams on what would be easier to understand, it was suggested that the project be wired up and connected to show it working and then let the builders put it into their own preferred box, whether it be aluminum or a plastic box with internal batteries for portable or operation in the field.

Here we can see that the Blue led is indicating that the message is running and that the relay is providing a PTT output to key the transmitter.

#2

The original plan was to have a radio microphone plugged into the front panel of the unit with a short pigtail going to the radio microphone input connector. The small relay that is used on the interface board provides switching between the different audio sources and provides a PTT function when ever the message is running.
The above wiring schematic will do just that. If the unit is going to be used as a audio source and connected to an accessory connector of a radio than the only connections that need to be made is to
ground, PTT and transmit audio in.

#3 ISD1820 Module external connections

The external switch connections are to actuate the PLAY sequence by grounding the GREEN wire momentarily.

By grounding the BROWN wire, the RECORD function is active. This connection needs to be grounded for the length of the message desired. This module has been set to 20 seconds record time.

The speaker output is taken from the WHITE 2-pin connector on the right of the screen. This goes to our interface board where a 1 to 1 Isolation transformer is used to separate the audio from the ISD chip and to the rest of the circuit.

This board doesn’t have a PTT function to key the transmitter, so a PTT switch is created by taking a DC voltage that is present on both speaker leads ( one side of the speaker is not grounded) and using a 2N4401 transistor we switch a relay giving the needed PTT connection and also switches the audio for us also.

In the metal chassis box I was going to use an additional audio amplifier and small speaker so that the message could be heard locally when ever it was running. The change of plans to the layout does not have that connected.

#4 Interface board

As you can see the Interface board is not complicated and easy to build on a piece of proto-typing PC board material.

The potentiometer is for adjusting the level to the transmitter so as not to not create a distorted signal.

The TQ2SA relay is a 4.5 volt relay, perfect for using a battery pack while operating in the field.

The isolation transformer is shown on the right.

The 2N4401 transistor is visable and the Blue Led. The layout is not critical.

#5 Connections

Video of the waveform after the relay to the radio Microphone input

I have put together a kit of all the parts needed to build this project, Minus the box or enclosure.

Everything is included to build what you see in photo #1 (even the battery box!) and can be had for $15. I can bring it to the meetings on Tuesday.

Please contact me if you have any additional questions about this project. The main ISD1820 Module is already built and tested so it will work without too much effort.

DE WB6AMT

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