Thursday, October 30, 2008 20 Second Recorder

We needed an audio source that was recordable, repeatable and could be remotely triggered. Our first thought of hacking a CD or MP3 player seemed impractical at the time based on cost.

Radio Shack's 20-Second Recording Module proved an affordable solution at $11. The unit is a small circuit board that comes with an attached 16-ohm speaker (for recording and playback), tiny circuit board mounted record switch and 9-volt battery clip.

The idea is to push and hold the record button (LED confirms status), speak loudly into the speaker then release record button. One push of the play button on the main board provides tinny playback.

To make this low-fi device useful, we need to:

  • tap the play button with two wires to trigger play
  • replace the speaker with a mini jack to connect to a PC (recording) and an a mp (playback)
  • attach power wires and wean this thing off 9-volt batteries
  • fit the whole shebang in some kind of protective box.

A little experimenting proved that the recorder operated fine from 5-volts. We will be using a PC power supply to power a 12-volt prop, so we already had 5-volts with nothing to do. Cutting off the 9-volt battery clip and splicing a length of wire on each lead takes care of power. The wire attached for power and ground will be about 18" long and should provide for many splices into different applications.

The play button can be visually traced. One pole connects to ground near the power connection, and the other pole connects to the switch side of resistor R3. So, a length of small gauge wire soldered to each location provide remote trigger inputs.

The mini jack connector that I had was salvaged from a PC. It was a stereo-mini and although this is a very mono device I soldered both positive leads from the stereo-mini to the positive speaker output of the board. The braided shield from the stereo-mini jack was soldered to the negative speaker output of the board. The connection nearest the "SPK" silkscreen was identified as positive based on a "+" marking on the speaker terminal.

An old Altoids tin seemed an appropriate enclosure. Three holes drilled in the sides and she was good to go. The stereo mini jack was threaded and mounted with a panel nut. I cut a grommet in half to protect the power and trigger wires where they passed through. The circuit board is secured by four big globs of hot melt glue. These adhere it in place and stand it off the tin a bit to eliminate any chance for shorts. Hot melt to each wire pass through provided a bit of stress relief and insulation.

At the last minute we extended the record switch wires so that it could be mounted to the lid. This keeps it protected through normal use (an accidental push erases your audio!), but still handy for recording.

The few modifications are simple and do improve functionality. Mounting it in a tin seemed to reduce the unit's susceptibility to interference. This had caused some erratic triggering from nearby devices, most notably a fog machine.

Practical application finds our little Altoids box hooked up to a PC power supply for 5-VDC, with the trigger wires extended to a relay mounted inside an animated prop, and its stereo mini output connected to a car amplifier driving two 6X9 speakers. What we lack in audio quality can be made up in volume.

In the end the results are satisfactory. Experimenting with PC level setting when recording the audio produced ample output volume. Sound quality is poor - think telephone - so selecting a simple audio track with limited or compressed frequency range gives better results. It is useful enough that we made two.

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