Invention Lab — Powered by RadioShack®

Throughout the years, I’ve built a lot of offbeat things from found objects and RadioShack parts. I wish I had taken pictures of the telephone answering machine I built in the late 70’s. It took up an entire table and was made from:

(2) RadioShack portable cassette recorders
(2) RadioShack Perf-Box VOX kits(2) RadioShack 4PDT relays
(1) RadioShack suction cup telephone pickup
An old telephone handset
An old telephone ringer
An 8 ohm transistor radio speaker
Various RadioShack plugs, jacks, wire and power supplies

Refresher: According to Wikipedia, “In telecommunications, a voice operated switch, also known as VOX or Voice Operated eXchange, is a switch that operates when sound over a certain threshold is detected. It is usually used to turn on a transmitter or recorder when someone speaks and turn it off when they stop speaking.” (Who can forget, “The Clapper”? Clap On! Clap Off!)

Each RadioShack VOX kit came with a very simple microphone and a SPDT (Single Pole Double Throw) relay so you could either turn something on or turn something off by producing a sound of a certain volume. That “sensitivity” level was adjustable by a potentiometer on the perf board. If I recall correctly, another pot was used to set how long the relay would stay energized after the sound level dropped below the sensitivity threshold. I rigged each VOX to drive a 4PDT relay, so I could control up to four things per VOX.

Each tape recorder came with an external mic input, an external speaker output and a remote control socket that could be used to start or stop the machine. In my answering machine, both machines were under remote control, so if there was no call coming in, they were both idle. In Tape Machine 1 (the “player”), I used a continuous loop cassette of about 30 seconds, typical for answering machines of the day. Machine 1 was set to “Play”. Tape Machine 2 (the “recorder”) had a regular cassette and was set to “Record”.

Here’s how it worked:

When a telephone call came in, the ringer would ring. VOX 1’s mic was close enough to the ringer to trigger it. VOX 1’s 4PDT relay would perform the following:

- Take the phone “off hook”, stopping the ringing and enabling the handset to send and receive sound
- Start Tape Machine 1 (play outgoing message)
- Arm Tape Machine 2 (ready to record)

The output of Tape Machine 1 was sent to the external speaker which was positioned in close proximity to the mouthpiece of the handset. Thus, the outgoing message could be heard over the phone by the caller. Also close to the external speaker was VOX 2’s mic. After the message played, a very loud tone on the tape followed. (In my case, the tone was actually generated during the recording of the outgoing message by a Perf Box kit electronic organ!) The tone had to be louder than the outgoing message so VOX 2 could be adjusted to be triggered only by the tone.

VOX 2’s 4PDT relay would:

- Stop Tape Machine 1 (outgoing message)
- Start Tape Machine 2 (record caller’s message)

The caller would more than likely begin with, “Man, that tone is really loud”, followed by the actual message. The caller’s voice was picked up by the suction cup coil on the earpiece, which was plugged into Tape Machine 2 (the “recorder”).

When the caller was through and hung up, VOX 1’s mic – which also had to be close to the telephone earpiece - would pick up the absence of sound and, after a few seconds:

- Hang up the phone
- Disarm and stop Tape Machine 2

VOX 2 – sensing no sound – would also reset, arming Tape Machine 1, ready for the next call.

To listen to your messages, simply unplug the remote control cable from Tape Machine 2, hit “Stop”, then “Rewind” and then “Play”. Before leaving, remember to leave the tape where you want new messages to be recorded, plug the remote control cable in and press “Play” and “Record” together.

The positioning of the mics and speakers, the relative volume of the outgoing message and the “tone” and the “sensitivity” and “release” adjustments of both VOXs made this a very finicky device. Besides taking up so much room, it also had to be in a relatively quiet environment or it might get accidentally triggered. I used it successfully for a few years – putting off the purchase of my first RadioShack TAD answering machine. Finally, one of the capacitors gave out and I couldn’t trouble shoot it. It was a fun and useful project. Given the time and resources, I might build a replica one day! If you do, please send me a picture.