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"Do Not Disturb" Telephone Timer

It takes your phone off the hook and automatically hangs up again after a preset time. A LED indicates when the phone is off the hook.

By John Clark

  • Start and Finish timer facility
  • LED indicator shows off-hook staus
  • Powered from phone line
  • Four time periods plus indefinite

There are times when you simply don’t want to be disturbed by anything, let alone a telephone – especially by that incessant "ring ring... ring ring... ring ring..."

Of course, the person at the other end of the line doesn’t know that you are, well, umm, let’s just say you’re otherwise engaged.

But you can imagine such times, can’t you?

Click for larger image
This almost-larger-than-life photograph shows the completed project. The leads emerging top and bottom plug into the phone and wall socket. It doesn't matter which way around they go.

What do you do? Take the phone off the hook? Sure, that works – until a couple of hours/days later you start to wonder why no-one’s ringing you.

What you need is either something to remind you to put the phone handset back on the hook – or better still, do it for you so the process is automatic.

And that is exactly what the SILICON CHIP "Do Not Disturb" Timer does.

Even better, it does it without you lifting the handset in the first place: you simply set the time period you’re going to be "busy" – whatever that is – and press the start button. The phone then effectively goes off-hook (ie, anyone ringing will get an engaged signal) until the time period is complete. The phone is then restored to its "normal" (ie, on-hook and waiting for calls) state.

As far as time periods go, you have everything from a sprint to a marathon – 7.5 minutes to a whopping two hours. And if the reason for your not wanting to be disturbed ends prematurely you can hit the "hang up" button at any time.

You can even make the off-hook time period indefinite by not selecting a time period. If that sort of sounds like you’re defeating the purpose for making the device in the first place, it’s a great little security feature if you don’t want anyone else to use the phone while you’re away from it – especially if the little box is hidden!

Speaking of little box, the timer is housed in a small plastic case with a 6P6C telephone socket located at each end. It has a 4-position DIP switch and two tiny pushbutton switches poking through the box top – one to start the timer and one to stop it manually. A LED indicates when the phone is off-hook.

Installation is simple: you just unplug the telephone line socket from the phone, plug it into the timer, then plug in a modular lead from the timer to the phone.

In case you’re feeling a sense of deja-vu, yes, we have described a similar device before – back in July 1992, in fact. But that used an LM3909 timer and alas, those devices are no more. Hence this new, improved model!


This Telephone Timer is NOT an Austel-approved device. The penalty for using a non-approved device, if detected and subsequent prosecution took place, could be a heavy fine, up to $10,000.

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