8 Tone generation

Every decent telephone system needs to give feedback to it's user as to it's current operational status. This has been done since time immemorial by using progress tones. I only need three types of progress tones; a steady tone for showing the exchange is ready for a call, a ringing cadence progress tone, usually two short bursts of tone separated by a second, and a "fast busy" cycling tone to show a false number has been dialed. So what we will need is some way to generate these tones and have them fed to the user operating the phone. 

I'm sure there are very many ways to generate a simple sine wave but I have a job lot of NE555 timer IC's so that's where I'm going to start. Using a NE555 timing calculator (this one is my favorite as you can enter the desired frequency and it will generate the options for the resister/capacitor combination to achieve it).

I'm not too concerned about the cycle lengths since I'm going to feed both outputs from the NE555 to a dual D-Type flip-flip which will "flatten" out the 555 square wave to a nice, regular square wave. This being the case, I want two wave forms, one at 400Hz and one at 420Hz, which when combined should be quite easy on the ear. The flip-flip needs double the input to generate the square waves at 400Hz and 420Hz, so set the NE555's to generate a wave of 800Hz and 840Hz. The picture shows the components of the Veroboard, two timers and the D-type.


A quick check with the scope shows all is good. Next I need to take the outputs, convert them to a sign wave, combine them and amplify them. I use a series of three resistors and capacitors on each output from the D-type to shape the square wave to a sine wave. The trade off here is it's going to vastly reduce the signal level so I use a dual op-amp configured as a voltage follower for each wave. Another op-amp is used to combine the signals and a final op-amp to amplify the signal slightly, about 1.5 times. 

I use my all time favorite ICL 7660 CPAZ voltage converter to generate the positive and negative power rails needed for the two op-amp IC's. When I do finally listen to the signal over a phone it's perhaps a bit high, sounds more like a flute than a dial tone, but that's ok, so long as the signal is clear and it's there, that will do me. It is, however, something of a moot point that I later find that the DTMF transceiver MT8880 I'm planning to implement to handle the DTMF codes to interface to house PABX can generate single tones and that would have worked perfectly for a dial tone. However the lowest frequency I can generate from the IC is 697Hz, so perhaps would have been a bit high any way. 

Here is the completed circuit, from left to right, voltage converter and two op-amps. The two cluster of capacitors to the right make up the square to sine array. The wave shape will feed to the other side of the source line isolation transformer via a relay to control the tone cadences.

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