Per the 555 Timer Chip Data Sheet Operation Description: The 555 operational time values depends mostly on the ratio of voltage set-points and remains relatively free from most of the typical issues experienced with external rc networks and feedback type oscillators.
Before there was really low cost crystal referenced tone generators there was a number of both well done 555 DTMF and 555 CTCSS tone generation circuits that worked without a hitch. 555 output Square waves should look just fine when filtered for audio at the end of a simple low cost rc network. Working 555 examples include the ctcss generator shown on the sonic web page. The Heathkit DTMF mic available with their early 2-meter kit radios. The gazillion made Heathkit Code Practice Oscillators. One nice thing about the 555 chip is the available output easily drives low impedance loads quite well. Also makes a nice oscillator to generate voltages independent of the package supply. Some 70's vintage radios used the 555 chip to generate a negative rail supply for those early eprom, prom and op-amp circuits. Quite the problem solving work-horse device and it has a practical use as a pretty good audio oscillator. cheers, s. > The 555 has been used for many timer applications. However, it timing is controlled with and RC network and these are difficult to keep within any tight accuracy like that of DTMF encoding. Besides there are plenty of xtal controlled devices for this. > > And as you said the output is a square wave, not always desirable for audio. However, does make a pretty clock for digital circuits. There are many applications the 555 can be used for. > > 73, ron, n9ee/r > > > > > > > >From: skipp025 <[EMAIL PROTECTED]> > >Date: 2007/08/28 Tue AM 06:27:32 CDT > >To: Repeater-Builder@yahoogroups.com > >Subject: [Repeater-Builder] Re: subaudibe tones.. > > > > >> W.r.t. early homebrew encoders, I never really liked the 555 version > >> because of the non-sinewave output. Since the encoder need to cover > >> more than an octave, fixed filtering of any kind couldn't be used to > >> clean it up. > > > >You can find the 555 used as a pretty good tone generator in many a > >circuit, including a fair number of dtmf pads. If you look at the > >circuit on the sonic web page you'll notice the low pass filter, > >which works pretty well. Even with the values shown I found the > >described circuit puts out a lot more audio than required for a > >typical radio so after all the filtering you still had a high value > >resistor in series to knock the level down. > > > >> So I went with the XR2206 function generator which put out a nice > >> clean sine wave. Only other problem was frequency stability: > > > >Advantage to the 555, which by nature of design and operation is > >relatively immune to voltage and temp drift with more than a > >reasonable amount of change. > > > >> only many years later after spending much time & effort looking > >> for the most stable Rs & Cs did I discover when trying to build > >> a 1000 Hz sine generator that the chip temperature affects the > >> operating frequency. > > > >Never had much of a a problem with the 555 and the circuit is dirt > >cheap to build. > > > >cheers, > >s. > > > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. >
