Jack.Winslade@f666.n285.z1.fidonet.org (Jack Winslade) (06/01/90)
A few years ago, I had reason to construct a simple ringing generator which would signal 'real' telephone bells, chirpers, and cause modems to answer. I tried a few things and here's a brief summary. 1. 60Hz line voltage through an isolation transformer. This would ring the (500 series) real bells, although more faintly at a faster frequency. The chirper I tried would kind of squawk. The modem (Hayes 1200) would answer, but when I added some nominal current-limiting resistors, it would fail to answer. 2. My second idea will sound like a real kludge -- it was. I half-wave rectified the output of the isolation transformer, and then using a circuit consisting of an RC network and a SCR, I passed every third cycle to the load. (I know this sounds exotic but it does work this way.) I level-shifted the output pulse to reverse-bias the gate of the SCR so that two of three pulses were blocked. This resulted in 20 100-some volt pulses out to the load. The chirper sounded fine, the modem answered, but the 'real' phone bell only tinkled a bit. :-( There obviously was not enough energy in the 20Hz portion of the spectrum to make it work properly. 3. My third attempt was to build a very simple two-transistor multivibrator using a center-tapped filament transformer backwards. I pre-loaded the output and tuned it by ear. (Scope -- manually selecting capacitors that gave a fairly stable 20Hz output.) With a load resistor and despiking capacitor across the output, I had a reasonably clean 20Hz almost-square wave signal. This worked with all three devices. I still use this today in combination with a simple plugboard when I have to key modems when I am testing software. With all of this conversation about ringers, I thought it might be interesting to some of yu. --- Through FidoNet gateway node 1:16/390 Jack.Winslade@f666.n285.z1.fidonet.org