12-14-2017, 12:25 PM
Here's the ringer:
https://imgur.com/a/EUOcj
It won't take you too many guesses to figure out what it does. The question riding your mind is why it takes so many chips to do it. The answer to that question lies in the primitive past.
The first telephone systems were developed in the 19th century, and thus they predate electronics by 30 to 40 years, depending on where you mark the beginning of the electronics age. Early systems were built from devices of two main types:
(1) Devices that exploit the electromagnetic phenomenon of magnetic fields induced in coils of wire, e.g., transformers, relays, and magneto generators.
(2) Devices that use little buckets of electrolytes to store an electrical charge, e.g., batteries and capacitors.
All of the signaling and voice transmissions were carried on the same pair of copper wires. This introduces a lot of complications when interfacing modern electronic equipment with the phone system. To make matters worse, an in-depth understanding of the system was the sole province of telephone men for many decades, and not generally disseminated in the larger engineering world. Consequently, sellers of modern telephone equipment usually incorporate ready-made interfaces into their products. That's where the subscriber line interface card (SLIC) comes in. That's the little circuit board on the right in the photo. It breaks out all of the voice and other signals into separate connections that our modern systems can easily deal with.
To ring an old phone, the telephone company puts a 90 volt, 20Hz to 25Hz AC signal on the line. It's 90 volts because it takes quite a bit of current to charge the electromagnetic coils that move the clapper that strikes the bells. (Modern phones with electronic noise makers for ringers process this signal in a different way, but it's still the same old high-voltage signal). The SLIC I'm using can (supposedly) generate enough DC current to do the job. However, we need to send a small AC signal to it to convert the DC current into an AC signal. There will be 8 SLICs in the system I'm building, one for each telephone set. Each SLIC has two inputs to ring the phone. One pin sets the SLIC to ringing mode. The other pin takes the aforementioned AC signal.
Now back to our ringer circuit. The little chip at upper right is a 555 timer that can be used to generate AC signals. Using our buddy Jeff's handy 555 calculator, we can figure out what components we need to add to generate the desired 20Hz to 25Hz signal. The calculator offers several choices for the desired frequency. I wanted one with a duty cycle as close to 50% as possible so the clapper will spend equal time hitting each of the two bells in the ringer. I found that plugging 22Hz into the calculator offers the best compromise.
The two chips on the left, above the voltage regulator, are 8-to-1 demultiplexers. They work about the same way as the line selector in our tone generator circuit, except they send a digital signal instead of an analog signal. One is used to select which SLIC will to be set to ringing mode. The other sends the digital square wave AC signal to the other pin on the selected SLIC.
That's my story, and I'm sticking with it.
https://imgur.com/a/EUOcj
It won't take you too many guesses to figure out what it does. The question riding your mind is why it takes so many chips to do it. The answer to that question lies in the primitive past.
The first telephone systems were developed in the 19th century, and thus they predate electronics by 30 to 40 years, depending on where you mark the beginning of the electronics age. Early systems were built from devices of two main types:
(1) Devices that exploit the electromagnetic phenomenon of magnetic fields induced in coils of wire, e.g., transformers, relays, and magneto generators.
(2) Devices that use little buckets of electrolytes to store an electrical charge, e.g., batteries and capacitors.
All of the signaling and voice transmissions were carried on the same pair of copper wires. This introduces a lot of complications when interfacing modern electronic equipment with the phone system. To make matters worse, an in-depth understanding of the system was the sole province of telephone men for many decades, and not generally disseminated in the larger engineering world. Consequently, sellers of modern telephone equipment usually incorporate ready-made interfaces into their products. That's where the subscriber line interface card (SLIC) comes in. That's the little circuit board on the right in the photo. It breaks out all of the voice and other signals into separate connections that our modern systems can easily deal with.
To ring an old phone, the telephone company puts a 90 volt, 20Hz to 25Hz AC signal on the line. It's 90 volts because it takes quite a bit of current to charge the electromagnetic coils that move the clapper that strikes the bells. (Modern phones with electronic noise makers for ringers process this signal in a different way, but it's still the same old high-voltage signal). The SLIC I'm using can (supposedly) generate enough DC current to do the job. However, we need to send a small AC signal to it to convert the DC current into an AC signal. There will be 8 SLICs in the system I'm building, one for each telephone set. Each SLIC has two inputs to ring the phone. One pin sets the SLIC to ringing mode. The other pin takes the aforementioned AC signal.
Now back to our ringer circuit. The little chip at upper right is a 555 timer that can be used to generate AC signals. Using our buddy Jeff's handy 555 calculator, we can figure out what components we need to add to generate the desired 20Hz to 25Hz signal. The calculator offers several choices for the desired frequency. I wanted one with a duty cycle as close to 50% as possible so the clapper will spend equal time hitting each of the two bells in the ringer. I found that plugging 22Hz into the calculator offers the best compromise.
The two chips on the left, above the voltage regulator, are 8-to-1 demultiplexers. They work about the same way as the line selector in our tone generator circuit, except they send a digital signal instead of an analog signal. One is used to select which SLIC will to be set to ringing mode. The other sends the digital square wave AC signal to the other pin on the selected SLIC.
That's my story, and I'm sticking with it.
