Analog Telephone Off-Hook Detector

A-Lead Control)

I spent over 30 years as a telecommunications tech. I worked for a large hospital and when I started, they had a Mountain Bell 300 line stepper system with a manual cordboard (switchboard) with two operators.

Shortly after I started, they contracted to have an ITT TE-400G electronic pbx installed and I was trained to maintain it and all of the 1A2 key systems attached to it. It basically replaced the mechanical stepper switch with one based on dedicated ttl logic circuits.

We outgrew it after a few years and installed a Northern Telecom (Nortel) SL-1 system. We reused all of the analog phones and key systems for a number of years and as budget allowed, eventually migrated to electronic sets. All of the key systems were phased out over a period of about 10 years from when I started.

The Nortel switches were pretty cool as they continually offered software and hardware upgrades to add more features. We had just started migrating it to VoIP when I left.

My retirement didn't last long as I was talked into teaching computer science classes at the local community college. I was instrumental in getting a class in VoIP implemented and we used Asterisk on a Linux host to build entire telecom networks in class.

Off Hook Detection
From my 1A2 key systems days of installing and maintaining analog telephone systems, I remembered that if you needed to place a single line phone on a key system line shared by multi-line phones, you needed to change the hook switch wiring to provide A-Lead control or else the single line phone might put the line on hold when you hung up and the light would not operate on the key phones indicating the line was in use.

Some phones did not have that extra set of contacts to provide that capability such as answering machines, credit card machines, modems, etc. If you wanted to share a key system line with them you needed an external adapter to detect when the line was seized and make a contact closure on the A-lead pair to the KSU.

One such device was the Melco S64-1 Loop Detector. It was a solid state device that you hard wired between the KSU and the external device. When the device seized the line, it operated the A-Lead control pair to light the lamps indicating that line was in use.

I recently decided to make some of my old 3 slot payphones more realistic by making the coin return functional. In order to do this, I needed a way to detect when the pay phone was off hook. I searched through my old phone stock and actually found a new Melco S64-1 in its original box. After analyzing the circuit, I determined that it would do most of what I needed but I needed to operate the coin relay on a higher voltage (100 VDC) and be able to reverse the polarity depending on whether I wanted it to collect or refund the money.

So based on the circuit of the Melco S64-1, I am designing a dedicated circuit board to make that function including the voltage quadrupler to raise 24 VAC up to the required 100 VDC to operate the coin relay. My goal is to have the payphone completely stock and not modified in any way.

Here are some images of the Melco S64-1 circuit and its existing board layout. This is the starting point for my own design for the payphone controller. Donít be confused to see that ground on the Melco unit is marked +24 VDC. Telephone systems, like early British cars used a positive ground system! Note that Ring-In and Ring-Out are just a loop through and do not make any connections to the circuit.

Basically the circuit is just a dual opto-isolator in series with a 22 ohm resistor, some diodes and caps that go between the phone device and the line. When the phone goes off hook, it draws current through the opto-isolator which triggers the transistor pair to provide 24 VDC on the A-lead wire to the KSU. This lights the lamps on the key phones to indicate that the line is in use. When you hang the phone up, the tip connection drops first, then the A-lead connection. If you drop the A-lead connection while the tip is still up, it would put the phone on hold. I am assuming that the 1000 mfd capacitor are there to prevent pulsing during rotary dialing and to provide a slow release so the A-lead connection drops last.

Once I get the rest of the circuit figured out, I will post the schematic and cad drawings for the complete unit.

This is a cad version of the layout of the existing board.