Automatic Answer Device (AAD)

An Advanced Project  - Corrections 28 Mar 08


This design is based on my United States Patent 4,527,015, here. It will answer a ringing telephone line, provide a contact closure or two and an isolated audio path. Disconnect is by CPC (Calling Party Control). This is a battery interrupt that occurs when the calling party hangs up. This must be present or the device will not disconnect. Generally speaking all Central Office lines have this, and PBXs do not. This is so important that you will need to build a simple tester and verify that you do get CPC on the line you want to attach this to.


Typical uses of the device are to provide an audio path to some source. Perhaps you have a speaking clock, or a message you want to play to the caller. NOTE: Rebroadcasting of radio stations, TV stations or recorded music is covered by copyright laws. Don’t do anything that might violate any of these laws or listen in to another phone line or monitor a room to hear conversations.


The picture on my Log page shows me drawing the schematic for this for several friends. They have some sort of custom chime they want people to be able to dial up and hear.  Because there is no timer in this circuit, the caller can listen as long as they want. You could add your own timer and force a disconnect by interrupting the telephone line allowing the SCR to release the line.


This is an advanced project, and you should be skilled in the art of electronic construction. Look at my page on Repeat Coils and decide which transformer you will use, either will work just fine.


First Build this:



Call the phone line you want to use the device on, connect the LED Tester to the phone line, one of the LEDs will light. Hang up the phone you placed the call from. The LED should blink when you get the CPC signal, this may take up to 20 seconds. For the Automatic Answer Device to work, you must get this CPC signal.





C1 - 0.1 MFD 250 VNP – DC isolation for the AC ring signal , 20 Hz

C2 - 0.022 MFD 250 VNP – RF suppression

C3 – 1 MFD to 2.2 MFD 250 VNP – Audio coupling to external circuit

C4 – 2.2 MFD 25 Volts – Audio Bypass & Back EMF suppressor

C5 – 10 MFD 100 Volts – Charges with HV from AC ringing

C6 – 0.022 MFD 250 VNP – RF suppression and transient suppression


R1 – 10 K – AC ringing resistor

R2 – 100 Ohms – Better audio matching on low Z

R3 – 100 K – Ring delay, can be up to 2 megs

R4 – 68K – Pulse stretcher (Corrected)

R5 – 68K – Keeps gate at a low Z to prevent stray pickup (Corrected)


CS – Click Suppressor & audio limiter or 4 each 1N4004

BR – 400 PIV 1A – Bridge rectifier or 4 each 1N4004

PC – Photocell – 1 Meg minimum dark resistance

TR – 600/600 Ohm Transformer – TU-016-RC or MP-305P

SCR – Silicon Controlled Rectifier – 200 PRV 4 Amp

Rly – 9 or 12 Volt Relay – maximum resistance 300 ohms (Corrected)

NE 1 – NE2 – Neon Lamp, C2A or similar



Theory of Operation


Ring Up:


 A neon lamp across the telephone line (NE1) lights up when there is ringing present. A capacitor (C1) and resistor (R1) limit the amount of ringing current used, about 0.1 or less of a standard bell, and the capacitor prevents DC from flowing in the circuit. This must be coupled to the photo cell (PC) in a light tight container, such as an old 35 mm film container.

It is best to glue the lamp to the photo cell with crazy glue, then paint the back of the neon lamp with white out, this provides a reflective coating increasing the amount of light falling on the photo cell. If you have room you can put the resistor and capacitor in the container.

You might test the photo cell first in the light. Connect a meter that measures resistance, test the photo cell in the dark and in the sunlight and measure the resistance. There should be quite a difference in resistance.

There are no real specifications for the neon lamp, just about any neon lamp will work. Official type would be C2A which is a high brightness type of bulb rated at 120 VAC at Ό watt or 1.8 Ma.

There are no specifications for the photo cell. It needs a dark resistance of at least 1 megohm or more, higher resistances are better. These cost about $2 each. I suggest Jameco ( as a source.

Power Supply:

The power supply for the AAD is the telephone line itself, including the AC ringing and the DC power on the line. This is rectified to DC of a single polarity (tip and ring make no difference in this case) with a bridge rectifier.

Ring Detection:

When the phone line rings the AC (ringing) rides on DC. This produces about 175 Volts DC (90 x 1.41 + 48).

The ringing “DC” flows thru R3 and the photo cell to charge C5. R3 is the ring delay resistor, increase the value and you can increase the ring delay. You could set the device to answer on 10 rings if you want.

Once C5 is charged to about 90 VDC, NE2 fires and lights up. The discharge path is R4 and the gate of the SCR. R5 and C6 keep the gate slightly desensitized so it will not respond to radio frequencies or stray pulses.

R4 also lengthens the time it takes to discharge C5. This keeps the SCR conducting until the ringing stops. It takes several Hz of shorting the ringing (the SCR conducting) before the central office trips the ringing, removes the ringing and establishes the voice path.

Audio Path:

When the voice path is established one winding of the transformer, the relay and the SCR form a series circuit. The voice or audio path is established, the relay closes and the SCR remains conducting.

C4 across the relay serves to bypass the audio across the coil.

The relay may be a 12 VDC relay, but it cannot be over 300 ohms. You will need to test this for operation by connecting it directly across the phone line. The relay should operate. Not all relays perform the same. If you can get one with adjustable contacts or a spring, then you can bend the spring to provide a bit more sensitivity.

Audio Path:

The transformer provides a low loss isolated connection. A click suppressor CS is installed so that clicks when the AAD answers are reduced and any high level audio you feed in is reduced (but it may be distorted so reduce the level).

C2 is to suppress any radio frequencies. C3 provides DC isolation from anything you might connect. R2 is used in case you feed an audio source from a low impedance source such as across a radio speaker.


This only happens when the caller hangs up. Remember, you built a simple tester and verified you had CPC.

As an option you can build a timer using a 555 circuit, operate a relay and use a contact on the relay to open the phone line.


This circuit works quite well and parts values are not too critical. Ringing can be 20 Hz, in the USA, or 25 Hz in other countries.

It is important the photo cell and neon be in a light tight case. If not the photocell will have a lower dark resistance and the while the circuit will work, the central office will see a resistance on the line and may indicate a fault.

Versions of this circuit, as manufactured under the patent, did have an FCC registration number. Most tariffs (those rules that the phone company operate under and impose on users) prohibit connections lasting over 24 hours, so make sure nobody will stay on the line that long, or build an external timer.

You can add a 1K resistor and high brightness LED in series across the relay to indicate the AAD is active.



Three coils are shown above. (1) The large Western Electric Type 120 Repeat Coil, (2) The small 1:1 Transformer (similar to the TU-016), and (3) A round Loading Coil, which is a 150 Ohm/150 Ohm Repeat Coil, but not designed as one. A House Key is shown for size comparison.