Sunday, October 7, 2012

Telephone Ringer

Caution: The circuit generates a high voltage which can shock.

The Phone Ringer circuit will work with any ordinary phone including older bell ringer types. The circuit rings the phone in a completely realistic manner until someone answers. When the receiver is lifted the user hears the audio of your choice. It might be another telephone, a tape recording, a favorite talk radio show, a fake busy-signal, a scanner tuned to weather or police, cues for the actor who forgot his next line, or whatever audio source strikes your fancy. DC current is passed through the phone to activate the phone’s electronics.

Provisions for experimenters include a ring inhibit control and an additional transistor will activate devices when the phone is answered. The ring inhibit control is used to start the ringing when a signal goes low and the activate-on-answer control can start a tape recording or other device when the phone is answered. For example, the ringer could be triggered by an alarm clock to make an artificial but realistic wake-up call. When you answer, your own voice instructs you about the importance of getting up. This wake-up caller is quite persistent, calling back the instant you hang up!

Do not connect this circuit or the phones used with this circuit to the phone lines.

The phone cable will have red and green wires which are simply connected to the points indicated by the schematic. Polarity should not matter. Other devices may be connected as described but no connection to a "real" phone line is intended.

The circuitry is simple and not particularly critical. The first two inverters form a slow pulse generator which controls the ringing rate. Change the 0.22 uf capacitor to change the ringing rate and change the 22 Meg. resistor in series with the diode to change the length of the ring. The second two inverters generate the 20 Hz ringing signal. This frequency can be changed by changing the .033uf capacitor. Mechanical bell ringers have a resonant clapper and should be driven with a frequency near 20 Hz but a slight variation may give a better ring. The last two inverters buffer the ringing signal and drive the two output transistors. Practically any transistors can be used for the output including 2N4401 and 2N4403 but power transistors in a TO-220 package might be more desirable if a lot of ringing is anticipated. The transistors should be capable of handling several hundred milliamperes. Any low-leakage signal diodes will work for the 1N914s.

The power transformer must handle 20 Hz with at least some efficiency so it is best to use larger units. Molded transformers will work fine but of course they cannot be DC types which have built-in rectifiers. Choose a transformer with a low voltage winding rated for an output voltage well below the DC power supply used. The circuit as shown runs on 12 volts with a 9 volt transformer. Some transformers have 220 volt windings which can give a stronger ring if necessary. A 6 volt filament transformer powered by the circuit as shown will give a quite strong ring. Reduce the 10 ohm emitter resistors to 4.7 ohms to get more ring power if power transistors are used . (Don't leave them out entirely since they help prevent high frequency oscillations.)

Ringing is inhibited by applying a voltage near VCC to the 1N914 diode. A simple transistor inverter can change the sense and increase the sensitivity so that a couple of volts will start the ringing (fig. 2). If the phone is to ring when the squelch of a modern scanner breaks try looking in the scanner for an analog switch integrated circuit. One of its pins will jump between 0 and 5 volts when the squelch breaks and this signal is fine for driving the inverter circuit. Fig. 2 also shows how to connect a photocell so that the phone rings only when the lights are off. (Record a dial tone so the victim concludes that the caller keeps hanging up just as he turns on the light.) The ringer control can also be used in a variety of other ways to automate the ringing. For example, a 470k pull-up resistor combined with a large electrolytic capacitor connected to ground makes an interesting doorbell. Just connect the doorbell switch across the capacitor and the phone will ring for a few seconds when the switch is pushed. (The capacitor discharges quickly but charges slowly.)

Fig. 3 shows how to add an answer activated control. The 1k resistor may be replaced with a relay for controlling a tape recorder. Put a diode across the winding to protect the transistor from inductive kick-back.

The phone ringer may be used to construct a pretend cellular phone system for the kids using an ordinary cordless phone and a regular phone wired in series. Keep the wiring neat and insulated so that the ring voltage doesn't "bite" anyone. Connect the ringer, ordinary phone and the base unit of the cordless phone in series. Wire a switch which enables the ringer and shorts the ordinary phone (two-pole switch). When the cordless phone is answered, flip the switch to talk. Shorting the ordinary phone is probably not necessary but be prepaired for a rather loud buzzing in the earpiece when the other phone rings! The advanced experimenter may wish to build an artificial phone system by adding a on-hook high voltage supply, dial tone oscillators, and appropriate switching circuitry. Quite a challange!



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