Convert a Ringer Box to a Bell Chime
Yes, it is Possible - Level Intermediate to Advance - Electronics Knowledge Needed
If you read the AAD (Automatic Answer Device) you noticed that a neon lamp and a photocell were used to detect ringing. The same type of ring detector for this circuit.
Standard Telephone Bells will work on Ringing Current, 20 or 30 Hz, half wave ringing with a diode matrix, used on a 1A2 Key System, or on DC (NOT common in Telephony today). For this project we will use DC to operate the ringer, and it will sound like a Bell Chime. There is a wide variation in old ringers, some were low resistance, some were high resistance.
Before starting this project you need to make sure your ringer works. The best way to do this is to use a standard ringer capacitor, 0.47 MFD 250 VDC, put it in series with the old bell, just like the wiring on a standard telephone, and connect it to the telephone line. When you call the line the bell should ring nice and loud, you may need to adjust the spring or the spacing of the gongs. Occasionally the ring will be weak, this could be caused by a weak magnet in the bell, especially in very old ringers. There is not a simple cure for this. Older materials used in the manufacture of magnets do not retain their magnetism compared to more modern materials and alloys.
If your bell is “weak” it probably will work, but with a very gentle sound. Old ringer boxes will make a "ding-ding" sound if the gongs are exactly the same. If you prefer a "ding-dong" sound use a modern ringer like a C4A from a 500 or 2500 telephone set.
This project is written to help you learn a little about electronics and will take you step by step to converting your ringer. Other variations will be presented after the basic project.
The heart of the circuit is the ring detector. The Neon Lamp and Photocell detect the ringing, this makes the Transistor conduct and current flows in the bell. When current starts to flow the clapper strikes one gong, when the current stops (silent interval) the spring in the bell pulls the clapper against the other gong. Careful adjustment of the spring and the gongs are necessary to achieve this effect.
NE – Neon Lamp, almost any type will work
PC - Photocell - dark resistance over 100K, light resistance 10K (use this as a guide)
C1 – 0.1 MFD 250 Volt capacitor – limits current and only lets AC light the neon lamp
C2 – 22 MFD 50 Volt capacitor – stores small charge for bell chime effect, value may be 10 MFD or 33 MFD, some adjustment may be necessary in the value
R1 – 10,000 Ohm resistor – limits current into neon lamp
R2 – 10,000 Ohm resistor – limits current to the transistor
Use 10,000 Ohms for 12 Volts and 22,000 Ohms for 24 Volts
R3 – 10,000 Ohm resistor – keeps base of transistor near ground level
R4 – 10,000 Ohm resistor – limits current to the transistor
TR – 2N4401 or 2N4410 – NPN Transistor
DR – 1N4004 – diode to stop back EMF voltage to the transistor
Light Tight Container – black 35 MM film container or equivalent
Parts Source: Jameco http://www.jameco.com
Theory of Operation
When ringing is present on the telephone line, the neon lamp glows. The light from the neon lamp makes the photocell conduct. The photocell is a photo-resistive device. The more light, the lower the resistance.
Current flows into the string of R1, PC, and R2. The photo cell lowers is resistance when the neon lamp glows. About half the power supply voltage appears across R2. R3 limits the current into the base of TR, the transistor. The transistor conducts when there is current on the base.
When the transistor conducts, the full power supply voltage is applied across the bell coils, and the clapper strikes one gong. Capacitor C2 keeps the clapper in one position as the transistor turns on an off in response to the AC ringing. C2 also acts as a dial pulse filter so dial pulses do not make the bell tinkle.
When the ringing stops the photocell goes back to a high resistance, and the voltage across R3 drops to a very very low value, the transistor no long conducts and there is no current in the bell coils. The spring on the bell pulls on the clapper and the clapper strikes the other gong.
The Neon Lamp and Photocell
Neon lamps operate on AC or DC, on AC both electrodes glow, on DC only one electrode glows. In general, the AC must exceed 65 Volts AC and the DC must exceed 90 Volts DC. Current must be limited with a resistor, if you stuck a neon lamp into a power outlet without a current limiting resistor it would explode. Telephone line ringing is composed of AC that “rides” on DC (with ringing this is called trip battery) or 90 Volts AC with 48 Volts DC.
The Neon Lamp and the Photocell must be sealed in a light tight case. You can use a 35 mm film container or something similar. The lamp and the cell should be in firm contact. I usually use crazy glue to stick the neon lamp to the photocell and when dry paint the back of the neon lamp with white out to increase the light reflectance. Or you can wrap them in a small strip of white paper.
I have used a variety of photocells with good results. I try to select a photo cell with a high resistance in the dark condition and a low resistance in the light condition. The Photocell is a resistive device, not a semiconductor junction. Cost of photocells is under $2.
You can test the photocell with a VOM, measuring the resistance in the light and the dark. Covering the cell with your finger or thumb to simulate the dark may not work. The cells are sensitive to infrared and your fingers are somewhat transparent to infrared, place a coin over the photocell to block the light or attach some leads and put it in the refrigerator (make use the light turns off when you close the door!).
The Bell and the Coils
The bell should be in working condition. The best way to test it is to put a ringing capacitor in series with the bell, the standard wiring for a telephone, and connect it to a telephone line and call it. This is the first step in testing it.
In round numbers there were at least 25 manufacturers of bells, and each one probably had 20 variations or more. So, 25 times 20 makes 500 or more varieties of bells. That’s just for starters! It is near impossible to come with one method of wiring the bell coils, series or parallel, and one specific voltage to use for a power supply.
A standard Western Electric C4A ringer, the one used in the 500’s and 2500’s has two coils. One coil is 1000 Ohms and the other is 2650 Ohms. If wired in series, the standard method, that is 3650 Ohms. If wired in parallel, what would be used for this project, it is about 726 Ohms. Older ringer coils varied between 300 and 1000 ohms each, for the common varieties. Coils in series would be 600 to 2000 Ohms and in parallel would be 150 to 500 Ohms. Anyway you look at it that is quite a variation.
Look at your box of old wall warts. The maximum voltage you should use is 24 Volts (well maybe 30). With a set of clip leads between the wall wart and the bell coils do some testing. What voltage will your bell work on? Remember telephone bells are polarized and usually only work with the correct polarity of the DC.
Before you start this, look at the spring or bias spring. Gently move the clapper with your finger and note the “tension” of the clapper. Adjust the spring for minimum tension, or so that any gentle pressure on the clapper with your finger will move it.
With the wall wart and clip leads see if you can get the bell coil to operate and sound like a bell chime. Adjust the gongs if necessary. If it works and sounds good, you should be able to build this without any problems. If it does not work, try a higher voltage wall wart, or wire the bell coils in parallel.
Wiring the bell coils in parallel is easy, but you get the wrong connections the magnetic field on one coil will cancel the other. Good thing you bought the package of 10 clip leads. Using clip leads connect the bell coils in parallel and try again to make the bell operate. If it does not, reverse the clip leads and try again.
Caution – when the magnetic field collapses (removing the power will do this) it will generate a voltage in the bell coil. This voltage could be 100 Volts or so. It can cause a mild shock, so use caution. To prevent the transistor from burning out, a diode shorts out this voltage when you remove the DC from the bell coil.
Substituting a Relay
By replacing the bell coils with a relay you can make a ring detector for switching heavy loads. With a 12 Volt relay, use a power supply of 12 to 16 Volts. You need to use the diode shown with the bell coils. You can operate a “fire bell” like the ones with 6” gongs or larger or a large light bulb. Just be sure the relay contacts will handle the load.
Substituting a Tone Generator
Many electronic stores offer a Piezo Siren or Piezo Tone Generator that will operate on 12 or 12 to 24 Volts DC. Replace the the bell coil and get a pierce sounding tone. Use two or three to them, in parallel, and get a really irritating sound that will drive you out of the house, or perfect for a factory with lots of noise.
You can try removing C2. This should make the Piezo device pulse with the AC ringing. It may also make a "tink" in the device when there are dial pulses on the phone line. The trick is to get the right value of C2 that stops dial pulses but allows the AC ringing to modulate the tone of the Piezo device.
Substituting an LED
Place three LED’s in series and add a 470 Ohm resistor to the series string of LED’s, this is for 12 Volts, use a 1000 Ohm resistor for 24 Volts. You can use 5 sets of these series strings. Not too useful, but there is no noise, just the glow of the LED’s.
Thanks to Bill Geurts for the the original bread boarding.