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Topic: Ringer circuit for vintage telephone bells. (Read 202 times) previous topic - next topic

6v6gt

Jun 18, 2019, 02:10 pm Last Edit: Aug 22, 2019, 11:40 am by 6v6gt Reason: Errata
A number of these vintage telephone bells have been turning up in online auctions here. These have been refurbished and look quite nice, and the electrical and mechanical parts are original even with cotton insulated copper wire coils.



So I bought two of these and designed and made a ringer circuit which generates the "traditional" ring pattern, so I could power them from either a USB charger or a 6 volt battery pack. It produces a 36 volt 20 Hz "square" AC which is ample for maximum volume



In principle, it is quite simple. The main elements are (a) a boost converter which can produce 36 volts from a 5 volt input, (b) an H bridge driver to generate the pseudo AC and (c) an Attiny841 to drive it all.

Since I intend it to be powered by batteries in conjuncion with a PIR or other movement detector, I designed it all for low power consumption. For example, the power to the boost converter and the H bridge is switched via a mosfet load switch which I have been experimenting with (TPS27801) and the ATtiny841 (which I have also been experimenting with) goes into sleep mode.

There is considerable scope for simplification if you are not interested in running it from batteries and low power is not an issue. For example, you can drop the load switch and connect the boost converter and the H bridge to Vcc directly and you can strip the sleep mode code from the sketch. If you use an alternative processor, say an ATtiny85 or a Uno, you'll have to review the pin allocation and maybe the setup of the pin change interrupt for the push button.

There are a number of published projects, some even integrating a vintage telephone with a mobile network, which may provide an alternative solution to this one.

The attached PDF file contains the schematic.


Errata:
C2 in the schematic should be between U4 pin 6 and U4 pin 2. Not as shown between U6 pin 6 and GND.

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