Build a Telephone Ringer: Circuit & Control

Circuit diagram:

Description

It’s conceivable that individuals possess a substantial residence, an expansive garden, and young children. This project potentially holds interest for such individuals. It represents a telephone ringer capable of activating any device powered by the mains. Using it, one can control a powerful siren or horn, as desired, to amplify and transmit the faint sound from a fixed-line telephone, making it audible within a large house or garden. Alternatively, it’s possible to make a lamp illuminate (or an indicator light), creating a ‘silent ringer’ useful when small children are resting.

A particularly noteworthy aspect of this straightforward and affordable project is the absence of a required power supply, unlike similar items marketed in stores. Prior to examining the drawing and grasping the underlying principles, it’s vital to understand that the ringer voltage on a standard telephone line is relatively high. Although Europe and the EU Commission haven’t yet intervened, the precise value and frequency of this voltage vary according to the nation. The line carries direct current irrespective of whether it’s in use or not.

Furthermore, only a small current—a few hundred mAs—needs to be drawn from an idle telephone line to persuade the PSTN exchange that the line is occupied. Consequently, capacitor C1 fulfills a dual function: it isolates the project from the direct current present on the line when it’s inactive or active, while simultaneously permitting the ringer current to flow. This latter current is rectified by D1 and limited by D2, producing approximately 6 V DC at the C2 terminals when a ringer signal is present.

This voltage activates LED D3, serving solely as a visual confirmation of proper operation, similar to the LED contained within IC1. This component is a high-power photo triac with zero crossing detection from the AC power supply, facilitating the switching of the load it controls without generating any discernible noise. This device, which could be referred to as a solid-state relay, was chosen because it’s packaged in a form resembling a TO220, slightly larger in size, and equipped with four terminals. The markings on the packaging correspond to the symbols depicted in our schematic diagram, preventing confusion. Because this circuit is not yet widely adopted, we must note that it can be obtained from the Conrad Electronics website (www1.uk.conrad.com).

To ensure safe operation, the circuit is safeguarded by a GeMOV on the AC side, commonly known as a Varistor, VDR or SiOV depending on the manufacturer. The specific model indicated here is generally available. The load is limited to 2 A, reflecting the specifications of IC1, which is more than adequate for the intended application. Finally, given that numerous components within this circuit are connected directly to the AC power supply, the assembly should be housed in a completely insulated enclosure for obvious safety considerations.