The other interesting part of this simple
and inexpensive project is that it doesn’t require a power supply,
contrary to similar items on sales in the shops. Before examining the
drawing and understanding the principle involved, it is important to
know that the ringer voltage on a fixed telephone line is pretty high.
Since Europe and the EU Commission have not yet interfered, the exact
value of this voltage and its frequency varies according to the country,
but that’s not important here. The line carries direct current whether
unoccupied or occupied.
Moreover, no more than a few hundred
mAs needs to be stolen from an unoccupied telephone line to make the
PSTN exchange believe the line is occupied. Therefore, capacitor C1 has
the dual role of insulating this project with respect to direct
current present on the line while unoccupied, or while occupied, while
also allowing the ringer current to pass. The latter is rectified by D1
and clipped by D2 which makes about 6 V DC available to the C2
terminals when a ringer signal is present.
This
voltage lights LED D3 which only serves as a visual indicator of
proper operation as does the LED contained in IC1. This is a high-power
photo triac with zero crossing detection from the mains, which allows
it to switch the load it controls without generating even the lowest
level of noise. This component, that we might just as well call a
solid-state relay, was selected because it is comes in the form of a
package similar to a TO220, a little bigger, and equipped with four
pins. The pinout will not cause confusion because the symbols shown on
our diagram are engraved or printed on the packaging. Since this
circuit is not yet very common, we need to mention that it’s available
from the Conrad Electronics website (www1.uk.conrad.com).
For
the purpose of safe operation, the circuit is protected by a GeMOV on
the mains side, called Varistor, VDR or SiOV depending on the
manufacturer. The model indicated here is generally available. The load
will be limited to 2 A, considering the model selected for IC1, which
is more than sufficient for the application planned here. Finally,
since a number of components in this circuit are connected directly to
the mains power supply, the assembly should be placed in a completely
insulated housing for obvious safety reasons.
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