circuit that wouldn’t affect the phone line?
— Ray Escue
borhoods, expect to find 100
volts at 20 Hz to
25 Hz. The cir- L
cuit consists of I
an input cap, a N
bridge rectifier E
(made of four
Zener diode, a filter cap, and a
RadioShack relay (Figure 12). In fact,
Phone Ring Detector
AThis is one of those cases
where, thankfully, you don’t
need optical line isolation. A
coupling capacitor is all the
isolation you need, plus the phone
company provides all the power needed to drive a sensitive relay directly. No
external battery needed! In most neigh-
■ FIGURE 12
with the exception of the 1 µF cap, you
can get all the parts from RadioShack.
The 1 µF cap is often found in small motors or ceiling fan speed controllers.
Check your local appliance repair shop
or do your shopping at one of the mail-order jobbers, like Jameco.
The ringer signal from the phone
company’s central office is a 20 to 60
Hz sine wave of 90 VAC to 150 VAC.
The capacitive reactance of the 1 µF
input capacitor is about 6K to 8K,
depending on the ringer frequency. This
limits the ringer current to 14 mA, more
or less — well within the pull-in range of
the relay. Don’t be concerned that the
input voltage is 100 volts plus and that
the relay is a mere seven volts. Relays are
current operated devices and by limiting
the current with the input cap, the relay
coil will seek its correct operating
voltage. But to be on the safe side, a 100
ohm input resistor and a 12 volt Zener
diode protects the relay from surges and
unexpected voltage spikes. NV
Electronic Parts by the Pound.
Let grab bags fill the empty bins
in your parts cabinet.