Mains Remote Switch

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This compact design
forms a remotely operated switch that receives its control signal via
the mains voltage. The switch is operated using the ‘mains remote
transmitter’ described elsewhere in this issue. With this transmitter, a
switch should be connected between pins 1 and 2 of K1. Depending on the
application, this must be either a press contact or a make contact. The
idea of the ‘mains remote switch’ is that a relay is energized in order
to connect the mains voltage on K1 through to K2. The ‘receiver’ (a
somewhat exaggerated term for such a simple design) is formed by Tr1 and
the tuned circuit L1/C4. The network C1/Tr1/C2 serves as a coupled
circuit tuned to the frequency of 143 kHz generated by the transmitter.

The selectivity is determined by L1/C4 and is primarily dependent on
the standard suppression coil L1. Gain for operating the relay is
provided by T1. The amplified signal is smoothed by C6 and provides the
voltage necessary to cause T2 to conduct and energize the relay. The
voltage divider formed by P1, R1 and R2 provides a bias voltage for T1
in order to increase the sensitivity of the receiver. This also allows
the relay to be energized without a received signal. D1ensures that C5
does not become charged and prevents T1 from conducting even more.

The operation of the circuit is based on the fact that the incoming
signal is sufficiently strong to overcome the hysteresis of the relay.
Once the signal is no longer present, the relay must naturally again
release. To be honest, it must be noted that the simple design of this
circuit has the disadvantage that its sensitivity may be somewhat
inadequate, depending on household circumstances. One possible solution
is to reduce the frequency of the transmitter to the region between 95
and 125 kHz. The values of C1, C2 and C4 will then have to be modified
to match, so this is something for readers who like to experiment.

Do not forget that just as with the transmitter, the entire circuit
(once it has been switched on, of course) is connected to the mains
potential. Power for the transistor stage and the relay is taken
directly from the mains voltage using a capacitive voltage divider; R5
is only necessary to limit the current through the diodes to a safe
value on switch-on. Rectification is provided by diodes D4–D7 and
filtering by C7. The impedance of C8 is low enough to provide sufficient
current. The no-load voltage (when T2 is not conducting and the relay is
not activated) is limited by zener diode D3.

R6 and R7 discharge C8 immediately after the circuit is disconnected
from the mains, in order to prevent any dangerous voltage from
remaining on the input terminals. Connections A and B are provided for
test purposes and also allow something other than the relay to be
energised (but keep in mind that the circuit is electrically connected
to the mains network!). The pinout of the relay is standard, so a type
other than that shown in the components list can also be used, as long
as you make sure that the operating voltage is 24 V and the operating
current does not exceed 28mA.

Resistors:
R1 = 1MΩ5
R2 = 220kΩ
R3 = 39kΩ
R4 = 6kΩ8
R5 = 220Ω
R6,R7 = 470kΩ
P1 = 10MΩ preset

Capacitors:
C1 = 22nF 275VAC Class X2, lead pitch 15mm
C2 = 22nF, lead pitch 5 mm
C3 = 220pF
C4 = 2nF2, lead pitch 5mm
C5 = 680pF
C6 = 100nF, lead pitch 5 mm
C7 = 100µF 40V radial
C8 = 330nF 275VAC, Class X2, lead pitch 22.5mm or 27.5mm

Inductors:
L1 = 470µH

Semiconductors:
D1 = BAT85
D2 = 1N4148
D3 = zener diode 24V 1.3W
D4-D7 = 1N4007
T1 = BC557B
T2 = BC547B

Miscellaneous:
K1,K2 = 2-way PCB header, lead pitch 7.5 mm
Tr1 = 5:5 turns 1mm dia. isolated wire on N30 ring core 16×6.3 mm, B64290L45X830 EPCOS
Re1 = PCB relay, 1 c/o contact, 8A 24V 1200Ω