Relay Switch Activated by Tone and Signal

The essence of the
circuit is for the input of tone and signal to provide an activation for
the relay switch.

  • Relay – an electrically operated switch where the current flowing
    through the coil of the relay is creating a magnetic field which
    attracts a lever and changes the switch contacts, thereby making its
    state open or close
  • BC214 – a complementary silicon planar epitaxial transistor used
    in AF small signal drivers and amplifiers as well as for low noise
    preamplifier applications due to its feature of good linearity of DC
    current gain
  • LM741 – a general purpose single operational amplifier with
    features such as offset null, compensated internal frequency, voltage
    range with high input, good stability of temperature, and protected from
    short circuit

The use of relay will allow the circuit to switch from one condition
to another. It can also be referred to as a form of an electrical
amplifier since it is able to control an output circuit of higher power
than the input circuit. There are many types of relays being used in
many electronic and electrical circuits, which include solid-state
relay, Buchholz relay, overload protection relay, latching relay,
forced-guided contacts relay, mercury-wetted relay, contactor relay,
machine tool relay, reed relay, polarized relay, and solid state
contactor relay.


Circuit diagram

The circuit created is sensitive enough to the AC signals in the
input stage, where the signals are ranging above 5 mV. It will also be
sensitive to react with the human voice signals having a range of
frequency from 50 Hz up to 3 KHz. The human voice is a part of the human
sound produced primarily by the vocal cords or vocal folds which in
turn produces a voice frequency that is used for the transmission of

During the absence of an input signal, the state of the 12 V relay RL1 is at OFF
condition as regulated by the 10K Ohms trimmer RV1. The circuit can be
made to react with its sensitivity in points A, B, & C, where a
negative feedback can be placed due to the addition of band pass filter.
The filter will operate only in the 1 KHz range and the circuit will
only correspond at this frequency.

Part list

Part list

The signal and tone activated relay switch were used in a wide range
of fields which includes measuring instruments, audio systems,
communications equipment, and factory-automation equipment. They can
also be found on telephone subscriber circuits for the polarity
reversing switch, testing, and ringing functions.

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