Using straight forward circuit and a standard PC-sound card, low cost on/off control of home appliances can be implemented via the Internet. Consider a system that consists of a remote computer and a local computer that is connected to an inexpensive and simple interface circuit with relay output.
A browser is needed by a local computer and also an application is needed which offers access to the remote computer through the web. Adding of Chrome Remote Desktop is available if you have installed Chrome as the browser. Free remote access is offered by Team Viewer if you use another browser. From the local computer checking is allowed by this circuit if the command is actually being executed.
At the remote computer the additional remote desktop access, the browser and an application is needed to generate an audio tone through the sound card. Sine wave generators are available in dozens i.e., Audio Testset from Darkwood Designs is one good choice. A level meter is incorporated by this software which measures the input-voltage level that is applied to the line/mic input of sound card of the computer.
Consider a level meter combined with a sine-wave test generator and a digital read out, such as this Darkwood Designs and operational setup is critical to it.
At the speaker output of the computer sound card, a 1000 Hz and 2-V sinusoidal voltage is available when the remote computer is accessed to the desktop and operates the tone generation; to achieve the highest value you need to adjust the reproduction audio volume.
To the input of the interface circuit this signal is connected and rectified via filtered by R1/C2 and germanium diode D1 in to a dc voltage, by which the non-inverting input of a comparator will be fed which is built around one half of op-amp U1.
By the divider formed by R3 and R2 a 0.5-V signal is developed at its investing input. The relay K1 gets activate via transistor Q1 and output of the comparators goes high when the tone generator is remotely activated. The appliance load on and off is switched by relay, so make sure that relay contacts have sufficient RMS and peak voltage and current ratings.