The circuit is fairly simple and works as follows:
The current consumed by the main load, goes thru the transformer (Tr1), where it induces small voltage across secondary winding. This voltage is amplified by 1st amplifier in opamp (OP1), rectified (by diode D5), filtered (by capacitor C5) and fed to user adjustable (via changing reference voltage at opamps pin with pot P1) comparator (built using 2nd opamp, OP2), which controls relay (Rel1) via transistor (T1), and relay controls auxiliary load. There are some additional components too — diodes D2 and D3 are used to provide protection of opamp input, in case of spike at circuit input, resistor R8 adjusts delay, after which relay is triggered. Resistor R6 protects opamp’s output from overloading and diode D4 protects transistor from high voltage spikes, which occur, when relay is de-enrgized. There are also two 0.1uf bypass capacitors on PCB (not shown in circuit) which are used for noise reduction at opamp’s power pins.
The circuit is built on single side PCB, which is designed to be home manufacturing method friendly — by CNC or etching. Of course, you can outsource manufacturing to any PCB manufacturing plant — gerber files are included in the «downloads» section. I’ve manufactured both versions, as you can see on the photos.
Before starting the assembly, you’ll need to modify an input filter. For this, you have to remove one of windings, and wind a new one, using 3-4 turns of relatively thick wire (18 or 20 AWG will work fine). This will be the «input» winding.
I’m using small 5VDC supply for powering the circuit. Current consumption with relay on is about 65mA, so almost any 5V power supply will work.
After assembly is completed, you can test the circuit in the following way. Set the pot P1 into middle position, do not connect load to the circuit, just power it on. If everything is done properly, relay should click and release in couple of seconds. If this does not happens, check your circuit for any errors or issues.
If everything is fine, you can connect your loads — both one to be monitored and one to be controlled. Put your main load into ON state, and by adjusting pot P1, make sure that controlled load is also powered on. Now put your main load into off/standby state (either with remote control or power button or whatever way you do it) and make sure that controlled load also went off (there might be a small delay of 2-3 seconds, before it goes off). If it stays on, adjust the pot in the opposite direction, until aux load turns off. Now wake up your main load and check whenever controlled load also goes on. That’s all.