Parts:
R1,R2,R8 1K 1/4W Resistors
R3,R4 220K 1/4W Resistors
R5 100R 1/4W Resistor (See Notes)
R6 10K 1/2W Trimmer Cermet
R7,R10 1M 1/4W Resistors
R9 22K 1/2W Resistor
R11 to R17 1K 1/4W Resistors
C1,C3 100µF 25V Electrolytic Capacitors
C2,C4 1µF 63V Electrolytic Capacitors
D1 5mm. Red LED
D3,D4 1N4002 100V 1A Diodes
D2,D5,D6,D7 LEDs (Any color and size)
Q1 BC327 45V 800mA PNP Transistor
IC1 TL061 Low current BIFET Op-Amp (First version)
IC1 LM358 Low Power Dual Op-amp (Second version)
IC1 LM324 Low Power Quad Op-amp (Third version)
L1 10mH miniature Inductor (See Notes)
RL1 Relay with SPDT 2A @ 220V switch
Coil Voltage 12V. Coil resistance 200-300 Ohm
J1 Two ways output socket
This circuit was designed on request, to remotely monitor when a couple of electric heaters have been left on. Its sensor must be placed in contact with the feeder to be able to monitor when the power cable is drawing current, thus causing the circuit to switch-on a LED. The circuit and its sensor coil can be placed very far from the actual load, provided an easy access to the power cable is available.
Any type of high-current load or group of loads can be monitored, e.g. heaters, motors, washing machines, dish-washers, electric ovens etc., provided they dissipate a power comprised at least in the 0.5 - 1KW range. This design features three versions. The basic one illuminates a LED when the load is on. The second version activates a Relay when a pre-set current value flows into the power cable. The third version switches-on D7 when the load power is about 1KW, D6 when the load power is about 2KW and D5 when the load power is about 3KW.
The basic circuit is shown top left in the drawing and must be used in all three versions. IC1 acts as a differential amplifier having a gain of 220. The small AC voltage picked-up by L1 is therefore amplified to a value capable of driving the LED D1.
The second version is drawn bottom left, must be connected to the basic circuit and uses a dual op-amp: therefore IC1 will be labeled IC1A and its pin layout varies slightly. IC1B acts as a voltage comparator and its threshold voltage can be precisely set by means of trimmer R6. Q1 is the Relay driver and D2 illuminates when the Relay is on. You can use the Relay contacts to drive an alarm or a lamp when the AC load exceeds a pre-set value, e.g. 2KW.
The third version is shown to the right of the drawing, must be connected to the basic circuit and uses a quad op-amp, therefore IC1 will be labeled IC1A and its pin layout varies slightly. IC1B, C and D are wired as comparators. They switch on and off the LEDs, referring to voltages at their non-inverting inputs set by the voltage divider resistor chain R11-R14.
