Automatic Load Sensing Power Switch

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This circuit will automatically switch on several mains-powered "slave" loads when a "master" load is turned on. For example, it will switch on the amplifier and CD player in a stereo system when the receiver is turned on. It works by sensing the current draw of the "master" device through a low value high wattage resistor using a comparator. The output of that comparator then switches on the "slave" relay. The circuit can be built into a power bar, extension cord or power center to provide a convenient set of "smart" outlets that switch on when the master appliance is powered (turn on the computer monitor and the computer, printer and other peripherals come on as well).


Schematic for Automatic Load Sensing Power Switch


Total Qty.
C1, C3210uF 35V Electrolytic Capacitor
C211uF 35V Electrolytic Capacitor
R110.1 Ohm 10W Resistor
R2127K 1/2W Resistor
R3, R411K 1/4W Resistor
R51470K 1/4W Resistor
R614.7K 1/2W Resistor
R7110K 1/4W Resistor
D1, D2, D431N4004 Rectifier Diode
D311N4744 15V 1 Watt Zener Diode
U11LM358N Dual Op Amp IC
Q112N3904 NPN Transistor
K11Relay, 12VDC Coil, 120VAC 10A Contacts
S11SPST Switch 120AVC, 10A
MISC1Board, Wire, Socket For U1, Case, Mains Plug, Socket


  1. This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed.

  2. A maximum of 5A can be used as the master unless the wattage of R1 is increased
  3. S1 provides a manual bypass switch.
  4. THis circuit is not isolated from the mains supply. Because of this, you must exercise extreme caution when working around the circuit if it is plugged in.

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Add A Comment

Hemant Dangi
Automatic Load Sensing Power Switch
Saturday, June 17, 2017 1:39:38 AM
Sir, this circuit is for 110vac, I want a circuit for same purpose but on 12vdc. I want to control a 12vdc motor which draws 0.8 amps at normal working, when it get jammed it draws 4amps and that time i want to stop it. please revert. thank you sir.
Automatic Load Sensing Power Switch
Monday, October 12, 2015 9:27:08 AM
I use a 40 amp bridge rectifier on the hot line - with the + & - terminals shorted - gives you a pair of high current diodes in series. Good voltage drop that serves as the diff. input to op amp. I add a small transformer (12 vdc) to remove the problem with high pk voltage and cap issues.
Brian Nebeker
Automatic Load Sensing Power Switch
Thursday, November 17, 2011 12:21:11 PM
Current sensing using a large resistor is wasteful of power and sensing power on the neutral side can cause dangerous conditions. If R1 fails it will leave the load side of the neutral line at the potential of the hot line. A far safer way of sensing current is to use a hall effort sensor such as those provided by Allegro. These sensors provide high voltage isolation and high power level handling due to the fact that they use a magnetic field instead of a resistance to sense the current. I have used these sensors on several projects and they make life much easier. If there is enough interest I could post of example of this circuit using a hall effect sensor.
Automatic Load Sensing Power Switch
Monday, April 25, 2011 9:43:10 PM
anybody has got this working??
Automatic Load Sensing Power Switch
Wednesday, November 04, 2009 12:42:46 AM
There's nothing in this circuit to keep the voltage across C3 from rising above its rated voltage when Q1 is off (i.e. when no current is being drawn by the master load). The capacitor will try to charge up to about 170V (the peak voltage of 120 VAC RMS), but it will eventually fail (and might literally explode!) once the voltage gets significantly above the nominal 35V. A zener or even a resistor in parallel with C3 could solve this problem.
Abdul Wahid
Automatic Load Sensing Power Switch
Sunday, March 01, 2009 5:42:16 AM
The values for 240VAC operation are R2 = 50K, 1W and R6 = 200K, 1W. these are calculated just using Ohms law by keeping the current at same level. if you want to increase the current then increase the wattage with same ratio, i.e doubling current will double the wattage of R2 and R6
Automatic Load Sensing Power Switch
Sunday, February 15, 2009 3:42:05 PM
Not sure this will work. To keep a 12V relay energised would need something like 50mA. So the power dissipated in R6 would be 0.050 * 0.050 * 4700 = 11.75W!
Eliud kabuga Gichaiya
Automatic Load Sensing Power Switch
Wednesday, September 24, 2008 6:58:46 AM
1. the value of r2 r6 need to be changed if you are in 240v .what is the value of this resistors. 2. if u need islorate .that is high version, about 15amp.what is rate of resistor.
Automatic Load Sensing
Thursday, June 05, 2008 12:40:02 AM
To make a "high power version" capable of sustaining ~15A total (Master + slave peak ), I understand that I need to change the W rating of R1 and also the relay/switch model but anything else ?
gagan kapoor
Automatic Load Sensing Power Switch
Monday, May 26, 2008 2:40:53 AM
what should be the value of r2 & r6 when connected to 240vac
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