1 KW Power (Watt) Meter

This watt-meter circuit has measurement range up to 1-KW. This circuit can give the complete (X)(Y) function although uses only one transistor. Actually, this circuit is used for 117 Vac±50 Vac operation. For lower or lower voltage, this circuit can be modified easily. This circuit only measure power on negative cycles. The advantages of this circuit is this circuit does not need external power supply. This circuit measures true power that is delivered to the load. Here is the schematic diagram of the  circuit:

At idle section, this circuit draw only 0.5W. This circuit has load current-sensing voltage of 10mV and load voltage loss of 0.01%. For linear loads, Rejection of reactive load currents is better than 100:1. When using a 50-μA meter movement, the nonlinearity of this circuit is about 1% full scale. Copper shunt can be used to give correct gain due to temperature. [Circuit's schematic diagram source: seekic.com]

Ref:http://freecircuitdiagram.com/2011/03/03/1-kw-power-watt-meter/

LM10 Battery Voltage Threshold Indicator

A battery threshold indicator circuit shown in the schematic diagram below has current regulation mechanism in driving the LED. A sufficient current should be satisfied at the minimum voltage but no excessive current when the voltage is at the highest level. Balance pin (5) is used as the reference  voltage for regulating the current. This pin generate 23 mV, which is internally temperature compensated.

When the voltage on the reference-feedback terminal (8) drops below 200 mV, the reference output (1) rises to supply the feedback voltage to the op amp through D2, so the LED current drops to zero. The minimum threshold voltage for these circuits is basically imited by the bias voltage for the LEDs. Typically, this is 1.7V for red, 2V for green and 2.5V for yellow. These two circuits can be made to operate satisfactorily for threshold voltages as low as 2V if a red diode is used. [National Semiconductor Application Note]

This battery voltage threshold indicator circuit  overcomes the difficulties caused by voltage change across the diode biasing resistor. [Circuit's schematic diagram source: National Semiconductor Application Note]

Ref:http://freecircuitdiagram.com/2011/03/11/lm10-battery-voltage-threshold-indicator/