Mains Voltage Indicator with a LED

This is a mains 230V AC voltage indicator and is a LIVE CIRCUIT, so take care. The Resistor has to be a fusible ceramic wire wound and the capacitor 630V AC or higher capacity.

This circuit has been drawn from my memory and i have not tried it out again, just see if it is ok and then try. You should use the fuse of 100mA a slow blow if you want but it is very important. This circuit has to be enclosed in a plastic sealed enclosure to avoid contact.

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Using LED As A Light Sensor

This circuit shows how to use an ordinary LED as a light sensor. It makes use of the photovoltaic voltage developed across the LED when it is exposed to light. LEDs are cheaper than photodiodes and come with a built-in filter, which is useful when the application involves colour discrimination. The photo-voltage of a red LED (its bandgap voltage) is typically about 2V. The source impedance of this voltage is about 800MΩ in daylight, rising to infinity in darkness. A TL071 JFET input op amp is used to amplify and buffer this extremely high impedance signal.

Circuit diagram:

LED As A Light Sensor Circuit Diagram

Resistor R1 ensures that the op amp "sees" a 0V input when the LED is in total darkness. To avoid undue loading of the signal, R1 would ideally be a 100MΩ or larger resistor but since such high values are rare and expensive I used a smaller value and increased the gain of the op amp to compensate for the voltage loss. To avoid the need for a second variable resistor to set the op amp’s input offset to zero, R1 must be large enough for the reduced voltage across the LED to swamp the op amp’s input offset voltage. With a 30MΩ resistor for R1, the voltage at the op amp input when the LED is exposed to bright light is reduced to about 60mV.

This is just over four times the 13mV maximum input offset of the TL071 op amp. R1 can be three 10MΩ resistors in series. Alternatively, I have found that a reverse-biased 1N4148 diode has an impedance of about 30MΩ (connect it in the circuit with the anode to ground). The output of the circuit is about 0V when the LED is in darkness. VR1 sets the gain of the op amp and it should be adjusted to give the required output voltage when the LED is exposed to bright light. http://www.hqew.net/circuit-diagram/Using-LED-As-A-Light-Sensor_12671.html

Direct Control of LED Current Boost Converter

To control the current of white  LED  white LED current detection, detection of white LED current is the easiest and most convenient way is to use a sense resistor in series with the white LED, the total output voltage of such a converter is white LED forward voltage and current-sense resistor and pressure drop. To simplify the circuit and reduce costs, designed to make the best sense resistor feedback voltage of the voltage drop across resistor selected converter. Galvanometer voltage drop across the resistor should be as small as possible, so as to minimize power consumption, too high a voltage will only increase the power consumption. White LED forward voltage and current-sense resistor voltage drop of usually higher than the mobile phone battery voltage, therefore, must use a boost converter to provide power. Assume that the converter's feedback voltage is 500mV, the total output voltage must be between 4.1 ~ 4.3V. Direct control of the white LED current boost converter shown in Fig.