This is because, during day time the LDR resistancedrops drastically causing the pin#2 potential to drop significantly and below pin#3 potential. This is the circuit you should get. If phototransistor Q2 does not draw current, then Q1's base gets current through R2. The base is at 0.7 V (always 0.7 V higher than the emitter), and the power supply is 3 V, so there's 3 V - 0.7 V = 2.3 V across R2. Then because of Ohm's Law the current through R2 = 2.3 V / 1 kΩ = 2.3 mA. Transistor Q1 will want to increase that 100-fold to get 230 mA collector current. R1 will limit that. If Q1 is on then the LED's cathode will be at around 0 V, and the anode 2 V higher, at 2 V (that's typical for a red LED). So there remains 1 V for resistor R1, and if we want 20 mA through it (and the LED) we apply Ohm's Law again: R = 1 V / 20 mA = 50 Ω. So R1 will make sure that the LED current won't go higher than 20 mA. As long as ambient light illuminates the phototransistor, the voltage at its emitter lead is sufficiently high for the base of PNP transistor Q1 to keep it shut off. Though a single NAND gate would be enough for implementing the actions, three gates have been engaged as buffers for getting better results and in a view of utilizing all of them as in any case three of them would be left idle.

The circuit may consume approximately 13 mA when the LED is illuminated and just a few hundred uA when its switched OFF. Circuit Operation Bill of material for the discussed automatic night operated LED lamp. The circuit is now transformed into a darkness activated switch for the load or the connected lamp. Once this is implemented, the LDR now gets associated with the positive line, and the R4 end gets connected with the negative line. Resistor R5 introduces some level of hysteresisto the op amp response so that the output of the op amp does behave erratically during the twilight or the transition periods where the light level on the LDR is at the threshold points. To make the circuit extermely compact one button battery type is preferred here, quite akin to those used in calculators, watches, etc.

Light Switch Circuit Diagram

Its non-inverting input pin#3 is clamped with a fix reference derived from the junction of the resistive divider formed by R2/R3.

The freewheeling diode connected across the relay coil is to protect the transistor from relay coil reverse EMF spikes, and this diode could be any silicon diode such as the 1N4148 or 1N41007. Automatic Light Sensitive Switch with Adjustable Dawn or Dusk Switching You can find lots of applications in which photoelectric detection is employed to turn a circuit on/off. This below shown straightforward circuit is configured like a bistable multivibrator. The phototransistor base is floating, and I swapped the 1k for a 22k in my circuit to bias it correctly (I arrived at this value roughly, see below) and used a BC337 npn. Since the BC337 has lots of gain the 22k works well for the base current.Referring to the schematic above, the working of this dual function light activated switch can be understood with the following points: To help with the breadboard I just threw together the little circuit shown in your question. I only had an IR phototransistor but it doesn't matter much for this, it still works the same. Anyway here are a couple of pictures of it working, hopefully you can see how the connections go: WARNING: The circuit is not isolated from the AC mains supply and will be floating at the mains level, which can be fatal for anybody who touches the circuit in powered ON condition, without an insulated enclosure. Circuit Description The reason the circuit is usually not the best way to do this is because it relies upon Hfe, which can vary quite widely in a transistor and is subject to temperature changes. This means the base resistor must be chosen according to the particular transistor used.

This fourth circuit is not only simple but very interesting and very easy to build. You might have seen the new flashlights manufactured with new high bright high efficiency LEDs. In other words the circuit can be used like aday activated automatic switch or a darkness activated automatic switch, depending upon the user preference or the specific application need. With the BJT turned OFF, the SCRand the load are also turned OFF in the presence of day light on the LDR. Some practical advice: if you want to use higher power lamps then diodes D1 …. D4 need to be replaced with other type 1N5404 and thyristor TH1 will be equipped with a heat sink. With these changes, the circuit can control currents up to 3A.Can be used to switch a lighting circuit 'ON' and 'OFF' based on light conditions (nightfall, daybreak) This variable resistor is used for setting the triggering point of the gate when the light falling over the LDR reaches the desired specified intensity. EDIT - the breadboard circuit you have added looks correct (though it's hard to read..) so go ahead and try it. If it doesn't work let us know. Maybe change the resistor to 2k or larger if you are worried about blowing the LED. This situation takes place when the R4 end is connected with the positive line, and the LDR is connected at point B which is the ground line, and illuminated by day light.

This last principle is used in the sensitive switch installation you will be presented next. Photocell is placed in a bridge circuit and a comparator is used as detector bridge balanced. The comparator’s output control thyristor through a transistor. In this circuit, protective measures must be taken because it is isolated from the network. Light Switch Circuit Diagram This prompts Q1 to begin getting the biasing via its base/ground resistor R and it starts to illuminate brightly as darkness gets deeper.Since the pin#3 is fixed at 5 V, means, as long as the pin#2 remains below this reference level, the op amp output remains high, enabling the T1 to remain switched ON, and the SCR/load switched OFF. To give an idea of why the 22k resistor, the BC337 I'm using has a gain of around ~400, and the voltage it will see is 3V - (Vled + collector-emitter drop) -> 3V - (1.8V + 0.7V) = ~0.5V. So 0.5V / 22k = 23uA into the base. Switching point of the light sensitive switch may be adjusted from P1. With the potentiometer set the minimum (minimal resistance) lamp will light a crepuscular light. If you want more flexibility, replace P1 with one that has a value of 1 MΩ. If desired operation reverse position in the scheme of the photoresistor can be changed with the group P1/R4. LA1 lamp will be extinguished, in this case, after dark. To make our circuit operative after dark, a phototransistor is employed, so that when the daylight is void, the LED gets switched ON.