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. 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. This prompts Q1 to begin getting the biasing via its base/ground resistor R and it starts to illuminate brightly as darkness gets deeper. 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.

With the BJT turned OFF, the SCRand the load are also turned OFF in the presence of day light on the LDR. 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. There are a wide range of applications for light sensitive switches: lighting, entrance door, automatic staircase, automatic opening of doors to the action of a ray of light, alarm systems, etc… Many of us are familiar with photosensitive switches made with a single transistor that is controlled by a photoresistor placed between the base and the recharger, depending on what is desired: normal functioning “closed” or normal “open” the switch. Once this is implemented, the LDR now gets associated with the positive line, and the R4 end gets connected with the negative line.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.

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. This is because, during day time the LDR resistancedrops drastically causing the pin#2 potential to drop significantly and below pin#3 potential. To make the circuit extermely compact one button battery type is preferred here, quite akin to those used in calculators, watches, etc. Can be used to switch a lighting circuit 'ON' and 'OFF' based on light conditions (nightfall, daybreak) The gate which is responsible for the sensing can be seen accompanied with the light sensing device LDR wired across its input and the positive via a variable resistor.

Light Switch Circuit Diagram

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. Now, the non-inverting pin of the op ampbeing fixed at around 5 V reference, the inverting input pin#2 is used for the detection of the light level via another resistive network formed by R1/P1 and the LDR. Using as Light Activated Switch