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This prompts Q1 to begin getting the biasing via its base/ground resistor R and it starts to illuminate brightly as darkness gets deeper. However when darkness sets in, the phototransistor starts losing conduction and the voltage at its emitter diminishes causing the phototransistor to slowly switch OFF. To make the circuit extermely compact one button battery type is preferred here, quite akin to those used in calculators, watches, etc.Īs 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.

To make our circuit operative after dark, a phototransistor is employed, so that when the daylight is void, the LED gets switched ON. The idea is to achieve something similar but with an added feature. You might have seen the new flashlights manufactured with new high bright high efficiency LEDs. This fourth circuit is not only simple but very interesting and very easy to build. The above actions are exactly replicated using the IC 4049 which is also wired with similar configuration and is quite explanatory.Ĥ) Automatic Night Operated LED Lamp Circuit The connected load over the relay now flips into the intended actions. The result is the triggering of the transistor and the relay assembly. 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.Īs this happens, the gate input goes high, the output consequently becomes low making the outputs of the buffer gates high. 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. 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 first circuit out of the two utilizes the IC 4093 which is quad two-input NAND gate IC.Įach of the gates are formed into inverters by shorting its both the inputs together, so that the input logic level of the gates now get effectively reversed at thie outputs. The second and the third figure incorporates CMOS ICs for executing the above functions and the concept remains rather similar.

LDR = any small LDR with around 10k to 50k resistance in day light (under shade)Ģ) Light Activated Day Dark Switch using CMOS NAND gates and NOT Gates.

The opposite happens when the light over the LDR falls or when darkness sets in. When T1 conducts, T2 is switched OFF ans so is the relay and the connected load or the light. The use of two transistors particularly helps to reduce the hysteresis of the circuit which would have otherwise affected the circuit if only a single transistor would have been incorporated. The LDR is used for sensing the ambient light conditions and is used for triggering T1 when the light level crosses a particular set threshold. The transistors T1 is wired as a comparator and consists of an LDR across its base and the positive supply via a preset. The transistors are rigged as inverters, meaning when T1 switches, T2 is switched OFF and vice versa. You will also Like: Automatic Street Light Circuit The first figure shows the use of a couple of transistors in association with a few other components lke resistors for the construction of proposed design. Let’s evaluate the circuits one by one with the following points: The first diagram shows how the circuit can be configured using transistors, the second and the third circuits demonstare the principle by using CMOS ICs while the last circuit explaines the same concept being implemented using the ubiquitous IC 555.

Using as Night or Darkness Activated Switchġ) Light Activated Day Night Switch using Transistors.Automatic Light Sensitive Switch with Adjustable Dawn or Dusk Switching.

4) Automatic Night Operated LED Lamp Circuit.3) Light Activated Relay Switch using IC 555.2) Light Activated Day Dark Switch using CMOS NAND gates and NOT Gates.1) Light Activated Day Night Switch using Transistors.