Candles have been very useful since ancient times, even before Edison came up with the idea of the light bulb to guide mankind at night. Today, from churches to kitchens, candles not only provide light when needed, but also add beauty and provide warmth. While ordinary candles work fine, they quickly melt, making the place dirty and sometimes a fire hazard if left unattended. Therefore, in this tutorial, we will use some simple electronics and LEDs to create a flameless electronic candle. In addition, this smart candle will turn on automatically at night or in the dark and turn off automatically during the day
Required materials
LM358 IC
LDR (Light Dependent Resistor)
1M and 1K resistors
Light-emitting diodes
10K pot
12V female DC power jack and 12V adapter
Card stock and perforated board
LM358 – Operational Amplifier Comparator
The brains behind this circuit is the LM358 IC, which acts as a comparator in this particular design. Before we dive in, let’s briefly examine the LM358, an operational amplifier (Op Amp) IC that consists of two op amps that can withstand voltages between 3.3V and 32V and has a very low supply current consumption of 500µA. The internal appearance of the IC is shown in the following figure.
It is often used to build simple comparator and amplifier circuits, but can also be found in active filter circuits, waveform shapers, etc. In this project, we will use the LM358 as a voltage comparator. A voltage comparator is used to compare two voltages and find out which one is greater than the other, and then change the output to high or low depending on that voltage. So, if we apply a voltage to the inverting and in-phase inputs and if the voltage on the in-phase input is greater than the voltage on the inverting input, the output goes high and vice versa, the output goes low. This project works exactly according to this principle. The voltage comparison equation is as follows.
V OUT =A O (V in+ – V in- )
Where A O is the open-loop gain of the op amp. V in+ is the input voltage at the in-phase input and V in- is the input voltage at the inverted input. Therefore, if V in+ is greater than V in- then the output will be high, otherwise it will be low.
LDR
If op-amps are the brains of our circuits, then LDRs are the sense organs. A light-dependent resistor (LDR) or photoresistor is a light-controlled resistor. Its resistance decreases as the intensity of light increases, and vice versa. In fact, when light is incident on an LDR, the semiconductor absorbs photons of light, bonding electrons leap to the conduction band, and the resistance decreases due to photoconductivity.
Circuit diagram and description
The circuit is not difficult. The complete circuit diagram for the electronic candle is given below
Connect a 1K resistor to pin 1 of the IC as shown in the circuit diagram, then connect the positive terminal of the LED to this resistor and the negative terminal to ground. Now connect the middle pin of the 10K potentiometer to pin 2 of the IC, and connect ground and 12V to the rest of the potentiometer pins. Connect a 1M resistor to 12V and connect the LDR in series with this resistor. Now connect the other end of the LDR to the ground side of the circuit. Connect the common of the LDR and 1M resistor to pin 3 of the IC. connect 12V to pin 8 and ground to pin 4 of the IC and you are ready to go. You don’t need to be very picky about the resistors we have connected. But make sure the resistors connected to the LDR are megohms, and the resistors with the LEDs are a few thousand.
We have built the complete circuit on a dummy board to make it compact and easy to use. This is really a simple circuit, you just need to improve your soldering skills and start designing it. First, mount the 12V female DC power jack on the perforated board. Keep the pinout of this jack in mind when designing the circuit. As shown in the diagram below.
The pinout of the op-amp has been discussed above, and the resistors and LDR are not polarized. After the soldering work is completed, the board should be as shown in the figure below.
Smart Electric Candles – Jobs
After designing the circuit on the perforated board and soldering it up, connect the 12V adapter to the female jack and your LED must glow. Now, to calibrate the comparator, adjust the 10K potentiometer to the level where the LED just turns off. Now cover the LDR with your hand and you will see the LED light up. You can adjust the sensitivity of the LDR by adjusting the potentiometer.
Now, let’s understand how this candle works. As we already know, in the dark, the resistance of the LDR increases to megohms and decreases to a few hundred ohms as the light intensity increases. Therefore, since the resistance is very low, the voltage at the ends of the same signal is very low compared to the inverting end, since we have a 10K potentiometer connected. So in this case, the output voltage is also very low, so the LED will not turn on. But in the dark, the resistance increases to megohms, which is very high compared to the 10K pot, so the LED will glow.
Adjusting the potentiometer will manipulate the sensitivity. By sensitivity, I mean the intensity of light at which your comparator turns on the LED. if you adjust the pot close to the LED, then it will also detect a tiny amount of darkness. But if you adjust it far away before the LED is on, then it will only detect high darkness. You can also test the sensitivity by placing your hand in front of the LDR. If it detects your hand far away, it is highly sensitive, if you need to cover it to light up the LED, it is not so sensitive.
If you want to use multiple LEDs, then this is not a problem. Connect two or three LEDs in series and finally connect them to where we connect a single LED and its perfect position. But make sure that your comparator can supply enough current to power all LEDs.
To make the candles, you can use anything to cover the LEDs. i have used cardstock and tissue paper. Roll the card up to the size of the LED and cut it from the top into a flame shape or whatever shape you want to make it look attractive. Cover the LED with this candle and you have made your own smart electronic candle.
