Making a Breathing LED Light With NE555

Everyone has heard of the NE555. It is a versatile chip that can quickly generate positive, triangle, and trapezoidal waves. Today we use NE555 to make a breathing LED light

We will need the following components to build this circuit. All these components are given with links from https://chipdatas.com. So, you can order the components easily.

NE555 IC

10Ω resistors * 2

47Ω resistors * 2

100Ω resistors

LED

8050 transistor

220uF 25V Electrolytic Capacitors * 2

CR2032 battery 6V * 2

Breadboard * 2

Some Circuit wire

Tools Needed:

Soldering Iron

Iron StandFluxNose pliers

Let’s take a look at [Waveform 1] first. If we express the breathing effect just described in words as a waveform, you will see several upside-down bowls. Okay, let’s describe it again: after powering on, the LED light gradually becomes brighter (point A to point B), stays brightest for a few seconds (point B to point C), and then gradually dims until it goes out (point C to point C). point D). After it goes out for a few seconds (point D to point E), it gradually changes from dark to bright (point E to point F), and this cycle continues (point F to more points later). If a digital-analog circuit is used, the way to change the brightness of the LED is to control the current flowing through the LED and also to control the voltage across the LED. Slowly rise from 0V to 6V (if a 6V power supply is used), and then slowly decrease from 6V to 0V. Think about what kind of circuit can achieve a gradual change in voltage?

Yes, it's a capacitor. Capacitors have the characteristics of charging and discharging, and every time it starts charging or discharging, it will gradually change the voltage. Capacitors are often used in voltage stabilizing circuits, because the voltage on the capacitor cannot change abruptly, but can only change slowly to achieve a stable voltage. And now we take advantage of this feature,

Let the LED slowly change brightness. So we designed the circuit of 【Analysis Circuit Diagram 1】. Transistor VT1 is used in the circuit to drive the LED, and R5 is a blocking resistor used to determine the maximum brightness of the LED. The reason why the triode is used is to control the LED with a small current. The advantage of a triode is that adding a small current to the base (b) can control the large current on the active electrode (c) and emitter (e). Just add a small capacitor to the base (b), otherwise, you have to connect a capacitor with a very large capacitance in series with the LED. The triode is like the gas pedal of a car, stepping on it lightly releases powerful power. Without the accelerator, I don’t know how many cows it would take to pull the car along the highway.

Two resistors R3 and R4 are connected in series to the base of the triode, and a capacitor C2 is connected between the two resistors. This is an artifact used for "slow change". The other end of the resistor R3 is connected to switch K1, and the other end of the switch is connected to the positive terminal of the power supply, where there is an inexhaustible and powerful current from the battery. Once the switch is closed, the current flows down to the capacitor C2 and the transistor VT1, which is the beginning of the circuit operation. Why add two resistors? Isn't one resistor enough to limit the current? In fact, the problem is not currently limiting. Pay attention to the position of the resistors. Resistors R3 and R4 are on both sides of capacitor C2. Resistor R3 is the resistor that limits the charging of capacitor C2. The current from switch K1 becomes a trickle after passing through R3, allowing the capacitor to charge slowly. Resistor R4 is located between the capacitor and the transistor. It is a resistor that limits the discharge of the capacitor. With R3 and R4, the process of charging and discharging the capacitor slows down, and choosing different resistor values will have different charging and discharging times. Now, when switch K1 is closed, a steady stream of current flows into the capacitor, charging the capacitor. The capacitor voltage slowly rises from 0V to 6V. At this time, the base voltage of the triode connected to the same line as the capacitor also rises together with the capacitor voltage. The phenomenon is that the LED goes from off to brightest, which takes a few seconds. Next, the switch K1 is turned off, the capacitor C2 loses the source of current, and the base of the triode VT1 continues to absorb current. As a result, the capacity of the capacitor continues to decrease, and the voltage also drops from 6V to 0V (in fact, it does not necessarily reach 0V, and the discharge stops when it drops below the minimum conduction voltage of the triode base). The phenomenon is that the LED goes from brightest to extinguished, and this process also takes a few seconds (the control of the time is the credit of resistors R3 and R4).

Okay, now we have achieved the effect of the breathing light. Turn the switch on and it will turn on, and turn off the switch and it will turn off and on. Then continue exploring and designing an automatic switching circuit.

The next job is to give life to the LED light and let it breathe in by itself. How do we do this? First, let’s take a look at the waveform diagram. You must know that before designing a circuit, it is a very effective way to express your design goals with some waveform diagrams and schematic diagrams. From the analysis of the drawings, we can find out what circuit we need. As will be studied below [Waveform 2]. Look, I've linked the desired behavior of the future "automatic switch" to the LED output waveform.

It can be seen from this correspondence that: at point A, the LED needs to gradually light up, and at this time the switch is closed, and the waveform is a process from low to high. Note that 6V is used when the switch is closed in the figure, which means that the voltage between the positive electrode of the capacitor and the base (b) of the transistor reaches 6V when it is closed, and the voltage is 0V when it is turned off. Why do you express it this way? Will understand later. OK, now to point B, where the LEDs are at maximum brightness for a few seconds. During this time the switch is still closed, yes. Then there is point C, the LED turns from bright to dark, and this time is the moment when the switch is turned off. When point D is reached, the LED goes off for a few seconds and the switch remains off. At point E, the LED gradually lights up again, and the switch needs to be closed again. And guess what? We get a "square wave", that is, the waveform is a square with right edges and right angles. The task of the waveform is completed, and then think about what can generate a square wave.

There are many circuits that generate square waves, just search "square wave circuit" on GOOGLE. You get the fruits of all human wisdom about square wave circuits. I searched, and one of them claimed to be "the simplest square wave circuit in the world". Click to see, it turns out that a square wave can be generated by using 2 NOT gate circuits (inverters) and some capacitors and resistors. This is indeed a good circuit design, but most NOT gate circuits are larger chips. In order to use a small and simple design, I found the NE555 chip. Everyone has heard of NE555. It seems to be a versatile chip. It can easily generate positive waves, triangle waves, and trapezoidal waves. So let's use NE555 to make the "automatic switch" part. You can also try to use other chips or circuits, as long as they can output square waves. Please look at [Analysis Circuit Diagram 2]. The circuit in the picture is very simple. This is the most classic peripheral circuit of NE555. It is recommended that you remember it. The circuit consists of a NE555 chip, 2 resistors, and 1 capacitor. The capacitor C1 is charged and discharged to generate the switching time length. R1 and R2 charge and discharge capacitor C1. When the circuit is working, C1 will charge through R1 and R2. See if the values of these two resistors look familiar. Their values are the same as R3 and R4 introduced earlier. As the saying goes: The circuits of happiness always have many similarities. While we are thinking, the voltage of capacitor C1 continues to rise. When the voltage reaches a higher value (usually 2/3 of the power supply voltage), pin 6 of the NE555 connected to it also reaches the same voltage. At this time, the internal operation of the NE555 chip starts, and the 3rd pin outputs a low level (that is, 0V), and the 7th pin also shows a low level (0V). Because the 7th pin becomes low level, the current from the positive pole of the power supply flows directly into the 7th pin through R1. Because the current always flows from the high-level end to the nearest low-level end with the least resistance. The current flows into pin 7, so there is no

A current flows through R2 to charge C2. Instead, only C2 discharges to pin 7 through R2. When the voltage on C2 is less than a certain value (usually 1/3 of the power supply voltage), the voltage of pin 2 connected to it is also lower than this value. At this time, the NE555 chip starts to act, making pin 3 output high level, while pin 7 is no longer low level. The circuit is now back to the way it was when it first started working, and C2 is recharged. If the cycle continues like this, the voltage of C2 is always between 1/3 and 2/3 of the power supply voltage.

Wandering between, but causing the NE555 chip's 3rd pin to output a stable local wave. Adjusting the value of R2 or C2 can adjust the cycle time of the square wave. Okay, now we have the "auto switch".

We have a breathing light circuit and an automatic switch circuit. The next step is to combine them effectively. The output produced by NE555 is only the third pin, and the input of the breathing part is the line connecting the capacitor C2 and the base of the transistor VT1.

Let's analyze, What is the output of NE555? What is the input to the breathing circuit? NE555 outputs a square wave, the low-level voltage is 0V, and the high-level voltage is 6V (power supply voltage). Friends who know the NE555 chip know that its third pin can output a large current when it is at a high level, which is very important because the input terminal of the breathing part needs a larger current to come in. If the input current is too small, the capacitor C2 cannot be charged in time. In addition, the switching voltages required for the breathing part are also 0V (when fading) and 6V (when fading). It seems that the output of NE555 is completely compatible with the input of the breathing part. If their voltages and output currents are different, they cannot be directly connected, and a conversion circuit needs to be added or a circuit scheme reselected. Please see [Analysis Circuit Diagram 3]. Just connect pin 3 of NE555 directly to one end of R3. At the same time, connect the power supplies in the circuit together to share a power supply. It should be noted here that if the two circuits are combined, they can use different power sources, but their ground wires (GND) must be connected together to ensure a common reference voltage, otherwise the circuit will not work.

[Official Circuit Diagram 1] is the complete circuit diagram after assembly, and uses a 6V battery power supply. In order to make the circuit diagram look beautiful, I messed up the pin positions of the NE555. During actual production, you can refer to the physical components under the circuit schematic diagram for comparison. Changing the value of resistor R2 in the circuit can change the delay time of the automatic switch, changing R3 can change the fading time (charging time), changing R4 can change the fading time (discharging time). Changing R5 changes the brightness of the LED light. Please see [Physical Photo 1], this is the breathing light circuit that I inserted on the small breadboard referring to [Official Circuit Diagram 1]. I used yellow LEDs, I like yellow, warm. The finished circuit is small and the two batteries are placed on another breadboard. I put it on the table and watched it breathe like a sleeping child. The coin cell battery should keep it alive for 3 or 4 days, or even longer if you switch to 4 alkaline batteries. The circuit for the breathing light is ready and it is automatic.