Make a simple LED flashing light

The LED flashing lights I want to introduce to beginners are divided into single group flashing, two groups of alternating flashing and three groups of cyclic flashing.

The circuit of a single flashing lamp is shown in Figure 1. It belongs to the Rc oscillation circuit. Its working principle is: when the power is turned on, the current charges C1 through RP, and the voltage on C1 cannot immediately reach the conduction voltage of VT1. VT1 is in the cut-off state at the beginning, and VT2 is of course also in the cut-off state, and the LED is not lit. When the voltage on C1 is charged to 0.7V, both complementary tubes are turned on, the LED is lit, and the charge stored on C1 is also released by the emitter junction of VT1 and reduced. When the base voltage of VT1 is reduced to 0.5V, both complementary tubes are cut off, and the LED is off. The circuit is repeated in this way. The LED presents a flashing state of turning on and off. If RP is adjusted to about 470kΩ, the flash frequency is close to 1 second. This circuit consumes very little power. If two No. 5 Nanfu batteries are used, it can be used for more than half a year to 1 year even if it is always turned on.

The circuit of two groups of alternating flashing lights is shown in Figure 2. It is a multivibrator composed of 555 time base integrated circuit. Its working principle is: at the moment of power on, the voltage on the timing capacitor C1 is 0V, the potential of pin ② is lower than (1/3) VCC, pin ③ outputs a high level, LED1 is off, and LED2 is on. After that, the power supply charges C1 through R1 and RP. When the voltage VC1 on C1 rises to (2/3) VCC greater than the reference voltage, pin ③ outputs a low level, LED1 is on, and LED2 is off. In this way, VC1 is periodically charged and discharged between 1/3 and 2/3 of Vcc, so that the level of pin ③ jumps periodically, forming self-excited oscillation. Its oscillation frequency is the number of times the two LEDs flash per second. f=1.443/(R1+2RP)C. The operating voltage of the 555 time base integrated circuit is 4.5~18V. The two current limiting resistors in Figure 2 refer to 100Ω when using a 5V power supply voltage.

The circuit of three sets of cyclic flashing lights is shown in Figure 3, which belongs to the astable self-excited multivibrator. Its working principle is: when the circuit is powered on, the positive voltage is charged through three LEDs, current limiting resistors R4~R6 and the emitter junctions of VT1~VT3 to C1~C3, providing the base voltage for the transistor. Although the values ​​of the three sets of circuits and components are the same, the parameters of the components cannot be exactly the same. Therefore, one transistor must be turned on first. Assuming that VT1 is turned on first, the LED1 group is lit, and C1 is reversely charged through R2 and the CE pole of VT1. VT2 is cut off, and LED2 will not light up. When the voltage on C2 is charged to 0.7V, VT3 is turned on and LED3 is lit. In this way, one of the three sets of LEDs will always be cyclically extinguished and two sets of LEDs will be cyclically lit. Its cycle period can be adjusted by changing the values ​​of C1~C3 and R1~R3; the larger the value, the longer the cycle.

The flashing lights introduced above can be connected in series or in parallel with multiple LEDs under the premise of selecting a suitable power supply voltage, but an appropriate current limiting resistor must be selected. The current limiting resistor selected when the LEDs are connected in series can be calculated according to the following formula:

R=(Uouter-UF・N)／IF

Where: R-current limiting resistor. Uexternal-external voltage, UF-forward starting voltage of each light-emitting diode (such as UF of φ5mm white LED is about 3~3.2v, limit voltage 3.9v; UF of red LED is about 1.6~1.8v). IF-working current of light-emitting diode, N-number of light-emitting diodes in series. If φ5mm high-brightness light-emitting diodes are used, the working current can be 15~20mA. Sometimes, the saturation voltage drop of the transistor (about 0.3V) should also be taken into account. If the LEDs are connected in parallel, IF should be multiplied by the number of parallel LEDs; at the same time, the value of the limit parameter ICM of the transistor should be considered, which should not exceed the standard and leave room. If the power supply voltage in Figure 3 is increased. The three base resistors can be reduced to 18kΩ~30kΩ, C1~C3 can be reduced to 22μF~30μF, and attention should be paid to the selection of suitable capacitors with higher withstand voltage values.

The flashing light circuit introduced above is simple and can be made by direct welding. The β value of the transistor used should not be too low, and should be above 80. If there is no high β value tube on hand, two tubes can be used to form a composite tube as a substitute.