Lower cost triode three-way circulating water lamp

The idea of ​​producing a red-themed audio
 
display effect according to the requirements of the event comes from the up owner of B station: Black Black Technology https://www.bilibili.com/video/BV19V4y1F7sf/?spm_id_from=333.999.0.0
 
Principle:
Self-excited multivibrator : An oscillator that self-excites and generates a square wave output by turning two electronic devices on and off alternately through resistance-capacitance coupling. Commonly used as square wave generator. A multivibrator is a self-excited oscillator that can generate rectangular waves, also called a rectangular wave generator. After the power is turned on, rectangular pulses can be automatically generated without the need for external pulses!
A multivibrator is a self-excited oscillator that can generate rectangular waves, also called a rectangular wave generator. "Multiharmonic" means that in addition to the fundamental wave component, the rectangular wave also contains rich higher-order harmonic components. A multivibrator has no stable state, only two transient stable states. During operation, the state of the circuit automatically alternates between these two transient states, thereby generating a rectangular wave pulse signal, which is often used as a pulse signal source and a clock signal in sequential circuits.
The essence is that capacitors C1 and C2 alternately charge and discharge. This process controls the conduction and cutoff of transistors BG1 and BG2.
The working status of transistors BG1 and BG2 determines the status of light-emitting diodes LED1 and LED2. Since we cannot directly see the saturated or cut-off state of the transistor, we choose "low frequency circuit" and add two light-emitting diodes to the circuit to indicate the two transient states of the circuit. At the moment when the circuit is powered on, transistors BG1 and BG2 obtain forward bias voltages through resistors R3 and R2 respectively, and both transistors tend to conduct. At this time, capacitor C1 is charged through resistor R1 and transistor BG2, and capacitor C2 is charged through resistor R4 and transistor BG1 charging. Although the circuit is "symmetrical" and the two transistors are of the same type, their characteristics are still not 100% identical.
If the current gain of transistor BG2 is higher than that of transistor BG1, it will enter the saturation state first.
When BG2 is saturated, capacitor C2 is discharged through resistor R3 and transistor BG2. Since "the voltage across the capacitor cannot change suddenly", at this time, the side of capacitor C2 coupled with the base of transistor BG1 generates a "negative jump pulse", causing the transistor to BG1 cuts off quickly. The circuit thus enters the process of self-oscillation, and the two transient states alternate. It is precisely because of the slight differences in the same type of devices that the "symmetrical" circuit begins to "oscillate". (1) Transient state 1: Transistor BG2 is saturated and conductive, capacitor C2 begins to discharge (reverse charge), transistor BG1 is cut off, and capacitor C1 begins to charge.
The phenomenon is that LED2 lights up and LED1 goes out. 
 At the previous moment, the discharge of C1 was completed. The discharge result caused the plate potential of the base coupling between C1 and BG2 to increase to 0.7V, causing BG2 to change from the cut-off state to the saturated conduction state. The saturated conduction of BG2 caused C2, which had been charged at the previous moment, to The positively charged plate of C2 is connected to the zero point of the potential (grounded),
because "the voltage across the capacitor cannot mutate", the negative plate of C2 will produce a "negative jump pulse", and the potential suddenly mutates to a negative value, due to the negative plate of C2 Coupled with the base of BG1, the base of BG1 is controlled by the negative potential, causing BG1 to turn from the on state to the off state.
At this time, C1 continues to charge through the loop composed of power supply E, LED1, R1, C1, and BG2, so that the pole connected to C1 and R1 is positively charged, and the potential of the pole coupled to BG2 is zero; C2 begins to charge via power supply E, The loop composed of R3, C2, and BG2 is discharged; because BG2 is turned on, the loop composed of E, LED2, R4, and BG2 causes LED2 to emit light. Transient one can be seen in the "dotted line" part in Figure 1.
(2) Transient 2: Transistor BG1 is saturated and conductive, capacitor C1 begins to discharge (reverse charge), transistor BG2 is cut off, and capacitor C2 begins to charge.
The phenomenon is that LED1 lights up and LED2 goes out.
In transient one, the continuous discharge of capacitor C2 makes the plate potential of C2 connected to the collector of BG2 zero, and the plate potential of C2 coupled to the base of BG1 continues to increase from the amplitude of the "negative jump pulse". When it crosses the zero point, When it rises to 0.7V, the base potential of BG1 also rises to 0.7V, causing BG1 to change from the cut-off state to the saturated conduction state.
The conduction of BG1 ends the charging process of C1 and connects the positively charged plate of C1 to the potential zero point (grounded) through BG1. The potential of this plate is zero instantaneously, but because "the voltage across the capacitor cannot change suddenly", The other plate of C1 will instantly generate a "negative jump pulse", causing the potential to suddenly change to a negative value. Since this plate is coupled to the base of BG2, the base of BG2 is controlled by the negative potential, causing BG2 to turn from a saturated conduction state to a cut-off state. state.
At this time, C2 continues to charge through the loop composed of power supply E, LED2, R4, C2, and BG1, so that the pole connected to C2 and R4 is positively charged, and the potential of the pole coupled to the base of BG1 is zero; C1 begins to charge via power supply E , R2, C1, and BG1 are discharged; since BG1 is turned on, a loop is formed by E, LED1, R1, and BG1, and LED1 emits light. Transient state two can be seen as the “dashed line” part in Figure 1.
 
To sum up, since the charging and discharging of the capacitor controls the working state of the triode, the alternating charging and discharging of the two capacitors creates two transient states in the circuit. This is the essence of self-oscillation of the self-excited multivibrator.
[1] Lu You. Circuit simulation and application of self-excited multivibrator [J]. Journal of Shandong Electric Power College, 2010, (1): 61-64
Taking advantage of the differences produced in the production process of transistors and other devices, LED Flashes regularly to achieve a flowing water effect.
3. Oscillation period T: Calculation In the above circuit, Q1 and Q2 act as two electronic switches. The switching of the electronic switches is determined by the RC time constant, which determines the pulse width of the square wave. Practice has proved that as long as T=0.7R2C1, the charge on C1 is basically discharged, and the base voltage of T2 changes from negative to 0. T1=0.7R2C1, T2=0.7R1C1 The sum of the pulse widths of the two square waves is the repetition period: T=T1+T2=0.7 (R2C1+R1C1) If R1=R2, C1=C2, T=1.4RC
we can pass Changing the resistance of R2 and R3 changes the speed of the running water lamp. In
 
this
 
 circuit, changing the resistance of the potentiometer affects the charging and discharging time of the capacitor and changes the on-off time of each of the three groups of LEDs. Actual picture