The third category is no steady work. Circuit is not stable multivibrator is the circuit 555 the most widely used category. Most forms of circuit changes . For simplicity , and it is divided into three types . The first one (see Figure 1 ) is the direct feedback , vibration resistance is connected to the output VO of . The second (see Figure 2 ) is the indirect feedback , vibration resistance is connected to the power supply on the VCC . The first of a unit circuit ( 3.2.1 ) is the most widely used . 2nd unit circuit ( 3.2.2 ) is a square wave oscillator circuit . S 3,4 -unit circuits are adjustable duty cycle pulse oscillation circuit, the same functionality but slightly different circuit , so each 3.2.3a and 3.2.3b of the Code . Third (see Figure 3) is voltage-controlled oscillator . As the circuit changes in the form of very complex, for simplicity , only divided into the simplest form ( 3.3.1 ) and with auxiliary devices ( 3.3.2 ) the two units . The figure cited two examples . No steady input of the circuit generally have two resistors and an oscillating capacitor oscillator . Resistance is only one oscillation can be considered a special case . For example: 3.1.2 omitted elements can be considered to be the result of RA . Sometimes encounter 7.6.2 3 -side parallel , only one non- stable circuit resistance RA , then it could be saved as a 3.2.1 unit circuit after the deformation RB . Summarized above Class 555 3 8 18 unit circuits , although they can not encompass all the application circuit 555 , the ancient words of : Stuck in which , I believe that we understand that most of its 555 circuits or helpful.