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Oct 14 2019

Four Kinds of Common Circuits of 555 Time-base Circuits


The 555 time-base circuit is a bipolar time base integrated circuit with a working power supply of 4.5v~18v. The output voltage is compatible with TTL, CMOS and HLT logic circuits and the output current is 200mA. It is easy to use and low in cost, and can directly push low-impedance loads such as speakers and inductors. It can also be used for timing and time delay in instrumentation, automation, and various appliances. At the same time, it can form a monostable trigger, an astable multivibrator, a pulse generator, a burglar alarm, a voltage monitor, etc.



I Internal Structure of the 555

Time-base Circuit

1.1 Voltage Comparato

1.2 Voltage Divider

1.3 SR Latch
1.4 Discharge Triode
1.5 Output Buffer

II Basic Working Mode of 555

Time-base Circuit

2.1 Monostable Working Mode
2.2 Bistable Mode of Operation
2.3 Unsteady Mode of Operation
2.4 Timing Mode

III Application of 555 Time-base


3.1 Motor Control Circuit
3.2 Photo Exposure Timing Circuit

I Internal Structure of the 555 Time-base Circuit

The circuit structure of the domestic bipolar timer CB555 is shown in Figure 1. It consists of a voltage divider, voltage comparators C1 and C2, an SR latch, a buffer output, and a collector open-circuit discharge transistor TD.

Internal Structure Diagram of 555 Time-base Circuit 

Figure 1. Internal Structure Diagram of 555 Time-base Circuit

1.1 Voltage Comparator

The voltage comparators C1 and C2 are two identical linear circuits, each having two signal inputs and one signal output. The same input terminal of C1 is connected to the reference comparison voltage VR1, the reverse input terminal (also called the threshold terminal TH) is externally connected to the input trigger signal voltage, and the reverse input terminal of C2 is connected to the reference comparison voltage VR2, and the same input terminal (also called the trigger TR') is externally connected to input trigger signal voltage.

1.2 Voltage Divider

The voltage divider is composed of three equal-value resistors connected in series, and the power supply voltage Vcc is divided to provide a reference comparison voltage for the two voltage comparators. When the control voltage input terminal Vco is floating, the reference comparison voltages of C1 and C2 are respectively:

图片8.png, Vco should normally be terminated with a high frequency interference bypass capacitor. If the Vco is externally fixed, then 图片9.png.

1.3 SR Latch

The SR latch is composed of two TTL and non-gate. Its logical state is controlled by the output potential of two voltage comparators, and there is an external direct reset control terminal R'D. As long as the low level is applied to the R'D terminal, the output terminal vo is immediately set to a low level, which is not affected by the state of other input terminals. The SR latch has three logical functions: setting 0 (reset), setting 1 (setting) and maintaining. The output signal of voltage comparator C1 is used as the reset control signal of SR latch, and the output signal of voltage comparator C2 is used as the position control signal of SR latch.

1.4 Discharge Triode

The discharge triode is actually a bipolar transistor switching circuit with a common emitter connection. Its working state is controlled by the Q' end of the SR latch, the collector is taken out of the chip, and the external RC charging and discharging circuit is connected. In general, the collector outside the lead-out sheet is referred to as a discharge end (DISC).

1.5 Output Buffer

The output buffer is composed of an inverter. The effect of the invention is to improve the load capacity of the time base integrated circuit and to isolate the influence between the load and the time base integrated circuit. The input signal of the output buffer is same as the output signal of the SR latch Q'

II Basic Working Mode of 555 Time-base Circuit

555 time-base circuit is widely used, it can easily used to form a variety of stable and practical circuits, but no matter how the circuit changes, if these practical circuits are classified according to their working principle, the basic working modes are monostable, bistable, non-steady and timing.

2.1 Monostable Working Mode

In practical application, continuous repetitive waves are not always needed. Sometimes, only the circuit needs to work in a certain length of time, and this kind of circuit only needs to work in monostable mode. The monostable mode refers to the fact that the circuit has only one stable state, also known as monostable trigger. In the steady state, the 555 time-base circuit is in the reset state, and the output low voltage is output. When the circuit is triggered by a low level, the 555 circuit is set to enter a transient state. During the transient time, the high voltage is output. After a period of delay, the circuit can automatically return to the steady state. The monostable operating mode can be divided into pulse-activated monostable and monostable voltage-controlled oscillators according to the working principle.

2.1.1 Pulse-activated Monostable Oscillators

The one-shot of the pulse start is based on the two terminals of the 555 time base circuit as the input of the trigger signal, and the output voltage of the inverter composed of TD and R is connected to the 6 terminal, and the capacitor C is connected to the ground at the 6th end at the same time, which constitutes monostable trigger , and the circuit is as shown in Fig. 2. At steady state, Vo=0; In a transient state, Vo=1. And the width of the output pulse tW is equal to the duration of the transient steady state.


Usually, the values of R range from several hundred ohms to several megabytes, the capacitance ranges from several hundred skin methods to hundreds of micrometers, and the range of tW ranges from a few microseconds to a few minutes. But as the width of tW increases, its accuracy and stability will decrease. In addition to the function of timing and delay, the pulse-activated monostable circuit can be used for shaking, frequency multiplication, pulse output and so on.

 Pulse-started Monostable Trigger

Figure 2. Pulse-started Monostable Trigger

2.1.2 Monostable Voltage Controlled Oscillator(VCO)

The voltage-controlled oscillator consisting of 555 time-base circuit is shown in Figure 3. In the circuit of Figure (a), the 2-terminal input is modulated with a pulse Vi, and the 5-terminal is modulated with a signal Vco. In the circuit of Figure (b), after the low-pass filtering and DC amplification, the closed-loop control 555 is controlled by using the output pulse, so that when the trigger frequency increases, the temporary stable width of the output waveform is automatically reduced, and the duty cycle of the output waveform remains unchanged. Monostable voltage controlled oscillator is mainly used for pulse width modulation, voltage frequency variation, A/D conversion, etc.

 Monostable Voltage Controlled Oscillator

Figure 3. Monostable Voltage Controlled Oscillator

2.2 Bistable Mode of Operation

The bistable mode of operation refers to a circuit with two inputs and two outputs, and its output has two stable states, that is, a set state and a reset state. This output state is determined by the input state, the original state of the output and the performance of the latch itself. The bistable operating mode can be divided into the SR latch and the Schmitt trigger according to the working principle.

2.2.1 SR Latch

For the 555 time base circuit, its logical function is completely equivalent to a SR latch, as shown in Figure 4, except that it is a special SR latch. It has two input TH (R) and TR' (S'), only one output Vo (Q) and no Q 'terminal. Because a Q-side can solve the connection to the load and explain the state of the latch, the Q 'side is omitted. This special SR latch has two special features: one is that its two inputs have different polarity requirements for the trigger level, the R terminal requires a high level, and the S' terminal requires a low level. Another is that the threshold levels of the two inputs are different, in normal state, R is 图片13.png. When 图片14.png, high voltage 1 will output; while 图片15.png, low voltage 0 will output. SR latches are commonly used in comparators, electronic switches, detection circuits, home appliance controllers and so on.

SR Latch & Schmitt Trigger 

Figure 4. SR Latch                         Figure 5. Schmitt Trigger

2.2.2 Schmitt Trigger

The two voltage comparators C1 and C2 in the 555 time base circuit, because their reference voltages are different, C1 is 1/3Vcc, and C2 is 2/3Vcc. And the set 0 and return 1 signals of the SR latch must occur in different voltages of the input signal. Therefore, the input signal has a different input signal value from a high level to a low level and a low level to a high level. With this characteristic, its two input terminals TH and TR are connected as a total input. The Schmidt contactor can be obtained at the end, as shown in Figure 5. Schmitt trigger is often used for electronic switches, monitoring alarms, pulse shaping, and the like.

2.3 Unsteady Mode of Operation

The non-steady state operation mode means that the circuit has no fixed steady state, and the 555 time-base circuit is in a state of repeated setting and resetting, that is to say, the output terminal alternately appears at a high level and a low level, and the output waveform is a rectangular wave. Since the harmonics of the rectangular wave are very rich, the unsteady mode of operation is also called a self-excited multivibrator. It can be divided into direct feedback type, indirect feedback type multivibrator and unstable type voltage controlled oscillator.

2.3.1 Direct Feedback Multivibrator.

The 555 time-base circuit can form Schmitt trigger. By using the hysteresis characteristic of the Schmitt trigger, a charge and discharge capacitor C is connected between the two input terminals of the circuit and the ground, and a feedback resistor Rf is connected between the output and the input end. Rf, which constitutes a direct feedback multivibrator, as shown in Figure 6(a). When the power is turned on, the charging and discharging process of the circuit after each flip is its temporary steady-state time. The two transient steady-state times are the charging time T1 and the discharging time T2 of the capacitor respectively. Among them, T1=t2=0.69, RC oscillation period T=T1+T2, oscillation frequency f=1/T and the duty cycle of the circuit is 50%. To change the values of R and C that can changes the charge and discharge time, that is, changes the oscillation frequency f of the circuit. The value of the charge and discharge resistor R in the circuit should generally be no less than 10K Ω. If the value is too small, the charge and discharge currents will be too large, which will cause the output voltage to drop too much, especially when the load is heavy.

2.3.2 Indirect Feedback Multivibrator

Because the direct feedback type multivibrator charges the capacitor C through the output terminal, the output is affected by the load factor, which causes instability of the oscillation frequency, the indirect feedback multivibrator is often used, and the circuit is as shown in Fig. 6(b). The working process of the circuit remains unchanged, but its working performance has been greatly improved. The circuit is charged with two resistors R1 and R2, and only the resistor R2 is used during discharge. The temporary steady-state times of the two resistors are not equal: T1=0.69(R1+R2)C, T2=0.69R2C, oscillating period T=T1+T2=0.69(R1+2R2)C and oscillation frequency f=1/T.

 Multi-vibrator Composed of 555

Figure 6. Multi-vibrator Composed of 555

If the circuit is improved and connected to the diodes D1 and D2, as shown in Figure 6 (c), the charging current and discharge current of the capacitor flow through different paths, and the charging current only flows through Rl, the discharge current only flows through R2. Therefore, the charge and discharge time of capacitance C is: T1=0.69R1C, T2=0.69R2C, oscillating period T=T1+T2=0.69(R1+R2)C and oscillation frequency f=1/T. If R1 = R2, the duty cycle is 50%.

Multivibrators have a wide range of applications in pulse output, audible alarms, home appliance control, electronic toys, instrumentation, power conversion, and timers, etc.

2.3.3 Unstable Voltage Controlled Oscillator

If the control voltage input of the indirect feedback multi-vibrator is not suspended, a steady-state voltage-controlled oscillator can be formed, as shown in figure 7. The charge and discharge time of capacitance C in Figure (a) circuit is as follows:


oscillating period T=T1+T2 and oscillation frequency f=1/T. When the input control voltage VI increases, the frequency f will decrease. The circuit of Figure (b) is a voltage-to-frequency conversion circuit (VFC) consisting of an operational amplifier and a 555 timer. By changing the voltage drop across the load resistor RL, the frequency of the 555 multivibrator can be changed. If the load is RL and the current is IO, then the voltage at both ends is Vi=IORL. The voltage is amplified 100 times by the differential amplifier A1, and the Al output is applied to the control terminal (5-pin)of 555 to modulate it. Thus, the 555 output(3-pin) signal frequency is proportional to the input voltage Vi. The unstable voltage controlled oscillator is mainly used for pulse width modulation, voltage frequency conversion, and A/D conversion.

 Unstable Voltage Controlled Oscillator

Figure 7. Unstable Voltage Controlled Oscillator

2.4 Timing Mode

The timing mode is essentially a kind of deformation of unistable mode. Its circuit is shown in Figure 8. Since this circuit is widely used in application circuits, it can be used as a basic working mode.

 Basic Circuit of Timing Operation Mode

Figure 8. Basic Circuit of Timing Operation Mode

The mode is mainly used in the timing or delay circuit. When the steady state is VO=0, the transient steady state Vo=1, the width tW of the output pulse is equal to the duration of the transient steady state, and the duration of the transient steady state depends on the external resistor R and capacitor C.


Figure (a) is a timing circuit that generates a high level when the power is turned on. After the delay time t, the output of the time base circuit will keep the output low level. If you want to make the 3 pin output high level again, just press the button SB, the stored charge of the capacitor C is discharged through the SB, the 2 pin is triggered by the low level, the 555 is set, and the 3 pin outputs the high voltage and then released SB button, and the timing is started. At this time, the power supply VDD is charged to the C through the timing resistor R, so that the voltage across the C (the threshold of the 555 ) is continuously raised. When the clock is raised to 2/3 VCC, the time base circuit is reset and the timing ends. And the output level of 3 feet is restored to a low level.

Figure (b) is a timing circuit that generates a low level when the power is turned on. After the delay time t, the output of the time base circuit will keep the output high level, because the voltage across the capacitor C cannot jump during power-on, so 555 TH terminal (6 pin) is high level, 555 reset, and 3 pin outputs low voltage. Then the power supply is charged to R through C, so that the voltage across C is continuously increased, and the level of the trigger terminal TR (2 feet) of 555 is continuously decreased. After the delay time t, the level of the 2 pin is reduced to 1/3 VCC. The base circuit is set, and the 3 pin keeps the output high level. If you want to output a low level with a delay time of t, just press the button SB.

III Application of 555 Time-base Circuit

In 1972, Signetics first introduced the NE555 bipolar time-base integrated circuit, which was originally designed to replace the mechanical timer with large volume and poor timing accuracy. However, after the device is put on the market, the IC is popular among electronics and electrical design and production personnel because of its low cost, convenient use and good stability. Its application range far exceeds the original intention of the designer, and its use involves almost all fields of electronic application. Since the birth of the first NE555 integrated circuit in the world for more than 30 years, its market has been enduring for a long time. Up to now, integrated circuit manufacturers all over the world are still competing for imitation.

3.1 Motor Control Circuit

The motor control circuit composed of NE556 timer is shown in Figure 9. In the circuit, NE556(1) constitutes an astable multivibrator, and NE556(2) constitutes a monostable multivibrator. R3 and C3 form a differential circuit, and VD1 is a limiting diode, which absorbs the positive peak pulse voltage generated by the differential circuit; The output of NE556(2) is excited by R5 and VT2 to reach the Linton transistor VT1, which makes it open/close, thus driving the motor to run. Among them, BP1 is used to adjust the period of excitation VT1, and BP2 is used to control the rotation speed of the motor.

The application of the 555 time-base circuit in the control circuit and the conversion circuit includes: automatic control circuit of water level, automatic control circuit of upper and lower limit temperature, voltage-frequency conversion circuit, frequency-voltage conversion circuit, and the like.

 Motor Control Circuit Composed of NE556 Time-base Circuit

Figure 9. Motor Control Circuit Composed of NE556 Time-base Circuit

3.2 Photo Exposure Timing Circuit

The photo exposure timer of the 555 timer is shown in Figure 10. The 555 time-base circuit has been connected to the timing mode. When the power is turned on, the timer enters a steady state. At this time, the voltage of the timing capacitor is VCT=VCC. In the equivalent trigger of 555, the two input terminals are high level, then the output is low level, VO=0, the relay does not pick up, the normally open electric shock is turned on, and the photo exposure light is not bright.

When the button switch SB is pressed, the timing capacitor CT is immediately discharged until the voltage is zero. At this time, the equivalent trigger input terminal of the 555 circuit is low level, then the output is high level, VO=1, the relay is closed, normally open The electric shock is closed and the photo exposure light is on. While the button switch is pressed, it is released immediately, and the power supply voltage is charged to the capacitor CT through the resistor RT, and the steady state starts. When the capacitance voltage rises to 2/3VCC, the timing time has arrived, and the input of 555equivalent circuit flip-flop is high, so the flip-flop is turned to low level, the VO=0, relay is released, and the transient steady state ends and returns to steady state. The exposure time of photo is tW=1.1RTCT, and the delay time can be adjusted and set by the potentiometer RP.

 Photo Exposure Timing Circuit Composed of 555 Time-base Circuit

Figure 10. Photo Exposure Timing Circuit Composed of 555 Time-base Circuit

The 555 time-base circuit can also form various types of boot delay circuits and various kinds of timing circuits in the application circuit of delay and timer, such as touch-type practical electronic timers, and wide-range adjustable timing for a long time. , a small timing sounder for quiz game, a telephone time limit timer, an automatic light timer, etc.

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