555 Circuits

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555 Review

555 is a Swiss Knife of timing circuits. In this post, I want to collect things related to 555 ICs. Why? Because timing circuits are not just used in toasters or ovens. They are also used in PWM (Pulse Width Modulation) circuits. PWM circuits are integral part of any SMPS (Switching Mode Power Supplies) power supplies. If you look around, just about any new gadget is powered by SMPS power supplies (as opposed to linear).

Please let me know if you notice any inaccuracies in this post so I can correct it.

555 Operation

555 Block Diagram

The functional block diagram above shows that 555 timer is built from one SR (Set Reset) FF (flip flop) and two voltage comparators.


SR FF work with input level changes (NOT the rising or falling edges like JK FFs). During normal operation, both the S (Set) and the R (Reset) inputs are at LOW levels.

If the S input goes HIGH, the output Q goes HIGH. After that, even if the S input goes LOW, the output Q will remain at HIGH.

If R input goes HIGH, the output Q goes LOW. After that, even if the R input goes
LOW, the output Q will remain at LOW.

At this point question comes to mind. What happens to Q, if both S and R goes HIGH? The answer is that it is Undefined. We have to make sure that S and R do not become HIGH at the same time.

Voltage Comparators

There are two voltage comparators shown as triangles in the block diagram above. Each comparator has two inputs labelled as Vin+ and Vin-, and one output Vout. Input voltages are analog and can take any value, however the Vout is digital. It is either HIGH or LOW.

Voltage comparator compares Vin+ to Vin-: If Vin+ > Vin-, then the Vout becomes HIGH. If Vin+ < Vin-, then the Vout becomes LOW.

Trigger / Threshold

Three resistors with equal values divide the supply voltage Vcc into 3 parts.

Vin+ input of the Trigger comparator is set to 1/3 of the Vcc. Vin- input is available as Trigger input (pin 2).

Vin- input of the Threshold comparator is set to 2/3 of the Vcc. Vin+ input is available as Threshold input (pin 6).


Based on all of the discussions above, there are two important results we have to keep in mind:

Trigger (pin 2) of the 555 should remain above 1/3 of the supply voltage. Momentarily lowering the pin 2 voltage less than 1/3 of the supply voltage will set the output (Pin 3) to HIGH.

Threshold (pin 6) of the 555 should remain below 2/3 of the supply voltage. Momentarily increasing the pin 6 voltage greater than 2/3 of the supply voltage will set the output (Pin 3) to LOW.

In most 555 circuits, the Trigger (pin 2) and the Threshold (pin 6) are connected together. In that case, remember the following:

If the connected voltage (Vconn) > 2/3 of Vcc, the Vout will be set to LOW.
If (Vconn < 2/3 Vcc) AND (Vconn > 1/3 Vcc), the Vout may be HIGH or LOW.
If Vconn < 1/3 Vcc, the Vout will be set to HIGH.

Other Pins

Connect Reset Pin (Pin 4) to Vcc for normal operation. If the Reset pin goes LOW, the output Q will go LOW irrespective of any other input.

Discharge pin (Pin 7) is used to discharge a capacitor. If the output is LOW, the switching transistor will turn on. This pin can be used for other purposes as well, like turning lights ON/OFF, etc.

Control Voltage (Pin 5) can be used to set Vin- of the Threshold comparator. This pin should not be connected to anything for normal operation. Note that Vin+ of the Trigger comparator will be 1/2 of the Control Voltage, if used.

555 Pinout

555 Pinout


If the TR (pin 2) and THR (pin 6) is connected together, 555 can be used as an inverter. If the combined input is HIGH, the output Q (pin 3) will be LOW. And vice versa.

Schmitt Trigger

Schmitt Trigger

Schmitt Trigger

This is probably the simplest 555 circuit that shows how 555 works. If the Vout (pin 3) is HIGH, LED1 comes on. If the Vout is LOW, the LED2 comes on. R2 and R3 are 220 ohms, R1 potentiometer is 5K or 10K ohms.

Assume the Vcc supply voltage is 6 volts. The potentiometer allows us to change the Vcommon voltage from 0 volts (ground) to Vcc supply voltage.

As we increase the voltage, as soon as Vcommon goes above 4 volts (2/3 Vsupply), the Vout will go LOW, and LED2 will light up. Increasing the voltage further does not cause any change.

As we decrease the voltage, as soon as Vcommon goes below 2 volts (1/3 Vsupply), the Vout will go HIGH, and LED1 will light up.


Oscillator 1

Oscillator 1

Oscillator 1

In the circuit above, R1 is 1K and C1 is 2 uF. One side of R1 is connected to the output. When the output goes HIGH, the capacitor starts to charge. When the capacitor voltage goes above 4 Volts, the output goes LOW, and the capacitor starts to discharge. When the C1 voltage goes below 2 Volts, the output goes HIGH, and C1 starts to charge again.

In the oscilloscope picture, the upper trace shows the voltage across the capacitor and the lower trace shows the output voltage.

Note that the output stays longer at HIGH than LOW. This is because the output voltage does not go all the way up to 6 Volts. Therefore it takes longer to charge the capacitor.

One variation to this circuit is that R1 is connected to Vcc supply voltage instead of output and the Discharge (pin 7) is connected to C1. In this case the output LOW is even shorter since the C1 is discharged very quickly.

If R1 is a potentiometer, the frequency of oscillation can be adjusted. Or if a thermistor is used in addition to a resistor, the frequency will depend on the temperature sensor value.

Monostable Multivibrator

Monostable multivibrator input is a short pulse, in this case a momentary LOW. The output then will go HIGH for a predetermined time, and then go LOW and stay LOW. For example, a circuit like this can be used in motion sensitive flood lights. Note that the opposite also is possible where the trigger pulse is a momentary HIGH. In that case the output will normally sit at HIGH, and go LOW for a predetermined amount of time.



Values are Vcc = 6v, R1 = 220, R2 = 10K, C1 = 47 uF, R3 = 1K.

To test it, take a capacitor and momentarily connect to Ground and TR (pin 2). The LED light will come on for about a second and then it will go off. On time can be adjusted by changing R2 and C1 values.





This is the first PWM circuit using two 555′s. The output of the first one is shown at the upper trace. It produces a very short LOW pulse. This signal is connected to the TR (pin 2) of the second 555. Every time it goes LOW, it sets the output Q (pin 3) HIGH, which allows C2 to charge. When the C2 voltage gets to 2/3 of the Vcc, the output is set to LOW. The potentiometer adjusts the time at which the output goes LOW. In the picture above, I set it to close to %50 HIGH. The lower trace shows the output of the second 555.




Only one 555 is used in this circuit. This one works well, but there is another version. Instead of connecting one side of R2 to Vcc, it is connected to the output Q (pin 3) and the discharge (pin 7) is disconnected.





This circuit allows voltage controlled PWM, the controlling voltage needs to be from 1/3 Vcc to 2/3 Vcc. LM311 is a single Voltage Comparator. R3 is a pull-up resistor. The potentiometer mid 1/3rd controls the full range of the PWM. Normally you would use two more 10K resistors on each leg of the potentiometer to Ground and Vcc. In this circuit, LM311 comparator is fed from the C1 capacitor voltage.

In the oscilloscope picture above the upper trace shows the voltage across the C1 capacitor, and the lower trace shows the LM311 output Q.

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