555 Timer IC – Integrated Circuit Electronic Component

555 timer integrated circuit IC pin layoutAfter learning about the basic resistor, diode/LED and capacitor components, it is then a good time to study the 555 timer in my opinion. You can quickly combine all of those components together to make fun circuits.

There are millions of 555 timer circuit schematic diagrams out there that can be found by doing a google search.  Once you have found a 555 timer circuit that you would like to build, all you have to do is wire the 555 component pin number as shown in the schematic.

  • ICs, like the 555, are usually a square/rectangle shaped box with the pin numbers labeled.
  • Part number is probably in the IC schematic symbol box. If it is just showing 555 or NE555 or µA555, then either a NE555 or µA555 should work just fine. You probably won’t come across a plain ole 555. If it lists a different variant, such as LM555 or LMC555, it might be required to use that exact variant or a suitable alternative.
  • Numbers inside the box , are the pin numbers of the physical IC. There’s usually 1 or 2 indents on the top of the component that indicates that it is the top of the component. Number 1 is the top left pin, and the numbers circle around counter clockwise, down the left side, then up the right side. The last number is the top right pin.

Each variant, indicated by different letters and/or numbers in the part number, may have a large difference in capabilities compared to the base model. Those differences must be taken into consideration while designing or modifying circuits. A google search of the exact part number and “datasheet” should result in the datasheets of multiple manufacturers.


Assorted integrated circuits (ICs) kit. Included is the NE555 timers and other commonly known ICs. I have previously covered some of the other ICs in YouTube videos. It is an Affiliate link ad that supports this channel.

Bistable Mode/Flip Flop:

Bistable mode 555 timer flip flop circuit schematic diagram by electronzap
Bistable mode 555 timer flip flop circuit schematic diagram by electronzap

Output is either high (close to positive supply voltage) or low (close to ground), depending on whether the set/trigger pin (output high), or the reset pin (output low) was last triggered. Trigger and reset pins are active low. That means that they respond to a low (voltage close to 0V/ground) signal.

The circuit is called bistable because it is stable (stays) in one of two (bi) output states until forced to change.

Bistable mode 555 timer flip flop set with light dependent resistor and reset with push button switch schematic diagram by electronzap
Bistable mode 555 timer flip flop set with light dependent resistor and reset with push button switch schematic diagram by electronzap

Second diagram is a modification made for the demonstration video I made in my current YouTube series. A light dependent resistor is used to switch the set pin, based on how much light is falling on the LDR, but the circuit must still be reset using the pushbutton switch connected to the reset pin.

Monostable Mode:

Monostable 555 timer one shot mode schematic diagram by electronzap
Monostable 555 timer one shot mode schematic diagram by electronzap

Output is low until a pulse (short low input) is given to the set/trigger pin. That pulse sets the output high for the time that it takes the capacitor at the threshold/6 pin to charge to 2/3 of the supply voltage (Vcc).

Called Monostable because it is stable (stays) in only one state (mono) of low until forced to change. After being forced into the high state, the monostable 555 circuit automatically sets itself back to low whenever the timing is done. Therefore, the high state is not stable, only the low state is.

555 timer monostable mode one shot controlled with magnetic switch learning electronics lesson 0009 schematic diagram by electronzap
555 timer monostable mode one shot controlled with magnetic switch learning electronics lesson 0009 schematic diagram by electronzap

I used a magnetic switch to set the monostable mode 555 timer in my recent video series.

Astable Mode:

Astable mode 555 timer multivibrator flashing LEDs with oscilloscope measurement locations schematic diagram by electronzap
Astable mode 555 timer multivibrator flashing LEDs with oscilloscope measurement locations schematic diagram by electronzap

Output keeps alternating between high and low for as long as the circuit is being powered. Timing is determined by how long the capacitor takes to charge (output high) to 2/3 supply voltage, and by how the long capacitor takes to discharge (output low) to 1/3 supply voltage.

Astable means the same thing as “not stable”. There is no stable state that the output will stay in until it is forced to change. It just keeping changing output states on it’s own.

Schmitt trigger inverted digital signal:

555 timer schmitt trigger with LEDs controlled by trimpot voltage divider circuit
555 timer schmitt trigger with LEDs controlled by trimpot voltage divider circuit

The output is set low when the signal gets to 2/3 Vcc (supply voltage) or higher. Whereas the output is set high when the signal gets to 1/3 Vcc or lower.  Between 1/3 and 2/3 supply voltage, the output stays in the last state that it was put into. The range of voltages where the output stays in whatever state it was last put into is called hysteresis.

  • Schmitt triggers are switch circuits or inputs with hysteresis.
  • Inverters (digital) are circuits that have a high output when the input signal is low, and a low output when the input signal is high.  Not to be confused with power inverters, which are devices that take a DC voltage and convert it into (usually to household voltage) AC.

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  • Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
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