Monostable or One Shot Mode 555 Timer – LEDs show output state

Monostable 555 timer has a low output unless it is forced to go high for a period of time. The output however returns to low when that time period is over. It stays low as long as it is left alone. Monostable means that there is one stable state, which is low.

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

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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.

  • Pin 4 (reset) is connected to the positive supply to prevent pin 4 from setting the output low. A low (close to 0V/ground) signal  to pin 4 will make it so the output is low, no matter what else is going on.
  • A low input to pin 2 (trigger) will set the output high, and start charging the timing capacitor through the timing resistor.
  • Once pin 6 (threshold) senses that the capacitor has charged to 2/3 of the supply voltage (2/3 of 5V in this case), then the output goes low, and the capacitor instantly discharges through pin 7 (discharge).
555 timer integrated circuit IC pin layout
555 timer integrated circuit IC pin layout
  • LED between +5V and output lights up when the output is low. As that is when there is a voltage difference across the LED and it’s protective resistor. I usually use a blue LED, to indicate a low output.
  • On the other hand, the LED between the output and ground (0V) lights up when the output is high. There will be close to 4V at the output when it is high, some voltage is lost due to the transistors used in most 555 timers. I usueally use a red LED to indicate that the output is high.

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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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