If the astable circuit is "keeps blinking", the monostable circuit turns on just once, for a set time, when you press a button, and then turns itself off. Typical uses are a stairway light timer, an alarm that sounds for a set time, and a pulse stretcher that extends a short input pulse to a comfortable length. This article covers how the 555 monostable circuit works and its time formula, choosing component values, the trigger condition, and common mistakes. For time calculations you can use the monostable tab of the 555 timer calculator.
How it works
In monostable, the output is normally Low, and when a trigger arrives the output goes High and stays that way for a set time. When the time ends, the output returns to Low and waits for the next trigger.
- Normally pin 7 (discharge) holds the capacitor to ground, so the capacitor voltage is 0 V. The output is Low.
- When the voltage on pin 2 (TRIG) briefly falls below 1/3 of the supply (the button is pressed), the output goes High, pin 7 opens, and the capacitor starts charging through R.
- When the capacitor voltage reaches 2/3 of the supply, the output returns to Low and pin 7 discharges the capacitor again.
The time formula
Output High time t = ln(3) × R × C ≈ 1.1 × R × C
An advantage is that the time barely changes even if the supply voltage changes. That is because the time for the capacitor to charge to 2/3 of the supply is set only by R·C.
| R | C | Output time |
|---|---|---|
| 10 kΩ | 10 µF | about 0.11 s |
| 100 kΩ | 1 µF | about 0.11 s |
| 100 kΩ | 10 µF | about 1.1 s |
| 47 kΩ | 100 µF | about 5.2 s |
| 470 kΩ | 100 µF | about 52 s |
| 1 MΩ | 10 µF | about 11 s |
If R is above 3 MΩ or C is a leaky electrolytic capacitor, the time becomes inaccurate. For long times of tens of seconds or more, a dedicated timer IC or a microcontroller is more accurate than a 555.
Circuit layout
- R: between +V and pins 7 and 6 (tied together)
- C: between pin 6 and ground
- Trigger: connect pin 2 to +V with a pull-up resistor (10 kΩ) and put a switch between pin 2 and ground. It is normally High and drops Low when pressed, triggering it.
- RESET (pin 4): connect to +V.
- CTRL (pin 5): attach 10 nF to ground to reduce noise.
- Output (pin 3): drives an LED with its resistor, or a relay or buzzer through a transistor.
Tying pins 7 and 6 together is what differs from astable. In astable, R2 was between pins 7 and 6, but in monostable a single R connects directly from +V to pins 7 and 6.
Trigger condition
The 555 monostable cannot be retriggered. Even if a trigger comes in again while the output is on, the time is not extended. Even with button bounce (brief jitter), only the first trigger is recognized, so there is the advantage that no separate debounce circuit is needed (Tact Switches and Pull-Up/Pull-Down Resistors).
What to watch is how long the trigger is held. If the Low state on pin 2 lasts longer than the output time, the output stays High for that duration. To make it turn on only for the set time even when the button is held down, make a differentiating circuit with a small capacitor (about 10 nF) and a resistor on the trigger input to turn it into a short pulse.
Application examples
- Delayed light: pressing the button lights an LED for about 10 seconds. Add a transistor to the output to drive a high-current lamp (Transistor Switch Circuits).
- Adjusting the alarm length: when a sensor signal comes in, the buzzer sounds for only 1 second.
- Pulse stretching: stretch a short pulse from a microcontroller to 0.1 seconds that a person can see, and show it with an LED.
Common mistakes
| Mistake | Symptom | Fix |
|---|---|---|
| Not tying pins 7 and 6 together | The output stays on and stops | Connect both pins to R together |
| Leaving out the pin 2 pull-up | It triggers by itself at startup | Connect to +V with 10 kΩ |
| Ignoring pin 4 | Resets at random | Connect to +V |
| Time far from the calculation | Electrolytic tolerance and leakage | Measure the value and trim with a potentiometer |
| Stays on while the button is held | The trigger is held longer than the output | Shorten the trigger width with a differentiating circuit |
Checking in Patterndo
The NE555 model in Patterndo is a timer model that handles both astable and monostable. Place the part, pull pin 2 to ground with a switch, and check that the HIGH and LOW state of the output pin changes after a set time. How to use simulation is in Using Patterndo Simulation.
Summary
The monostable output time is t ≈ 1.1 × R × C, and the key is tying pins 7 and 6 together and connecting them to +V through R. Do not forget the trigger pull-up and handling pin 4. To understand the same R·C idea more generally, see Understanding RC Time Constants and Capacitor Charging.