Monostable 555 Timer Calculator Formula
Understand the math behind the monostable 555 timer calculator. Each variable explained with a worked example.
Formulas Used
Pulse Width
pulse_width_s = 1.1 * r_ohm * c_faradsPulse Width (ms)
pulse_width_ms = 1.1 * r_ohm * c_farads * 1000Variables
| Variable | Description | Default |
|---|---|---|
r_kohm | Timing Resistor(kΩ) | 100 |
c_uf | Timing Capacitor(µF) | 10 |
r_ohm | Derived value= r_kohm * 1000 | calculated |
c_farads | Derived value= c_uf / 1000000 | calculated |
How It Works
555 Timer Monostable Mode
Formula
T = 1.1 x R x C
A trigger pulse starts timing. The output goes high for time T then returns low. The circuit requires a new trigger for each pulse.
Worked Example
R = 100k, C = 10 uF.
r_kohm = 100c_uf = 10
- 01T = 1.1 x 100000 x 0.00001
- 02T = 1.1 seconds
- 03T = 1100 ms
Frequently Asked Questions
What triggers the monostable?
A negative-going pulse on pin 2 (trigger) that drops below 1/3 Vcc starts the timing cycle.
Can I retrigger during the pulse?
Standard 555 is not retriggerable. The pulse runs its full duration regardless of new triggers.
How do I make longer delays?
Increase R or C. For very long delays (minutes) use large capacitors or cascade two timers.
Ready to run the numbers?
Open Monostable 555 Timer Calculator