01
Waiting state
Between pulses OUT (pin 3) is low and the discharge transistor on DISCH (pin 7) is on, holding C1 near 0 V. R2 keeps TRIG (pin 2) at VCC. RESET (pin 4) is tied to VCC, because a low RESET overrides everything else.
Timing & Oscillators
NE555 Monostable Delay Timer (One-Shot)
The one-shot circuit from the TI NE555 datasheet: press START and the output stays high for a fixed time set by one resistor and one capacitor, about 1.1 s with 100 kΩ and 10 µF.
- Operating Supply Voltage
- 4.5 V – 16 V (TI §7.3)
- Components
- 7
- Formulas
- 2
- Gotchas
- 4
Circuit overview
- Operating Supply Voltage
- 4.5 V – 16 V (TI §7.3)
- Components
- 7
- Formulas
- 2
- Gotchas
- 4
Click any component in the schematic above to inspect its electrical ratings, function, and connections.
Circuit overview
This is Figure 6-2 of the Texas Instruments 555 timer datasheet (SLFS022K), the monostable or one-shot mode. A short low pulse on the trigger input starts a single output pulse whose length depends only on R1 and C1: tw = 1.1 × RA × C. Because the trigger and threshold levels are both fixed fractions of the supply, the pulse length does not change with the supply voltage as long as the supply stays constant during the pulse. TI draws a bare input line; here a START button with a 10 kΩ pull-up drives it, and the values RA = 100 kΩ and C = 10 µF give a pulse of about 1.1 s. A 10 nF capacitor on CONT and a 100 nF supply bypass complete the circuit, the latter as TI recommends in its power-supply section.
Key Technical Specifications
| Operating Supply Voltage | 4.5 V – 16 V (TI §7.3) |
|---|---|
| Pulse Length | tw = 1.1 × RA × C ≈ 1.1 s with 100 kΩ and 10 µF (calculated) |
| Trigger Level | 1/3 VCC; 1.67 V typical at VCC = 5 V (TI §5.5) |
| Threshold Level | 2/3 VCC; 3.3 V typical at VCC = 5 V (TI §5.5) |
| Supply Sensitivity of Timing | 0.1 %/V typical, 0.5 %/V maximum, monostable (TI §5.6) |
| Temperature Coefficient of Timing | 50 ppm/°C typical, monostable (TI §5.6) |
| Output Current | ±200 mA recommended (TI §5.3) |
Theory of Operation & Working Principle
02
Trigger
Pressing START pulls TRIG below the trigger level, one third of VCC (1.67 V typical at VCC = 5 V). The internal flip-flop sets: OUT goes high and the discharge transistor turns off, so C1 is free to charge.
03
Timing
C1 charges through R1 toward VCC:
VC(t) = VCC(1 - e^(-t/(RA C)))The pulse ends when V_C reaches the threshold of two thirds of VCC, which happens after ln 3 ≈ 1.1 time constants:
tw = RA C ln 3 ≈ 1.1 × RA C = 1.1 × 100 kΩ × 10\,µF = 1.1 sBoth levels scale with VCC, so the pulse length does not depend on the supply voltage while it stays constant. TI gives a supply sensitivity of 0.1 %/V typical for this mode.
04
End of the pulse
At the threshold the flip-flop resets: OUT goes low and the discharge transistor turns on and empties C1, ready for the next trigger. TI notes that the sequence ends only if TRIG has been high for at least 10 µs before the end of the interval; if START is still held, OUT stays high until it is released.
05
Output
OUT can source or sink up to 200 mA, enough for an LED with a series resistor, a small relay through a flyback diode, or a logic input.
Design Equations & Calculations
Pulse Length
tw = 1.1 · RA · C
Duration of the output pulse (TI §6.3.1).
| Symbol | Parameter | Typical value |
|---|---|---|
| RA | Timing resistor | 100kΩ |
| C | Timing capacitor | 10µF |
| tw | Pulse length (calculated) | 1.1s |
Capacitor Voltage During the Pulse
VC(t) = VCC(1 − e − t/(RA C))
C1 charges toward VCC; the pulse ends when V_C reaches 2/3 VCC, at t = ln 3 · RA·C.
| Symbol | Parameter | Typical value |
|---|---|---|
| VCC | Supply voltage | 5V |
| RA C | Time constant | 1s |
Bill of Materials (BOM)
Every part needed to build this circuit. Select a reference designator to inspect it on the schematic.
| RefDes | Component | Value / rating | Package | Role in circuit | Reference |
|---|---|---|---|---|---|
| Precision Timer TI NE555P; other 555 variants need their own supply and output checks. | NE555P | PDIP-8 | Monostable timer | Precision Timer pinout & datasheet | |
| Resistor | 100kΩ | Axial 1/4 W | Timing resistor RA | — | |
| Aluminium Electrolytic Capacitor Low-leakage type; + to THRES/DISCH; voltage rating above VCC. | 10µF | Radial | Timing capacitor | — | |
| Ceramic Capacitor | 10nF | 0805 / radial | CONT bypass | — | |
| Ceramic Capacitor Close to pins 8 and 1. | 100nF | 0805 / radial | Supply bypass | — | |
| Resistor | 10kΩ | Axial 1/4 W | Trigger pull-up | — | |
| Tactile Switch | START | SMD / THT | Trigger | — |
Practical Applications & Target Domains
Delay-off timers for lights and fans
Debouncing a switch into one clean pulse
Missing-pulse and timeout detection
Fixed-length pulses for relays, buzzers and logic
Critical Design Traps & Engineering Gotchas
Traps found in real builds, with the fix for each.
Holding the Button Stretches the Pulse
The pulse ends only if TRIG has been high for at least 10 µs before the timing interval runs out. If START is held longer than tw, OUT stays high until it is released.
Mitigation
Keep the trigger shorter than the pulse. Where the input can stay low, couple it through a small capacitor so that only its falling edge reaches TRIG.
Electrolytic Leakage with a Large RA
Near the threshold only (VCC − 2/3 VCC)/RA flows into C1, about 17 µA with 100 kΩ at 5 V (calculated). An electrolytic with high leakage takes a noticeable part of that current, lengthens the pulse, and with a very large RA may never reach the threshold.
Mitigation
Keep RA within the 2 kΩ – 100 kΩ range TI uses for its timing specifications and make the time longer with C, using a low-leakage capacitor; use film capacitors for short, accurate pulses.
Floating CONT Pin
CONT carries the internal threshold. Left open, noise coupled into it moves the threshold and makes the pulse length jitter.
Mitigation
Bypass CONT to ground with a small capacitor (10 nF here), as TI draws in Figure 6-2.
RESET Left Unconnected
RESET is active low and overrides the trigger. Left floating it can pick up noise and end or block pulses.
Mitigation
Tie RESET to VCC as drawn, or drive it from logic if the pulse must be cancelled.
Primary sources
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