LMC555 CMOS timer: one-shot pulses and an LED blinker

The 555 timer is one of the most widely used chips in electronics: with one resistor and one capacitor it makes a time delay, and with two resistors it becomes an oscillator. Texas Instruments' LMC555 is a CMOS version with the same pinout as the NE555/LM555 but far lower supply current. This guide explains what happens inside the chip and follows one button press from start to finish.

The basics: two voltage limits and a latch

Inside the LMC555, a chain of three equal resistors divides the supply to create two references: 1/3 V+ and 2/3 V+. Two comparators each compare an input with one of these references, and a latch (a one-bit memory) remembers which one fired last.

When TRIGGER (pin 2) falls below 1/3 V+, the latch is set: OUTPUT (pin 3) goes HIGH and the discharge transistor on DISCHARGE (pin 7) lets go. When THRESHOLD (pin 6) reaches 2/3 V+, the latch is reset: OUTPUT goes LOW and the discharge transistor connects pin 7 to GND, emptying the timing capacitor. Pulling RESET (pin 4) LOW forces the output LOW regardless of the other pins. The datasheet gives the priority: RESET overrides TRIGGER, which overrides THRESHOLD.

Because both the capacitor's charging and the two limits scale with the supply, the time hardly depends on the voltage. That is the whole trick of the 555: R and C set the time, not the exact supply voltage.

The eight pins

The LMC555CMMX comes in an 8-pin VSSOP package (TI package DGK). Its pinout matches the DIP and SOIC versions:

  • 1 GND – ground.
  • 2 TRIGGER – below 1/3 V+ starts a pulse (active-LOW pulse).
  • 3 OUTPUT – the driven output.
  • 4 RESET – LOW stops the timer; tie to V+ when unused.
  • 5 CONTROL VOLTAGE – the 2/3 reference; typically 3.3 V on a 5 V supply.
  • 6 THRESHOLD – reaching 2/3 V+ ends the pulse.
  • 7 DISCHARGE – transistor to GND that empties the timing capacitor.
  • 8 V+ – supply, 1.5–15 V.
Top-view pinout of the LMC555CMMX in VSSOP-8: left side pin 1 GND, 2 TRIGGER, 3 OUTPUT, 4 RESET; right side pin 8 V+, 7 DISCHARGE, 6 THRESHOLD, 5 CONTROL. Pin 1 is marked with a dot.
LMC555CMMX in VSSOP-8, top view. The dot marks pin 1; pins are counted anticlockwise.

The circuit: an LED that lights for 1.1 seconds

We build the classic monostable circuit (a "one-shot"): every press of S1 gives one output pulse of fixed length, however short the press. R2 holds TRIGGER HIGH at rest and S1 pulls it LOW. R1 and C1 set the time; THRESHOLD and DISCHARGE share the same node. RESET is tied to +5 V, and CONTROL has a 10 nF capacitor to GND against noise. The supply is decoupled close to the chip with 100 nF in parallel with 1 µF, as TI recommends.

Schematic of a monostable LMC555: R2 10 kΩ from +5 V to TRIGGER pin 2 and push button S1 to GND; R1 100 kΩ from +5 V to pins 6 and 7, C1 10 µF from pins 6/7 to GND; OUTPUT pin 3 via 1 kΩ and an LED to GND; RESET pin 4 to +5 V; CONTROL pin 5 via 10 nF to GND; decoupling 100 nF and 1 µF.
One-shot LED timer with the LMC555. The pins are drawn in the same order as the package pinout.

The pulse length is tH = 1.1 · R1 · C1 = 1.1 · 100 kΩ · 10 µF ≈ 1.1 s. The factor 1.1 comes from the capacitor charging from 0 V to 2/3 V+, which takes ln 3 ≈ 1.1 time constants. The LED current is about (4.6 V − 2 V) / 1 kΩ ≈ 2.6 mA; the datasheet guarantees at least 4.4 V at the output with a 2 mA load on a 5 V supply, plenty for a modern LED.

One complete cycle, step by step

  1. Rest: OUTPUT is LOW, the LED is off, and the discharge transistor holds C1 near 0 V. R2 keeps TRIGGER at +5 V.
  2. Press: S1 pulls TRIGGER below 1/3 V+ (about 1.7 V at 5 V). The latch is set, OUTPUT goes HIGH, the LED lights, and pin 7 releases C1.
  3. Charging: C1 charges through R1 and the voltage on pins 6/7 rises. Release the button: TI requires TRIGGER to be back HIGH at least 10 µs before the timing interval ends.
  4. End: When pin 6 reaches 2/3 V+ (about 3.3 V), the threshold comparator resets the latch. OUTPUT goes LOW, the LED turns off, and pin 7 quickly empties C1.
  5. Ready again: The chip waits for the next press. An extra press during the pulse does not extend it, as long as the button is released in time.
Timing diagram: TRIGGER on pin 2 gets a short LOW pulse; the capacitor voltage on pins 6/7 rises exponentially from 0 V to the dashed 2/3 V+ line and then drops to 0; OUTPUT on pin 3 is HIGH from the trigger pulse until the 2/3 limit, tH = 1.1 · R1 · C1 ≈ 1.1 s.
One monostable cycle: the trigger pulse starts it, 2/3 V+ on the capacitor ends it.

Building it: VSSOP-8 needs an adapter

The LMC555CMMX is a surface-mount chip with only 0.65 mm between pins, so it will not fit a breadboard directly. Solder it to an MSOP/VSSOP-8-to-DIP adapter with 0.65 mm pitch, and check the orientation from the pin 1 dot before soldering. TI's ordering table lists the marking ZC5. Parts for the example:

  • LMC555CMMX on a VSSOP-8 adapter
  • R1 100 kΩ, R2 10 kΩ, LED resistor 1 kΩ
  • C1 10 µF (electrolytic: plus towards pins 6/7)
  • 10 nF ceramic on pin 5, 100 nF ceramic + 1 µF at pin 8
  • Push button S1, an LED and a regulated 5 V supply

Turning it into a blinker (astable mode)

Connect TRIGGER and THRESHOLD (pins 2 and 6) together, remove the button, and fit a resistor RA from V+ to pin 7 and a resistor RB from pin 7 to pins 2/6. Capacitor C goes from pins 2/6 to GND. C now charges through RA + RB and discharges through RB, and the chip runs freely between 1/3 and 2/3 V+.

According to the datasheet, the HIGH time is t1 = 0.693 · (RA + RB) · C and the LOW time is t2 = 0.693 · RB · C. With RA = 10 kΩ, RB = 68 kΩ and C = 10 µF, t1 ≈ 0.54 s and t2 ≈ 0.47 s: about one blink per second. In this circuit the HIGH time is always slightly longer than the LOW time; the datasheet shows a separate 50 % duty-cycle variant.

Limits and common mistakes

  • Keep the supply between 1.5 and 15 V; 15 V is also the absolute maximum.
  • The output is not a power driver. The absolute limit is 100 mA, but the datasheet only specifies output levels at 2 mA (HIGH) and 8 mA (LOW) on 5 V. Use a transistor or MOSFET – and a flyback diode – for relays and motors.
  • The VSSOP package may dissipate at most 555 mW at 25 °C and has the highest thermal resistance of TI's package options.
  • Never leave RESET floating; tie it to V+ when unused, or the timer may trigger falsely.
  • If the button is held longer than tH, OUTPUT stays HIGH until TRIGGER is released. If you need a fixed pulse regardless of the press, make a short trigger pulse, for example with a capacitor coupling.
  • Electrolytic capacitors often have ±20 % tolerance and leakage that lengthens the time. The LMC555's own input currents are tiny (typically 10 pA), so for accurate timing a film or ceramic capacitor and a larger resistor work better.
  • At 5 V, TI states the output is compatible with TTL and CMOS logic. At lower supplies the limits scale with V+, but the LED current drops.

Related guides

An RC oscillator can also be built from Schmitt-trigger gates; see the MC14093B Schmitt trigger guide. To divide the 555 clock down, the MC14027B J-K flip-flop guide shows how a flip-flop halves a frequency.

Product and source

The stocked part is Texas Instruments LMC555CMMX/NOPB: the LMC555 in an 8-pin VSSOP (package DGK) on a large reel, −40 to 85 °C. Pinout, thresholds, timing formulas, supply and output limits were checked against TI datasheet SNAS558N (revised March 2024).

View the LMC555CMMX/NOPB product · Read the TI datasheet