NE555 timer: pinout, ratings, CMOS versions, the NE556 and a worked flasher

TI makes the same bipolar timer in four temperature grades from one data sheet: NE555 (0 to 70 °C), NA555 (−40 to 105 °C), SA555 (−40 to 85 °C) and SE555 (−55 to 125 °C, and up to 18 V). The letter after the number is the package: an NE555P is the 8-pin DIP and is marked NE555P; the NE555D is the SOIC-8, marked NE555; the PS (SO-8) and PW (TSSOP-8) versions are marked just N555. The LM555 is the National part that is now TI's, with the same pins and near-identical numbers.

A C in the name or a 7555 means CMOS: TLC555, LMC555, ICM7555. They sit in the same socket and run the same circuits, but they draw tens to hundreds of microamps instead of milliamps, and they cannot drive a load the way the NE555 can. Loose NE555s sold by the hundred are often unbranded, and the print does not tell you who made them.

TI gives the NE555 and NE556 the same limits: 18 V on the supply, ±225 mA from the output, nothing above VCC on the inputs. The LM555 table lists only dissipation, soldering and storage; its 18 V appears as the recommended maximum, and its electrical table runs from 4.5 to 16 V. All three are rated 4.5 to 16 V in use and 0 to 70 °C, so design to 16 V and to 200 mA, not to the absolute figures.

The 4.5 V floor is the line people miss. A 3.3 V rail is outside every bipolar 555's rating; the CMOS versions below are the ones for it.

The capacitor swings between one third and two thirds of the supply, set by three equal internal resistors, so timing does not depend on the supply voltage: TI puts the astable drift at 0.3 % per volt. At 5 V the thresholds can sit anywhere from 2.4 to 4.2 V on one chip, which is why two 555s built with the same parts rarely run at quite the same rate.

The output is a bipolar push-pull stage that loses about 1.7 V when high: 13.3 V typical from a 15 V supply at 100 mA, 12.75 V at worst. From 5 V at 100 mA it can fall to 2.75 V, which is too low for some 5 V logic inputs at that current. In return it sources and sinks 200 mA, enough for a small relay or a string of LEDs without a transistor.

The supply current is the other cost: 3 mA typical at 5 V with the output low, 10 mA at 15 V, before any load. The bipolar output also draws a short current spike as it switches, which is why TI asks for a 0.1 µF ceramic capacitor right across pins 8 and 1.

The CMOS timers keep the pinout and the timing formulas, and TI calls the TLC555 "functionally interchangeable with the NE555". What changes is everything around the timing: they run from lower supplies, draw microamps, take tiny input currents so large timing resistors work, and run faster. What they give up is output drive.

So a CMOS timer drops into an NE555 circuit when the output drives logic, a transistor base or a MOSFET gate. It does not when the output drives an LED, a buzzer or a relay directly: the TLC555 may source no more than 15 mA at all, and at 5 V none of the three promises a high level beyond 2 mA. Drive the load through a transistor, or connect it between the supply and the output so the stronger sink side does the work (the LED then lights while the output is low). Two more differences: the ICM7555 needs no capacitor on CONT, and the TLC555 in TSSOP has 14 pins with a different map.

The NE556 holds two independent NE555 timers that share only the supply (pin 14) and ground (pin 7). Timer 1 uses pins 1 to 6 (discharge, threshold, control, reset, output, trigger, in that order) and timer 2 mirrors it on pins 8 to 13. The ratings and the per-timer figures match the NE555: 4.5 to 16 V, ±200 mA output current, the same thresholds and the same 2.75 V minimum output high from 5 V.

The supply current doubles: 6 mA typical and 12 mA at most at 5 V with both outputs low. TI also gives the matching between the two halves: their timing drifts with temperature within ±10 ppm/°C of each other, and with supply within ±0.2 % per volt. That makes the NE556 the better choice for a pair of timers that must track, such as a pulse generator with adjustable delay and width. It is not pin-compatible with the NE555, so it only goes on boards laid out for it.

• Flashers, tone generators and clocks from a fraction of a hertz to tens of kilohertz, set by two resistors and a capacitor. The 555 astable lesson goes through the cycle.
• Delays and debounced one-shots: a press or a pulse in, one clean pulse of a set length out. See the monostable lesson.
• Driving a relay, a buzzer or LEDs straight from pin 3, up to about 200 mA. For a bigger load, pin 3 drives a transistor such as the BC547.
• Simple PWM for motor speed and LED dimming; see PWM with a 555.

It is the wrong part for battery-powered timing over hours (a CMOS timer draws far less), for 3.3 V boards, and for anything above about 100 kHz, where TI itself points to the TLC555.

An LED that blinks about once a second from a 9 V supply: RA 10 kΩ from the supply to pin 7, RB 68 kΩ from pin 7 to pins 6 and 2, a 10 µF capacitor from there to ground, and the LED on pin 3 through 390 Ω.

The very first flash after power-up lasts longer, about 0.857 s, because the capacitor starts from 0 V instead of 3 V. The 555 timer calculator works out other values, and the 555 LED flasher build takes this circuit onto a board.

• Fit 100 nF ceramic across pins 8 and 1, as close to the chip as it will go. Without it, the current spike at each output edge can retrigger the timer or upset a neighbouring chip. The decoupling lesson explains why.
• Put 10 nF from pin 5 to ground on a bipolar 555. TI says it can improve operation; on a noisy supply it keeps the thresholds still.
• For timing that has to be right, use a film capacitor and a larger resistor rather than a big electrolytic: an electrolytic's tolerance and leakage move the timing more than the 555 does. For a flasher like the one above it does not matter.
• Keep RA + RB under about 3.4 MΩ at 5 V and 10 MΩ at 15 V: beyond that the threshold current, up to 250 nA, is no longer small next to the charging current.
• This circuit always runs above 50 % duty, because the high time includes RA and the low time does not. Keep RA small next to RB, as in the example, to get close to half.

The LM555 and the NA, SA and SE grades of TI's own 555 are true equals. The CMOS timers share the pins but not the output, so they are not in this table; see above for when they work.

1. Build the flasher above on a breadboard at 9 V. A good chip flashes at about once a second from the moment power goes on.
2. With a multimeter on pin 6, the reading should swing between about 3 and 6 V, one third and two thirds of the supply. If it sits still instead, check the wiring at pins 2, 6 and 7 before you blame the chip.
3. Measure the supply current with the LED out of circuit. At 5 V, TI allows 6 mA at most with the output low; a chip that draws far more, or gets warm, is damaged.
4. Short pin 4 to ground: the output must go low and stay low until you release it.

The 555 timer lesson shows what each pin is doing inside the chip while you measure.

Use an NE555 or LM555 for anything on 5 to 15 V that drives a load straight from pin 3, with 100 nF across the supply, 10 nF on pin 5 and pin 4 tied high. On 3.3 V or a battery, use a TLC555 or ICM7555 and put a transistor between it and anything heavier than a logic input. Reach for the NE556 only when two timers must track each other on one board.

The NE555 is one of the cheapest ICs in any Indian market, sold loose or in tens. Ask for the maker by name and check one from every bag with the flasher above before it goes on a board. If you need a CMOS timer, ask for the TLC555 or ICM7555 by its full number. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.