NE556 Reference Tables, Pins & Specifications

Pin reference

Signal names, electrical functions and connection guidance

PDIP-14

Physical pin numbers14 pins

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NE556 pin functions and connections
Signal	Type	Function & connection
1DISCH	Open-collector discharge	Discharges the external timing capacitor when this channel is LOW. Connection guidance Connect to the junction of RA and RB. This is a switching node, not a second push-pull output.
1THRES	Threshold input	Ends the HIGH interval when the timing node exceeds the upper reference. Connection guidance Join to 1TRIG and the timing capacitor. Nominal reference: 2/3 VCC with CONT unmodulated.
1CONT	Control voltage	Sets the comparator reference for timer 1. Connection guidance Bypass to ground with 10 nF in the example. Changing CONT changes both thresholds and the timing equations.
1RESET	Active-low reset	Forces this output LOW and turns on its discharge switch. Connection guidance Tie to VCC for continuous operation. Do not leave reset floating; it overrides trigger and threshold.
1OUT	Push-pull output	Timer 1 output, independent of the other channel. Connection guidance Connect a high-impedance scope or a current-limited LED. Output swing depends on load; it is not rail-to-rail.
1TRIG	Trigger input	Starts the HIGH interval when the input falls below the lower reference. Connection guidance Join to 1THRES in the astable example. Nominal reference: 1/3 VCC. A sustained LOW trigger can hold the output HIGH.
GND	Ground	Common return for both channels. Connection guidance Connect all local ground symbols to supply return.
2TRIG	Trigger input	Starts the HIGH interval when the input falls below the lower reference. Connection guidance Join to 2THRES in the astable example. Nominal reference: 1/3 VCC. A sustained LOW trigger can hold the output HIGH.
2OUT	Push-pull output	Timer 2 output, independent of the other channel. Connection guidance Connect a high-impedance scope or a current-limited LED. Output swing depends on load; it is not rail-to-rail.
2RESET	Active-low reset	Forces this output LOW and turns on its discharge switch. Connection guidance Tie to VCC for continuous operation. Do not leave reset floating; it overrides trigger and threshold.
2CONT	Control voltage	Sets the comparator reference for timer 2. Connection guidance Bypass to ground with 10 nF in the example. Changing CONT changes both thresholds and the timing equations.
2THRES	Threshold input	Ends the HIGH interval when the timing node exceeds the upper reference. Connection guidance Join to 2TRIG and the timing capacitor. Nominal reference: 2/3 VCC with CONT unmodulated.
2DISCH	Open-collector discharge	Discharges the external timing capacitor when this channel is LOW. Connection guidance Connect to the junction of RA and RB. This is a switching node, not a second push-pull output.
VCC	Supply	Positive supply shared by the two timers. Connection guidance Apply regulated +5 V; place 100 nF directly between pins 14 and 7.

Operating conditions

NE556 grade

Manufacturer-referenced values5 parameters

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Parameter	Value	Conditions / notes
Supply	4.5–16 V	Recommended operating range
Ambient temperature	0 to +70°C	NE556N / NE556DR
Output loading	Up to ±200 mA specified	Check output drop and package dissipation; not a promise of rail-to-rail swing
Example supply	5 V	Common to both channels
Local decoupling	100 nF	Directly across pins 14 and 7

Recommended operation is distinct from absolute stress limits.

Thresholds and output levels

25°C · stated test conditions

Manufacturer-referenced values5 parameters

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Parameter	Value	Conditions / notes
Trigger at 5 V	1.67 V typical; 1.1–2.2 V limits	Each channel
Threshold at 5 V	3.3 V typical; 2.4–4.2 V limits	Each channel
LOW output	0.1 V typical; 0.25 V maximum	5 V supply; sinking 5 mA
HIGH output	3.3 V typical; 2.75 V minimum	5 V supply; sourcing 100 mA
Supply current	6 mA typical; 12 mA maximum	5 V; outputs LOW; no external load

These test points must not be extrapolated to every load or temperature.

Timing equations

Nominal RC relationships

Manufacturer-referenced values5 parameters

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Parameter	Value	Conditions / notes
Astable HIGH	ln(2) × (RA + RB) × C	Capacitor charges through both resistors
Astable LOW	ln(2) × RB × C	Capacitor discharges through RB
Frequency	1 / [ln(2) × (RA + 2RB) × C]	Use Ω, F and seconds
HIGH duty cycle	(RA + RB) / (RA + 2RB)	Above 50% in the ordinary two-resistor circuit
Monostable pulse	Approximately 1.1 × R × C	Trigger must return HIGH before the timing interval ends

Ideal ratios assume unmodulated CONT. The initial power-on interval differs from steady-state cycles.

Two-clock circuit

5 V · independent outputs

Manufacturer-referenced values8 parameters

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Parameter	Value	Conditions / notes
RA1 / RA2	10 kΩ	VCC to respective DISCH pin
RB1 / RB2	100 kΩ	DISCH to respective timing node
C1	10 µF	Timing node 1 to GND; positive terminal to timing node
C2	1 µF	Timing node 2 to GND; positive terminal to timing node
Nominal f1 / f2	0.687 Hz / 6.87 Hz	Ideal values; component tolerances omitted
HIGH duty cycle	52.38%	Both channels
Control bypass	10 nF per channel	Pins 3 and 11 to GND
Reset	Pins 4 and 10 to +5 V	Neither input floats

The illustrated scope is a high-impedance load. Use a series resistor for any LED.

Control behavior

Applied independently to each timer

Manufacturer-referenced values5 parameters

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Parameter	Value	Conditions / notes
RESET LOW	OUT LOW; DISCH conducting	Overrides other inputs
TRIG below lower reference	OUT HIGH; DISCH open	When RESET is inactive
THRES above upper reference	OUT LOW; DISCH conducting	When trigger is above its reference
Both timing inputs between references	Retain latch state	Hysteresis prevents chatter in a clean RC cycle
Unused channel	Hold RESET LOW and define its inputs	Do not assume floating pins are harmless

Input priority follows RESET, then trigger, then threshold.

Package and pin reference

NE556N · PDIP-14

Manufacturer-referenced values5 parameters

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Parameter	Value	Conditions / notes
Pitch	2.54 mm	Through-hole
Timer 1	DISCH 1; THRES 2; CONT 3	RESET 4; OUT 5; TRIG 6
Timer 2	TRIG 8; OUT 9; RESET 10	CONT 11; THRES 12; DISCH 13
Shared power	GND 7; VCC 14	Never reuse the eight-pin NE555 numbering
Orientation	Top view	Pin-one mark and notch identify the numbering

Match the exact mechanical package before manufacturing a PCB.