LM555/NE555 timer

Functional diagram of 555 timer, with pinout

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LM556/NE556 dual timer

Functional diagram of 556 dual time, wtih pintout (left half)

Functional diagram of 556 dual time, wtih pintout (right half)

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Truth Table for LM555/NE555 timer and LM556/NE556 dual timer

	¬Trigger	¬Reset	Threshold	Output
555 pin	2	4	6	3
556 pin (L)	6	4	2	5
556 pin (R)	8	10	12	9
	0*	0*	0	0	¬reset overrides ¬trigger
	0*	0*	1*	0	¬reset overrides ¬trigger
	0*	1	0	1	= ¬trigger
	0*	1	1*	1	¬trigger overrides threshold
	1	0*	0	0	= ¬reset
	1	0*	1*	0	= ¬reset
	1	1	0	?	current flip-flop state
	1	1	1*	0	= threshold

¬trigger sets the flip-flop.

threshold or ¬reset resets the flip-flop.

¬reset overrides ¬trigger, which overrides threshold.

* = active state (¬trigger & ¬reset are active-low, threshold is active-high)

¬trigger is relative to 1/3 Vcc and threshold is relative to 2/3 Vcc, unless changed via pin 5 (control voltage)

Data Sheets etc.

TI NE556 (or LM556 with erroneous block diagram)

Fairchild LM555

SGS-Thomson LM556

TI LMC555 CMOS low-power timer

TI TLC555 CMOS low-power timer

Intersil ICM7555 & ICM7556 CMOS low-power timers

CSS555 micropower programmable timer

more 555 & 556 datasheets

555 & 556 timer application notes

Rob Paisley's 555 timer circuits

The seldom-used Control Voltage pin can occasionally be useful