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NE555

Timer

Philips Electronics 

Philips Semiconductors Linear Products

Timer

Product specification

NE/SA/SE555/SE555C

TYPICAL APPLICATIONS

VCC

VCC

VCC

10k

.001µF

2 555

1/3 VCC

OVOLTS

NOTE: All resistor values are in Ω

SWITCH GROUNDED

AT THIS POINT

1

DURATION OF

TRIGGER PULSE AS

SEEN BY THE TIMER

Figure 1. AC Coupling of the Trigger Pulse

Trigger Pulse Width Requirements and Time

Delays

Due to the nature of the trigger circuitry, the timer will trigger on the

negative going edge of the input pulse. For the device to time out

properly, it is necessary that the trigger voltage level be returned to

some voltage greater than one third of the supply before the time out

period. This can be achieved by making either the trigger pulse

sufficiently short or by AC coupling into the trigger. By AC coupling

the trigger, see Figure 1, a short negative going pulse is achieved

when the trigger signal goes to ground. AC coupling is most

frequently used in conjunction with a switch or a signal that goes to

ground which initiates the timing cycle. Should the trigger be held

low, without AC coupling, for a longer duration than the timing cycle

the output will remain in a high state for the duration of the low

trigger signal, without regard to the threshold comparator state. This

is due to the predominance of Q15 on the base of Q16, controlling

the state of the bi-stable flip-flop. When the trigger signal then

returns to a high level, the output will fall immediately. Thus, the

output signal will follow the trigger signal in this case.

Another consideration is the “turn-off time”. This is the measurement

of the amount of time required after the threshold reaches 2/3 VCC

to turn the output low. To explain further, Q1 at the threshold input

turns on after reaching 2/3 VCC, which then turns on Q5, which turns

on Q6. Current from Q6 turns on Q16 which turns Q17 off. This

allows current from Q19 to turn on Q20 and Q24 to given an output

low. These steps cause the 2µs max. delay as stated in the data

sheet.

Also, a delay comparable to the turn-off time is the trigger release

time. When the trigger is low, Q10 is on and turns on Q11 which turns

on Q15. Q15 turns off Q16 and allows Q17 to turn on. This turns off

current to Q20 and Q24, which results in output high. When the

trigger is released, Q10 and Q11 shut off, Q15 turns off, Q16 turns on

and the circuit then follows the same path and time delay explained

as “turn off time”. This trigger release time is very important in

designing the trigger pulse width so as not to interfere with the

output signal as explained previously.

August 31, 1994

352

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