Triggering with Negative Edge

Although the LM122 is triggered by a positive going trigger

signal a differentiator tied to a normally ‘‘high’’ trigger will

result in negative edge triggering In

Figure 19 R1 serves

the dual purpose of holding the trigger pin normally high and

differentiating the input trigger pulse coupled through C1

The timing diagram included with

Figure 21 shows that trig-

gering actually occurs a short time after the negative going

trigger while positive going triggers have no effect The de-

lay time between a negative trigger signal and actual starts

of timing is approximately (05 to 15) (R1

on the trigger amplitude or about 25 to 75 ms with the

er the trigger pin is kept high and must reset itself during the

short time that the trigger pin voltage is low A conservative

value for C1 is

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Chain of Timers

The LM122 can be connected as a chain of timers quite

easily with no interface required In

Figure 20A and 20B two

TLH7768 – 30

FIGURE 19 Timer Triggered by Negative

Edge of Input Pulse

possible connections are shown In both cases the output

of the timer is low during the timing period so that the posi-

tive going signal at the end of the timing period can trigger

the next timer There is no limitation on the timing period of

one timer with respect to any other timer before or after it

because the trigger input to any timer can be high or low

when that timer ends its timing period

TLH7768 – 31

(a)

TLH7768 – 32

TLH7768 – 33

(b)

FIGURE 20 Chain of Timers

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