# 555 monostable multivibrator leading edge trailing edge

#### john74

Joined Aug 22, 2015
9
Im studying timer circuits...and it seems to be a bit confusing. Studying the 555 monostable multivibrator in particular the input and output pulses...it states that , with a negative going input pulse, the leading edge of the negative input pulse starts the leading edge of the output pulse which is positive...ok...fine but is this the same as saying - the output pulse starts when the input (trigger) pulse starts?

Or is it a matter of terminology...or how its worded...it seems like it should be simple but yet its confusing.Because it seems like I read other places where its not clear, sort of contradictory.

Also what if the input trigger pulse was positive...how would that relate to the output pulse? Would the leading edge of the input still start the leading edge of the output. I appreciate any thoughts, thank you so much

#### hp1729

Joined Nov 23, 2015
2,304
Im studying timer circuits...and it seems to be a bit confusing. Studying the 555 monostable multivibrator in particular the input and output pulses...it states that , with a negative going input pulse, the leading edge of the negative input pulse starts the leading edge of the output pulse which is positive...ok...fine but is this the same as saying - the output pulse starts when the input (trigger) pulse starts?

Or is it a matter of terminology...or how its worded...it seems like it should be simple but yet its confusing.Because it seems like I read other places where its not clear, sort of contradictory.

Also what if the input trigger pulse was positive...how would that relate to the output pulse? Would the leading edge of the input still start the leading edge of the output. I appreciate any thoughts, thank you so much
Yes, that could be worded better. Your understanding is correct. The trigger input is a low edge clock (1/3 VCC). The output goes high, Discharge* is released and the capacitor starts to charge. When the charge gets up to 2/3 VCC the output goes low. In astable operation Discharge* goes low and the capacitor starts discharging. When the charge on the capacitor gets down to 1/3 VCC the trigger point is reached again, the output goes high, Discharge* is released and a charge cycle starts again.

Inside the 555 is an S-R latch with voltage comparators on the inputs. The Set side is the Trigger input and a voltage divider set to 1/3 VCC, The Reset side is the Threshold input and a voltage divider set at 2/3 VCC. This 2/3 VCC point is available at the Control input. Discharge* is an NPN transistor coming from the output of the latch.

#### ericgibbs

Joined Jan 29, 2010
18,841
hi John,
Look at page #4 of this 555 App for info regarding the Trigger.
E

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#### crutschow

Joined Mar 14, 2008
34,408
The negative going (falling) edge of the trigger input starts the positive going (rising) edge of the output pulse.
Note that the trigger pulse can't stay low longer then the output pulse period or the output will stay high as long as the input is held low.

#### dl324

Joined Mar 30, 2015
16,909
what if the input trigger pulse was positive...how would that relate to the output pulse? Would the leading edge of the input still start the leading edge of the output.
The timer requires a voltage lower than 1/3Vcc to trigger. If you apply a positive pulse, triggering would occur on the falling edge of that pulse.

#### hp1729

Joined Nov 23, 2015
2,304
hi John,
Look at page #4 of this 555 App for info regarding the Trigger.
E
Good ap note. Thanks.

#### john74

Joined Aug 22, 2015
9
The timer requires a voltage lower than 1/3Vcc to trigger. If you apply a positive pulse, triggering would occur on the falling edge of that pulse.

#### john74

Joined Aug 22, 2015
9

#### chalky

Joined Aug 20, 2017
1
Hi all, is the trigger actually level, rather than edge? Matters if it's low at power-on time.

#### crutschow

Joined Mar 14, 2008
34,408
Hi all, is the trigger actually level, rather than edge? Matters if it's low at power-on time.
The 555 is level triggered.

#### Tonyr1084

Joined Sep 24, 2015
7,894
Positive and negative triggering simply means this: When a digital signal goes from 0 to 1 (low to high) that is the positive going edge of the wave form. When the signal goes from 1 to 0 (high to low) that's the negative going edge. So if a device requires a positive edge then as soon as the digital signal comes in - in this case - the timer begins. But if a device requires a negative edge then suppose you push a button and hold it for 10 seconds. The timer won't start counting until you release the button. At that moment when the clock pulse goes low (negative edge) the timer starts.

What the output is doing may be the same or it may be the opposite, depending on what you're working with. Most of MY experiences have revolved around positive edge triggering. However, negative edge is not uncommon. Just saying it's been my (me personally) experience positive edge has been more predominate. Such as the triggering pulse of a decade counter or a flip flop. But I'm sure there are devices that require the negative edge.