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Delay off after the input pulse completed with 555ic

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Mindmywork

Member
Hai.
I just want some help in this school project circuit.im using 555ic to get delay off effect of lights.

But the problem comes when trigger is longer than delay time, then lights going instantly off without any delay. So can u please help me in building the right circuit. Im just using monostable mode to get the result,but in vain
 

crutschow

Well-Known Member
Most Helpful Member
Show the circuit of what you have and how it's being triggered.
 

Mindmywork

Member
This is the circuit. Triggered with IR circuit when there is a blockage between IR led and photo diode. Replaced the load with a led,cause i want to use it in the street lights06-20-42-IR-security-alarm-circuit.gif theme.
 

crutschow

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Most Helpful Member
If I understand correctly you want the one-shot time to start at the beginning of the trigger and time out after the delay time after the trigger stops.
Below is the LTspice simulation of a modification to the circuit that does that.
I added two transistors, one to trigger the 555, and the other to keep the voltage on the timing cap C1 (Vc) at zero volts until the end of the trigger, at which point the 555 times out.
You can use other NPN transistors but they should be rated for a least 200mA maximum to minimize their saturation voltage.
Note that the input polarity of the LM358 is reversed, since the transistors add a phase reversal.

As can be seen, the pulse OUT is extended a little over 1 second after the end of the trigger as determined by the R1C1 values shown.

upload_2016-10-23_22-52-9.png
 
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Mindmywork

Member
Thank you very much. That's the exact thing that I wanted.
Sir.do I need to add that 50k resistors and two transistors only or some extra components to the circuit that I'm working on. .
Once again thank you very much.
 

Les Jones

Well-Known Member
Most Helpful Member
I would suggest modifying your original circuit as follows. Remove the connection between pin 1 of the LM358 and pin 2 of the 555. connect a capacitor between pin 1 of the LM358 and pin 2 of the 555. (I would suggest between 100 nF and 1 uF) Connect a resistor of between 100K and 470K between pin 2 of the 555 and +5V. Also connect a diode between pin 2 of the 555 and +5V with the + end to +5V. Almost any diode will do. 1n4148 or any of the 1N400x series. (x is any number between 1 and 7)

Les.
 

crutschow

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Most Helpful Member
do I need to add that 50k resistors and two transistors only or some extra components to the circuit that I'm working on.
You just need to add the two 50k resistors (R2, R4 in my schematic) and the two transistors to the circuit you posted, and reverse the inputs to the op amp.

The pot and input resistor R3 in my circuit are part of the circuit you already have.
V2 is just to simulate the switching of the photodiode.

Note that LJ's suggested modifications won't give you the extended delay pulse you want after the end of a long trigger pulse.
 

Mindmywork

Member
Thank you very much crutshow. I will try it out. I can't express how happy I am. Looking at your suggestions im sure to get the desired result. Thank you once again sir.
 

Les Jones

Well-Known Member
Most Helpful Member
I misunderstood your reqirement so my suggestion would not do what you need as Crut's says.

Les.
 

Mindmywork

Member
You just need to add the two 50k resistors (R2, R4 in my schematic) and the two transistors to the circuit you posted, and reverse the inputs to the op amp.

The pot and input resistor R3 in my circuit are part of the circuit you already have.
V2 is just to simulate the switching of the photodiode.

Note that LJ's suggested modifications won't give you the extended delay pulse you want after the end of a long trigger pulse.
I did reverse the inputs but circuit failed to recognise the blockage between ir led and photodiode?
What to do?
And I don't get the v2 . I already connected the photodiode to 5v as in circuit. Im nooby. So plz bare with me
 

crutschow

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Most Helpful Member
Bingo. Im getting what I want. Thank you crutschow.
Great.
I assume you realized that V2 in my schematic was just for simulation purposes and is not needed in your actual circuit.

Edit: Below is a variation on the circuit I posted that only requires one transistor instead of two.
Note that the connection to DIS (pin 7) must also be removed.

It could actually be reduced to no transistors if the LM358 opamp is replace with an LM339/LM393 comparator as shown in the second attachment, since the LM339/LM393 has an open collector output.

upload_2016-10-24_10-40-12.png

upload_2016-10-24_10-59-9.png
 
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Mindmywork

Member
Sir. Another question .
Im getting the output pulse right now.
I want to improve it more.
Like say 3 short input pulses given in the delay time which are shorter than the delay. Can the delay be reset to the last input pulse? Just want to experiment it more.
 

crutschow

Well-Known Member
Most Helpful Member
Sir. Another question .
Im getting the output pulse right now.
I want to improve it more.
Like say 3 short input pulses given in the delay time which are shorter than the delay. Can the delay be reset to the last input pulse? Just want to experiment it more.
Yes.
All the circuits I posted will start the delay after the trailing edge of the last pulse as long as the time between pulses is shorter than the one-shot time, as shown below.

upload_2016-10-24_11-14-34.png
 

Mindmywork

Member
sir. u r the best. i tried to find out a solution for the first problem googling two days, but in vain. if i were know this site earlier, lots of my time could have saved.
ill try it out and post the results tomorrow. thank you sir.
 

crutschow

Well-Known Member
Most Helpful Member
What you needed to look for is "retriggerable one-shot".
 

spec

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Hi MMW,

Which part of India are you from? If you put it next to 'Location' on your user page, it will show in the box at the left of your posts.

Some of us are interested to know where members are from. It also helps us to know what components you can access and to establish your mains supply characteristics and the temperature range in your local, all of which helps us reply to your questions.:)

spec
 
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