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555 timer circuit reset function

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Uman

New Member
I am designing a circuit with a 555 monstable one shot timer to auto off the circuit power if the time expires. How can a 555 timer be reset to zero with an external positive pulse and automatically restart the count? The reset function on the timer only stops the count and the output goes low. I need the output to stay high and the count refreshed to zero and continue counting to a predetermined value, then if it achieves this value, the timer output goes low.
Any suggestions?
Thanks,
Jeff
 

Roff

Well-Known Member
The circuit below will reset and hold the voltage on the timing cap to VCC/3 so long as reset is high. VCC/3 is the voltage on the cap at the beginning of the timing cycle, so when you release reset, the length of the first cycle will be very close to all those that follow. The output (pin 3) will be high while reset is high.
A 555 doesn't actually have a count, its just an RC multivibrator.
 

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sonicysa

New Member
would it be possible for you to explain the reason for the other sorta circuit in here? for example, if you were to just use only 1 npn to basically short the capacitor vs. the two pnps and one npn?
I was trying to do the same thing to a monostable sorta circuit and the resistance of the npn across the capacitor really throws things off. Like the cap never charges all the way up.
I'm trying to use a much larger value though than your example.. Like 3Mohm resistor and 100uF cap. to get about 6minute time.
what I need is to have the 555 put out a high or low for 6min then have another circuit attached feed in a high voltage to reset/start the timer over. I'm thinking this is basically what your answer to the guy above was but I'm guessing I would need to re-calculate the values for the left hand side of your circuit(the 200k ohm parts etc. if I was changing the RC time constant values on the right.
Thoughts etc. ?
Also in my instance when the circuit that is feeding back that high value to do the reset is switched off it may look like a pin floating or may be like direct ground. not sure.
Thanks.
 

MikeMl

Well-Known Member
Most Helpful Member
I am designing a circuit ... to auto off the circuit power if the time expires. ...

Is this to turn off something like in a car after the key is turn off? If so, there are much better ways of doing it than using a 555.
 

ronv

Well-Known Member
Most Helpful Member
His circuit holds the voltage at Vcc/3 so that the 1st pulse will be accurate. If you just pull it to ground the 1st pulse will be long. You are using very large values so sometimes leakage current can be a problem. I'm thinking you must be using a CMOS 555? Like LMC555? You can fix your floating input by adding a resistor - say 47k from base to ground on the first transistor.
 

sonicysa

New Member
I understand that he said it would hold it to 1/3 while reset is held down. But I don't really understand how that works with the mix of resistors and npn and pnp transistors on that left side circuit. Maybe it is too difficult to explain but I was hoping somebody might find that not too difficult.
In my case maybe I don't even really need to worry about that complexity either--not sure. Because I really don't need consistent after pulses, I really just want to discharge the time cap and start over. Too bad "reset" doesn't actually do just that. Basically I have another circuit that I want to sleep for 6 min so I was thinking to use the 555 as a monostable. The initial pulse of 6 min would keep the other circuit off and then turn it on using the out pulse change after six minutes over a low current relay. At first I thought mosfet but I need to keep the ground of the other circuit the same as the first and it seemed to be more trouble to do a high side mosfet.

[You are using very large values so sometimes leakage current can be a problem. ]
Any ways around that?

[I'm thinking you must be using a CMOS 555? Like LMC555?]
Yea NTE955MC does that have anything to do with extra leakage? any suggestions there? also using a 9014 for the npn transistor... was what was lying around.

[You can fix your floating input by adding a resistor - say 47k from base to ground on the first transistor.]
So in my attached picture, I have a switch attached from +volts to a resistor attached to the base of the npn. This was to simulate the other circuit resetting the time cap.
And in the simulator it worked fine with the high values 3Mohm and even on the breadboard from what I remember it seemed to work and reset. but when actually soldered all together it doesn't seem to work the same. I see voltage over the time resistors dropping and then they get to that almost trigger point and then start going backward. like the cap starts to discharge a bit through the npn. without the npn in the circuit it works fine.
One other thing to note is that it would be an arduino digital pin that I was using as the input to the reset and that it would look like ground part of the time vs an open like the switch seems to be. so I had just put in a 1k between arduino out pin and the npn base.
 

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sonicysa

New Member
Is this to turn off something like in a car after the key is turn off? If so, there are much better ways of doing it than using a 555.
No. But if you have other ideas feel free to mention. I'm open to anything. But this is just an arduino running on a battery and arduino seems to suck at sleeping.
 

sonicysa

New Member
His circuit holds the voltage at Vcc/3 so that the 1st pulse will be accurate. If you just pull it to ground the 1st pulse will be long. You are using very large values so sometimes leakage current can be a problem. I'm thinking you must be using a CMOS 555? Like LMC555? You can fix your floating input by adding a resistor - say 47k from base to ground on the first transistor.

I understand that he said it would hold it to 1/3 while reset is held down. But I don't really understand how that works with the mix of resistors and npn and pnp transistors on that left side circuit. Maybe it is too difficult to explain but I was hoping somebody might find that not too difficult.
In my case maybe I don't even really need to worry about that complexity either--not sure. Because I really don't need consistent after pulses, I really just want to discharge the time cap and start over. Too bad "reset" doesn't actually do just that. Basically I have another circuit that I want to sleep for 6 min so I was thinking to use the 555 as a monostable. The initial pulse of 6 min would keep the other circuit off and then turn it on using the out pulse change after six minutes over a low current relay. At first I thought mosfet but I need to keep the ground of the other circuit the same as the first and it seemed to be more trouble to do a high side mosfet.

[You are using very large values so sometimes leakage current can be a problem. ]
Any ways around that?

[I'm thinking you must be using a CMOS 555? Like LMC555?]
Yea NTE955MC does that have anything to do with extra leakage? any suggestions there? also using a 9014 for the npn transistor... was what was lying around.

[You can fix your floating input by adding a resistor - say 47k from base to ground on the first transistor.]
So in my attached picture, I have a switch attached from +volts to a resistor attached to the base of the npn. This was to simulate the other circuit resetting the time cap.
And in the simulator it worked fine with the high values 3Mohm and even on the breadboard from what I remember it seemed to work and reset. but when actually soldered all together it doesn't seem to work the same. I see voltage over the time resistors dropping and then they get to that almost trigger point and then start going backward. like the cap starts to discharge a bit through the npn. without the npn in the circuit it works fine.
One other thing to note is that it would be an arduino digital pin that I was using as the input to the reset and that it would look like ground part of the time vs an open like the switch seems to be. so I had just put in a 1k between arduino out pin and the npn base.
 

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ronv

Well-Known Member
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I think this does what you want.
 

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MikeMl

Well-Known Member
Most Helpful Member
No. But if you have other ideas feel free to mention. I'm open to anything. But this is just an arduino running on a battery and arduino seems to suck at sleeping.


So can you program the Arduino to change the state of a port pin to turn itself off?

What turns it on?
 

sonicysa

New Member
Mike, the arduino is on as soon as power is supplied to it. So if that power source is coming from a 555 circuit and the 555circuit can be reset to trigger another off state of 6 minutes then yes essensially it would be turning itself off but doesn't need to worry about turning itself on because the other circuit will just end up supplying power again a few min later.

ronv I'll experiment with your circuit idea. thanks. Doesn't look like it has large resistor values though so hopefully the swap will result ok.
 

sonicysa

New Member
Yes, I just didn't use the large values.
I ended up just switching out the transistor for a low voltage relay. Just went with overall two relays and one cmos 555 timer. Seems that the nand gate was a waste and burned an extra 2-3mA. Didn't realize I could just wire up one of the relays the other way to invert the 555 output. : )
Thanks for the suggestions.
 
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