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1 second timer

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Hello Guys,

I was wondering how to generate a 1 second negative pulse that would turn on a relay I am using to power up a remote PC. (relay joins two wires for one second, then disconnects)

I have successfully done this with a microcontroller (89C52),a relay and a 2N2907. The relay is only connected for 1 second, then disconnected, using a delay loop within the microcontroller code.

I feel this is overkill and I am looking for an easier way to do this without the microcontroller.

The 1 second pulse does not have to be accurate. The PC is switched on by a momentary push button switch joining two wires. So it can be a 1/2 second pulse or longer, accuracy here is not important. The relay does this as soon as the circuit is powered up, which is hooked to a simple timer.

The circuit I have works perfectly however I am just curious to know how to do it with a few capacitors or resistors if possible.

Anyone have any ideas or sample circuits?
 
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You can build a 555-based monostable circuit, but for a minimum-part solution I would use a small microcontroller with internal oscillator and pull-up resistors. You would need a few extern components. The PIC10F200 is a 6-pin microcotroller, the PIC12F508 or 629 are fine too.
 
1 second pulse

eng1 said:
but for a minimum-part solution I would use a small microcontroller with internal oscillator and pull-up resistors.

I already did this with a microcontroller, I wanted to find a way to eliminate the microcontroller.
 
I designed this for someone recently. It does more than what you want but can be easily adapted.
 

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1 second pulse

thank you for your quick responses.

On powerup, the pin controlling the base of the 2N2907 should be low for one second,(or whatever, 1/2 second, etc) then high permanently.

I was just wondering if there was a simpler way to do this with hardly any components.

A small microcontroller like a tiny11 would do the task with no external components at all. Since I had an old 89C52, I tried it, and it works perfectly.

Please forgive my hardware ignorance, but can this be done with just a capacitor and resistor?
 
johnsmith123 said:
I was just wondering if there was a simpler way to do this with hardly any components.
Then change it so you need a positive pulse, then you won't need a transistor, all you'lll need is a 555 and a couple of resistors and capacitors.
 
1 second pulse

Ron H said:
Where will the pulse that triggers the relay be coming from?

In my working board it is coming from a microcontroller with a 1 second delay between toggling the pin.

In the 555 circuit, the pulse would be coming directly from the 555 timer.

I found this circuit. This should be what I am looking for.

Thanks to everyone for your help.
 

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Evidently you can't just use the switch. Is that because it is actuated by something that doesn't allow it to be closed for the required amount of time?
 
johnsmith123 said:
In my working board it is coming from a microcontroller with a 1 second delay between toggling the pin.

In the 555 circuit, the pulse would be coming directly from the 555 timer.

I found this circuit. This should be what I am looking for.

Thanks to everyone for your help.
I don't think you answered Ron's question.

What initiates the pulse? Do you want it to start when power is applied to the unit or after you press a button?

The 555 circuit you posted only gives a short pulse after the button is pressed. But you wrote that you want the pulse to go high and remain high after about 1 second. So what starts the 1 s timer?
 
ljcox said:
I don't think you answered Ron's question.

What initiates the pulse? Do you want it to start when power is applied to the unit or after you press a button?

The 555 circuit you posted only gives a short pulse after the button is pressed. But you wrote that you want the pulse to go high and remain high after about 1 second. So what starts the 1 s timer?
After rereading his posts, I think the switch in the schematic is there because the circuit was designed for something else, and he hopes to adapt it to his purposes, but without the switch. I think he wants his laptop to turn on when he turns on the power to another circuit.
 
Here's a raw circuit that might do the job.

The PNP transistor is normally off due to the positive current at its base. When the normally open switch is closed, Capacitor C1 discharges through R2, and the tranisistor is turned on, activating the relay (RLoad).

I'll leave it to you to determine the RC time components that suit your purpose.
 

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Re: 1 second pulse

ljcox said:
I don't think you answered Ron's question.

What initiates the pulse? Do you want it to start when power is applied to the unit or after you press a button?

Yes. As soon as the circuit is powered up, the pulse would go high [or low] for one second, then stay low[or high].

It doesn't matter whether or not the pulse is high or low, I can adjust the connections to the relay accordingly. There is no push button in the circuit.
The circuit is powered by an appliance timer that turns on at 8:30 am every morning.(sounds crude, but its the simplest ($4.00) way short of building an appliance timer myself) The appliance timer turns off 5 minutes later since the circuit already did what it needed to do.
(just like myself physically going to the PC and holding in the momentary on/off switch for 1 second)
The relay just briefly joins two pins on the motherboard, that powers up the PC.

At night, the PC is shut down automatically by software. The next day, the circuit is powered up at 8:30 am, and the PC is turned on again.

ljcox said:
The 555 circuit you posted only gives a short pulse after the button is pressed. But you wrote that you want the pulse to go high and remain high after about 1 second. So what starts the 1 s timer?

The capacitor and resistor value in the circuit I posted needs to be changed to 100uF and 10K. This will give a one second pulse.

Ron H said:
Evidently you can't just use the switch. Is that because it is actuated by something that doesn't allow it to be closed for the required amount of time?

Yes the PC is powered up by a momentary push switch. This switch simply joins two wires together on the motherboard for as long as the human holds in the switch, the momentary switch must be closed, then open. The time that the switch needs to be closed is not important. A human would press the switch for under 1 second, so that is the reason for the 1 second pulse. (give or take 0.3 seconds)


I hope I answered most of your questions.

Remember: I have this working PERFECTLY with a microcontroller.
Out of shear curiosity, I would like to know of a REALLY simple way (least amount of components without the microcontroller) to make this work.

I have seen simple reset circuits for the 8051 microcontroller that delay the time period while the reset is low. A simple resistor and a capacitor. I was thinking that this type of setup could somehow be modified to work with what I am doing.

I included a basic schematic (please forgive me, I'm NEW)
Cheers
 

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My solution: a self-triggered monostable with an NE555, a resistors, a capacitor (or two 1uF polyester caps in parallel). On power-up it produces a 1 second pulse (high). You need a current limiting reistor between the output and the base of the transistor. I used 12 V because I didn't know the voltage from your timer.
 

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working circuit

AllVol said:
Here's a raw circuit that might do the job.

The PNP transistor is normally off due to the positive current at its base. When the normally open switch is closed, Capacitor C1 discharges through R2, and the tranisistor is turned on, activating the relay (RLoad).

I'll leave it to you to determine the RC time components that suit your purpose.

Thank you AllVol that circuit works perfectly. Extremely simple, hardly any components, best of all NO 555 timer and no microcontroller. I eliminated the switch, and for R1 I used 100K (hope that is ok) The circuit just powers up and turns on the transistor for almost 1 second, then it stays off. PERFECT!

I used a pot to determine what R2 would be. Best results are R2 = 30K and C1 100uF.

Is there a formula where I can determine the values of C1 & R2?

Many thanks,
JS
 
johnsmith123 said:
Thank you AllVol that circuit works perfectly. Extremely simple, hardly any components, best of all NO 555 timer and no microcontroller. Many thanks,JS

Those were your specifications. Glad it worked. Good luck.

EDIT: You're pretty close. OR maybe I was. In the original design, I guessed 47K for R2 and a 100uF cap. R1 is not critical.
 
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1 second pulse

AllVol said:
You're pretty close. OR maybe I was. In the original design, I guessed 47K for R2 and a 100uF cap.[/I]

In the end I changed the capacitor to 220uF because the 100uF cap gave a pulse that was too narrow. The pulse is now about 1/2 second and it works perfectly.

Thanks again,
Cheers!
 
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