Change DC Logic Output to Pulse

[diggbayte]

Hello everyone....

I have a simple (I think) question. I need to change a continuous DC logic output to a single pulse output using a simple circuit or switch of somekind. I am planning on splitting the DC logic output of my circuit into two seperate leads. One lead works perfectly already with the DC out, but I need the second lead to change into a single pulse, like pushing a button once. Each time the DC out is true, I need a switch or circuit (flip-flop???) to send a single logic pulse. I would prefer to purchase something that could accomplish this as opposed to building it, but I have no idea what direction to look for a solution.... any help???


Thanks,
Scott

 

[melone]

http://www.national.com

Look for 555 timer....

 

[nbucska]

Better think over what do you want. A continuous DC can't
be converted to a pulse for you don't know when the pulse
is to be sent.

Usually you send the pulse when the DC rising exceeds
a certain threshold. The circuit is : Comparator with reference voltage -- triggering a single shot.

<nbucska@pcperipherals.com>

 

[cbarn24050]

Hi, a capacitor in series should do it.

 

[xyzz]

Diggbayte, what is your logic voltage, currents and frequencies?
Is it for industry use?
Is it a budget project?
For industrial use a special timer relay can make the job,
but the cost will be comparable to cheapest PLCs.

In the relay logic it may be done as follows:
- the continuous signal operates relay 1 coil,
- relay 1 switches a capacitor from charging to discharge on relay 2 coil,
- relay 2 is active until the capacitor is discharged to some voltage.
The pulse time depends on capacity and discharge current.
In reality the relay 1 contacts have to be protected by limiting the charging current (by resistor).
This determines minimum time between pulses.

Cbarn24050, not exactly.
You have to provide means to discharge the capacitor while the signal is off.

 

[tonymac]

It's possible to use a capacitor to hold the old logic level momentarily while inputting to a logic gate or comparator to cause a pulse output. The logic level could be tied to both pins, but with a capacitor hooked on the one to slow its response, causing a dissimilar input and a momentary output until the capacitor charged/discharged in the presence of the new signal.

 

[phastmike]

Depending on application a simple RC (passive) High-pass filter should be enough.
At input the transition from Low to High should make a quick pulse (Discharge time depends on RC values).

 

[plcsavvy]

If you are working with TTL level signals, why not try a 74121 monostable multivibrator IC. The monostable multivibrator, more commonly known as a one-shot has one input (trigger) and two out puts Q and Q not. You will need both a resistor and a capaitor to set up the time constant.
When a logic low DC signal is applied to the trigger input the Q output will transition from a logic low to a logic high. The duration of the pulse is controlled by the rc time constant expressed as 0.7 RtCt. If your signal input to the Trigger input of the 74121 is alogic high just use a 74704 inverter IC to convert the signal to a logic low.
Best regards, PLCSAVVY.

 

[EMRAH]

I Think you can put a capasitor serial to a resistor and put it to the base of a npn transistor. the collector would be the dc output and a 2.2K resistor between emitor and the ground.
The output of the pulse is from the emitor. Put a resistor parallel to the capacitor. The length of the pulse would be from the capacitor.