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# How do you smooth out voltage pulses using a capacitor? Answered

I am making a project using an alarm clock, and basically for this part, I am trying to make the alarm turn on an array of leds and I have it working, I have the voltage regulated right and everything, I just need to know how to make the leds stay lit instead of turn on and off with the pulse of the alarm... I thought that I could do it with a capacitor but I haven't figured it out yet... Thanks

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## 11 Replies

These things usually output to a piezo buzzer? Get the alarm to trip the LEDs via a transistor, and like you suggest add in a big capacitor, think big.

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faham (author)2010-02-18

Look fo a circuit called sample and hold circuit, it is basically a transistor (or op amp) with a cap and resistor at the base. See also my website:

http://www.geocities.com/fifaham/NegPulse.htm

and peak detector just for an idea:

http://www.geocities.com/fifaham/UCircuits.htm

steveastrouk (author)2009-09-09

Sounds like your powersupply may also be a little weak. What have you used ? How much current can the LEDs draw ?

arhodes18 (author)2009-09-10

I just used an old cigarette lighter plug in regulator deal... Im not entirely sure how much current they can draw because I got them all out of old electronic devices...

steveastrouk (author)2009-09-10

Measure the current with a meter then. A capacitor may help, it may not. What volts drive the clock ? Try throwing a 1000uF capacitor across the input to the regulator.

arhodes18 (author)2009-09-10

Well, I did have a 470 uF cap after the regulator but before the leds on the positive line, so it was in series I believe? I did notice however that the higher the uF the the better it worked, meaning that it caused the flash on and off turn to a "fade" on and off...

steveastrouk (author)2009-09-10

Putting a large cap in parallel - which is what you are doing is helping. Try putting it before the regulator - because you might be stressing the thing severely.

arhodes18 (author)2009-09-11

oh ok will do... thank you

Re-design (author)2009-09-10

Put it on a timer circuit so that when the alarm goes of the timer will power the leds for a minute or what ever time you want. I think what you're seeing it the pulsing of the buzzer going on and off. A timer can be made easily with a 555 i c. Google 555 timer.

arhodes18 (author)2009-09-10

I tried doing something similar to that, only using an op amp as a comparator instead of a 555 timer, but I couldn't figure out how to wire it right... Do you know how you could do it using the comparator?

orksecurity (author)2009-09-09

Look up "rc filter".