# Newbie circuit modification

Discussion in 'General Electronics Discussion' started by gwfami, Sep 13, 2012.

1. ### gwfami

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0
Sep 13, 2012
Hi,
I'm kind of a newbie at circuit modification, understand most of the basics, but some things elude me.

I've been asked by my son-in-law to modify this circuit so that it will drive a couple of led strips for his band (strobe like)

Each strip is about 0.05 amps at 12 volts. I checked on the specs of the op amp shown, and it doesn't look like it can handle the load (http://www.jameco.com/Jameco/Products/ProdDS/1745688.pdf).

My initial thought was getting a large enough op amp that could handle 1/2 amp or so at 12 volts, but I'm not sure what to look for here.

My second thought, probably the best, is to use a transistor to switch the led's on and off based on the op amps output, but once again, I don't know which one to use.

Any and all help will be greatly appreciated.

Thanks,

GWFAMI.

2. ### Harald KappModeratorModerator

11,794
2,749
Nov 17, 2011
Have a look here, link to page 2.

A modification of your circuit could look something like this:

For the PNP transistor any type with at least 12 V Vce and enough Ice (0.05A per strip) is suitable.

For R 1 use R1=12V/(Ice*100). Use the next lower standard value you can get.

Using R2 you can control the current though the LED strips. If the strips have theit own built-in current limiting you can omit R2. If not, set the LED current by adjusting R2 to a suitable value. You may need to experiment, the exact value of R2 depends on the LED current and the characteristics of the LED strip.

D1...D2 represent (symbolically) one single LED strip. If you want top drive ore than one strip I suggest you use a singe resistor-transistor combination for each strip. Thus a failure of one strip will not influence the other strips. Also you will not experience any hassle due to small differences in teh V-I characteristics of the strips.

Also the whole circuit must be operated from 12 V (not 9V as in your example). Other wise the PNP will never be turned off even if the OpAmp output is at its maximum (this would be 9V for a 9V operating voltage and thus the base of the PNP would still be at a lower potential than the emitter,m this the transistor would be on),

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3. ### gwfami

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Sep 13, 2012

I'm thinking that your modification is from the op amp on, and doesn't include the peizo trigger and associated circuitry, right?

GWFAMI

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
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Jan 21, 2010
that would be right. His circuit is just a modification of what the op-amp drives.

5. ### gwfami

25
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Sep 13, 2012
I'm looking at getting an assortment of resistors, don't want to buy them 1 at a time, for this project. What should the wattage be? I figured that it should be at least 1-1.2 watts, but then, you guys are the experts.

Thanks,

GWFAMI

1,114
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Aug 13, 2011
¼W will be sufficient.

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
There is a thread here concerning one member's search for such a collection of resistors. Let me see if I can find it...

This isn't the one I was thinking about, but it's got some (possibly) useful links

8. ### CocaCola

3,635
5
Apr 7, 2012
The one you are talking about is probably the one from vick5821 awhile back... I'm remote desk topping from my phone right now so it's a hassle to search and what not or else I would have found the actual thread...

But, on that note I was browsing the Dealtime Extreme website today under new additions and they have just added a bunch of resistor kits to their inventory... They are not always the quickest to ship but they have never let me down and they are my go to place for cheap Asian marketplace type items... Can't comment on the price either, I just glanced over the offering...

9. ### davennModerator

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1,969
Sep 5, 2009
Here is the thread in question

Dave

10. ### gwfami

25
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Sep 13, 2012
I have modified the original circuit to look like this.

The problem that I'm having now is that the LED remains lit even when IC1 is removed (IC1 sends a signal to the transistor to power up and allow the LED to light up). It's pretty dim, but the LED strip is still lit. Is it possible that the transistor is leaking some electricity though? I've tried 2 different types of transistors, both new, and they all allow enough current to flow through to light the LED strip.

Also, when the chip activates the transistor, the LED strip only lights up at about 1/2 power. Shorting the leads to the LED strip results in a very bright light, which is what I am wanting.

Any ideas or critiques will be greatly appreciated.

Thanks,

GWFAMI

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5,164
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Dec 18, 2013
First thing is you don't have a resistor protecting the base from over current and the LEDs. See the first example of circuit. You may need a pull up from the base to the +V supply. This would explain the LED lighting with the chip removed. Always fit a pull up if you are planning on removing the chip to stop the base floating.

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12. ### gwfami

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Sep 13, 2012
Thanks Adam, but I'm still quite the noob. Not certain what you mean by "need a pull up from the base to the +V supply" Is that the resistor? I really don't plan on removing the chip once everything is working.

GWFAMI

1,114
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Aug 13, 2011
In Arouse1973's schematic, R1 is the base resistor and R2 is the pull up resistor.

14. ### gwfami

25
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Sep 13, 2012
So I modified the circuit to include the 2 resistors as described. Does this look correct? I didn't put in the LED resistor, as the strip has it's own resistors built in.

I'm also concerned that the LED strip doesn't light up as brightly as expected, perhaps this is due to the transistor I'm using. I've got some TIP127 PNP darlington transistors that could handle more current, would they be more appropriate?

Thanks,

GWFAMI

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Last edited: Feb 7, 2014