# 555 and 9v batt.

Discussion in 'General Electronics Chat' started by adamclark, Nov 30, 2013.

Oct 4, 2013
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hey, I am going to build a 555 oscillator for the star on our Christmas tree with my kids and we are going to use a 9v battery. the LEDs are 3vdc and they pull 500ma of current, What value of resistor would I use to drop the voltage down to 3v for the LEDs but still have enough to power the 555 timer? I forgot to add that its a string of 20 leds paralleled, I plan on cutting them in half so when they are put on the star Ive got,ive got 10 on and 10 off when the 555 is working..

Last edited: Nov 30, 2013
2. ### tracecom AAC Fanatic!

Apr 16, 2010
3,878
1,396
There are a couple of issues. First is that the 555 can only sink and source 200 mA, so there's not enough current available to power 500 mA of LEDs even if they are divided in half. Secondly, LEDs aren't rated by voltage; they are rated by current. If the 20 LEDs are really in parallel, they should have one current limiting resistor for each LED, which is quite inefficient.

The questions that really need to be answered are (1) what is the voltage drop across LED, (2) what is the maximum current rating of each LED, and (3) what voltage are you planning on using to power the 555 and the LEDs.

ETA: I just reread your post and saw the 9V battery. That will not be enough to power the LEDs for more than a very few minutes.

Oct 4, 2013
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ok,, this has to be powered by battery. the LEDs came all connected together from the store. they are actually a string of LED Christmas lights and the battery box that came connected to them holds 2 1.5v aa batteries and they ran for days on the aa's before they died. but I don't think the 555 would run on 3v from the aa batteries. the batt. box on the string has enough room for a little 555 circuit and it has a switch in it too. I was going to cut the divider to make room for the 9v.
So I cant answer number 1 because I don't know how to measure it.
and for number 2. the psu I have them connected to has an amp meter and it can go to 40vdc but when I hook anything to it and turn it up, it will stop at the max voltage the device ive connected needs. so as long as the LEDs are connected to it, I can turn the psu up as high as I want and it just stops at 3v. wont let me give it more. Then I turn on the amp meter and the needle is on 500ma. Im not sure if that answers anything for number 2.
and for number 3. I will use whatever voltage I need to power the 555 and the LEDs. That's why I posted here before I built anything.. I just need to build this tonight on a breadboard so when my kids get back from their mothers tomorrow, we can solder it together on the protoboard and set up the tree.. So I have to use what I have here to build it. but I have a good variety of resistors and caps and I have a couple dozen 555's so Ive gotta make this work with what ive got.. thanks again

Oct 4, 2013
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Ive built a flashing circuit from a npn transistor and capacitor before, but It wont oscillate like I want and it wont work unless I give it 10vdc or more.

5. ### tracecom AAC Fanatic!

Apr 16, 2010
3,878
1,396
Okay, you can use a 555 for your oscillator, power it from a 9V battery, and power the LEDs with AA batteries. The easiest way to do that is with a couple of MOSFETs; do you have any? And if so, what are they?

Oct 4, 2013
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I might. let me dig trough my box of stuff..

Oct 4, 2013
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I wish Bill_Marsden would chime in,, dudes a master of the 555.. he could tell us 10 different ways to do this with a 555..lol..

Oct 4, 2013
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I have one of these. http://pdf1.alldatasheet.com/datasheet-pdf/view/212744/TOSHIBA/2SC4544_07.html.. it says its for high voltage switching and amplifier, and I have a k2996 600v silicon n channel power mosfet http://pdf1.alldatasheet.com/datasheet-pdf/view/172746/UTC/UK2996-TA3-T.html ... I have this one, im not sure if its what I need but heres the datasheet for it.. http://pdf1.alldatasheet.com/datasheet-pdf/view/39006/SANYO/2SC4458.html .. or one of these http://pdf1.alldatasheet.com/datasheet-pdf/view/170798/STMICROELECTRONICS/D882.html ... or one of these... http://pdf.datasheetcatalog.com/datasheets/90/233094_DS.pdf... and IO think that's about it in my box..

9. ### inwo Well-Known Member

Nov 7, 2013
2,435
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As far as driving the led's. Put a few in series.
Will use less current overall.

How about two 9 volts in series? That might run most of them and only draw 25ma each leg.

Oct 4, 2013
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I need this to be simple, my 4 yr old and my 12 yr old are helping build this for the tree.. Ive built simple oscillating circuits with a 555 with just a cap and a couple resistors. Why would they drain a 9v so fast when they run for days now on 2 1.5v aa batteries? I just need to know what resistor I would need to drop the voltage to 5v to run the 555 and what resistor to drop the output to 3v to run the LEDs? then I find out that the 555 wont handle the current (500ma).. now I need a simple solution for that. When I thought this up, it seemed very simple to use the 555 to turn on the LEDs.

Oct 4, 2013
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Could I use a Transistor after the 555 to handle the current?

Oct 4, 2013
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ok,, I think Ive got it.. could I use the output of the 555 to a resistor to the adj. leg of an lm317t and run the 9v to the in leg of the 317t and the resistor between the 555 and the adj. leg would determine the 3v output to the LEDs? it would take 2 317t's and one 555 and 1 cap. with no more than 5 resistors.. This would work right?

13. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
It takes me a while to catch up around here sometimes.

I'm not really sure what you are trying to do. Do you have a preliminary schematic?

You mention 500ma, that is ½A. Having shorted 9V batteries before (new ones) they can only provide around 1A at no voltage. Take a look at my Long Duration 555 series of articles, it can be done pretty simply.

Last edited: Dec 1, 2013

Oct 4, 2013
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I have a store bought string of 20 blue LEDs. It has a battery box for 2 1.5v aa batteries and a switch. I want to put these in my star for our xmas tree. I am going to place them in the star 1 on 1 off and so on so it will be 10 on and 10 off staggered in the star. I want to be able to play with the speed so I can get the effect I want. Ive built the 555 tester circuit found everywhere on the net, where 2 LEDs oscillate to show a 555 is good. I could change the speed by changing either the cap. or the resistor. that's all I need to do, but I want to use a 9v battery in the case instead of 2 aa's. I measured the LED's on my psu and at 3vdc it pulls 500ma exactly. it was said that's to much current for a 555 to handle so I thought that adding 2 317t adjustable regulators to be triggered by the out of the 555 to send 3v to the LED's.. I have no schematic and I want to build it with what I got on a breadboard tonight and let my kids actually assemble it on the pcb tomorrow before we set up our tree..

Oct 4, 2013
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here is the circuit I would like to use. if it wont handle 500ma, what could I do or if I use the 317's where would I place them?

16. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
You don't need LM317, that is nothing but a voltage or current regulator. What you need is some transistor.

Could you show some picture please? It sounds like you want to do a complete rewire of the whole thing. The 3VDC, AAA batteries or what?

Oct 4, 2013
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I just want to cut the string in half so I have 2 sets of 10 LED's in parallel. I want to use a 555 to oscillate and have 10 on and 10 off then switch to 10 off and 10 on. I want to be able to adjust the speed of the oscillation. can I use the 2 aa 3v. batteries to do this? can you provide a simple schematic for me? The one I found doesn't work. Ive wired it 10 times on my breadboard and one LED lights with the switch on or off and the 555 gets hot, so I think it might not be right. ill attach the schematic for you to see.. thanks again

• ###### 555-timer-tester-circuit.jpg
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Oct 4, 2013
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ok,,, so the schematic works, the problem was op..lol

19. ### MrChips Moderator

Oct 2, 2009
12,635
3,453
As Bill says, you need a transistor to deliver the extra current.
Why not use a CMOS TLC555 which can run off 2V.

You can also build an oscillator using two transistors or a single UJT.