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Automatic Bawlslight v2?

 
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LiesOfXIII
HG Supreme


Joined: 13 Dec 2006
Posts: 62

PostPosted: Sun Jan 14, 2007 2:37 am    Post subject: Automatic Bawlslight v2? Reply with quote


Can anyone tell me if that schematic would work?

I have an old power plug from a caller id, 9v 200ma.
I'm going to have 4 bawls bottles, 4 13000mcd leds per bottle, a single photocell under one bottle, and a little toggle switch. I'm going to fill each one with broken glass to dim them some, gonna put a bottle in each corner of my ceiling as a nice calming blue light Smile

Already ordered the bawls, leds, and a nice new keyboard for me Smile
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BronzeG3
HG Master


Joined: 30 Apr 2006
Posts: 134

PostPosted: Sun Jan 14, 2007 8:56 am    Post subject: Reply with quote

I'm horrible with transistors, so I can't check that for you, but you seem to have a couple of problems. I'm not sure the power requirements of the LEDs, but I'm assuming 3v @ 20 mA, which gives .06 watts each. The power supply is 9v @ 200mA giving 1.8 watts. The maximum LEDs the power supply can power would be 30, (1.8/0.06), and that is without any loss from other resistors or transistors.

Another problem I see is the 100k ohm resistor. Following the current: Start at power, go through LED network, through the transistor, through the 100k resistor, and back to the power supply. Even assuming the LEDs and transistor have 0ohm resistance (which they do not), the current would still have to go through the 100kohm resistor. Using ohm's law V=IR, 9V = I*100kohm, 9/100000 = 0.00009 or 0.09 mA. That is the maximum current allowed by putting that resistor there.

Summary: Too many sets of LEDs, you should lose a few sets. Use much lower value resistor.

If you have any other questions, let me know. Good luck.
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Alan
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Joined: 29 Jan 2006
Posts: 1399
Location: Winnipeg, MB

PostPosted: Sun Jan 14, 2007 12:21 pm    Post subject: Re: Automatic Bawlslight v2? Reply with quote

LiesOfXIII wrote:

Can anyone tell me if that schematic would work?

I have an old power plug from a caller id, 9v 200ma.
I'm going to have 4 bawls bottles, 4 13000mcd leds per bottle, a single photocell under one bottle, and a little toggle switch. I'm going to fill each one with broken glass to dim them some, gonna put a bottle in each corner of my ceiling as a nice calming blue light Smile

Already ordered the bawls, leds, and a nice new keyboard for me Smile


Hi LiesOfXIII,

There are a few issues with this, fist your voltage supply is only 9 volts and you have 16 LED drops in the circuit, not counting the drops across the transistor this would mean that you would need a power supply of at least:
3.2 * 16 = 51.6 Volts (assuming 3.2 Volt drop LEDs)

It is also a bad idea to common LEDs together, assuming these are 30mA LEDs you would think that if the current limiting was adjusted to 60mA for the series parallel circuit all would be fine however you will not get a perfect current divide between all of the parallel LEDs meaning that one could be slightly over driven and one could be slightly under driven. Now if the over driven one burns out (and opens) because of this situation the one beside it will be driven at double its current rating and also burn out very fast. This would also have the effect of turning off all of the other LED in the chain...

In general the operating voltage of an LED should be thought of as an approximate value and they should be placed in a circuit where the value is not needed to be an exact value.

If you want to stick with a 9 Volt source I would go with 16 parallel sets of two LEDs and a resistor in series. Your power supply would be under rated for this though, I would use a 1 Amp or greater supply. As for the transistor A large darlington would do the trick.
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