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Author Topic: web cams  (Read 5024 times)

BritBrat

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web cams
« on: November 05, 2010, 04:24:15 PM »

I am working on doing my own bird cams and maybe web cam later.

Been making a test nest box today but will be making a viewing feeder box next that will hold a colour camera then another nest box with a colour/B&W camera and infared led's.

Any electronic/electrical guys here?

Say I have an LED that uses a 150 ohm resistor with a 12 volt supply, if I reduce the voltage to 9v what resitor value would I need for the same LED?

These are the LED specs:
http://www.maplin.co.uk/media/pdfs/yh70m.pdf

And the 3 Watt Wirewound Resistor:
http://www.maplin.co.uk/Module.aspx?ModuleNo=2173

I think I worked it out at 100 ohm
« Last Edit: November 05, 2010, 04:36:08 PM by BritBrat »
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silversurfer44

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web cams
« Reply #1 on: November 05, 2010, 04:52:48 PM »

If I remember my ohms law you would need a 112.5 ohm resistor to maintain the same current.
Plus or minus 25% as near as damn it 150 ohms tops.
« Last Edit: November 05, 2010, 05:00:22 PM by silversurfer44 »
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tonyappuk

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web cams
« Reply #2 on: November 05, 2010, 05:14:49 PM »

100 ohm is a preferred value but 1 watt would be enough. No harm in using a higher wattage one if you've got room - it might run cooler.
Tony
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geep

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web cams
« Reply #3 on: November 05, 2010, 05:26:08 PM »

I reckon 108.3 ohms.
Assume that the diode forward voltage is 1.2v.
Then there's 10.8 V across the 150 R resistor, which gives current of .072 A.

To push the same .072 A via the 9v supply:
We have 7.8 V across a resistor and a current of .072 A,
so R = V/I gives us 7.8/.072 = 108.3

100 ohm should give .078 A.
As it's rated @ 100mA max @ 25 Deg C, 100 ohm should be fine.
Diode power dissipation is then 93.6 mW (1.2V * 78 mA) which is below the 100 mW rated maximum @ 25 C.
The resistor should be dissipating 608 mW (7.8 V * 78 mA).

Cheers,
Peter



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BritBrat

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Re: web cams
« Reply #4 on: November 05, 2010, 10:00:07 PM »

Thanks for the detailed reply just what I needed.

Next question:

If I had two in series what resitor would I use then?

200 ohm or 50?
« Last Edit: November 05, 2010, 10:03:56 PM by BritBrat »
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geep

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Re: web cams
« Reply #5 on: November 07, 2010, 10:20:12 PM »

For 2 diodes in series the volts drop will be 2.4 V.
With a 9 V battery and a current of .078 A the resistor will drop 6.6 volts.
So R = V/i = 6.6/.078 = 85 ohms.

I would go for 91 ohms.
This will give a current of 73 mA, and power of 87 mW (per diode)
or 82 ohms with 81 mA  / 97 mW (getting close to max rating - take care that 9v supply isn't exceeded).

I you go for 50 ohms - nearest preferred is 51 (5%) - you'll blow up your diodes.
51 ohms will try to push 129 mA through your 100 mA max diode.
Also power dissipation too high - 155mW per diode, but 100 mW max.

Cheers,
Peter
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BritBrat

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Re: web cams
« Reply #6 on: November 08, 2010, 07:21:11 AM »

Thanks for the reply. I was way out that time.

What would be the effect if I kept to the 100 ohm and had two in series?

Less brightness but more spread on the light?

Or would they just not work at all, not enough current.
« Last Edit: November 08, 2010, 07:24:24 AM by BritBrat »
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silversurfer44

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Re: web cams
« Reply #7 on: November 08, 2010, 07:33:26 AM »

In parallel would be a better way to go, that way you would supply the correct amount of volts/mA to each diode.
I have forgotten the formula for calculating series resistance but I'm sure geep will come back on that.
« Last Edit: November 08, 2010, 07:35:51 AM by silversurfer44 »
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geep

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Re: web cams
« Reply #8 on: November 08, 2010, 09:13:55 AM »

Not a good idea to put diodes in parallel.
It's not possible to make identical diodes, so one of them will take more current than the other.
Then you could start to get into a thermal runaway situation.
As it gets warmer, its "resistance" decreases, so it takes more current, so its "resistance" decreases, so... until pop!
If you must put them in parallel, each diode needs its own small series resistor to swamp this effect,
which rather negates putting them in parallel to start with.

Edit - added this: with 2 diodes in series, 100 ohms, 9v supply:
Current: 66 mA, Power:  436 mW in resistor, 79 mW in each diode, being 158 mW total
With 1 diode, 100 ohms, 9v supply:
Current: 78 mA, Power:  608 mW in resistor, 93.6 mW in diode, being 93.6 mW total
And before you ask  :) - with 3 diodes, 100 ohms, 9v supply:
Current: 54 mA, Power:  292 mW in resistor, 64.8 mW in each diode, being 194.4 mW total
with 4 diodes, 100 ohms, 9v supply:
Current: 42 mA, Power:  176 mW in resistor, 50.4 mW in each diode, being 201.6 mW total

So I would expect to get more light out from 2 diodes.
Assuming it's proportional to the power:
2 diodes 158.4/93.6 =  1.7 brighter,
3 diodes 194.4/93.6 = 2.1 x brighter
4 diodes 201.6/93.6 = 2.2 x brighter

The datasheet doesn't give any curves for performance.

Cheers,
Peter

PS - For £19.99 you can get the works, with sound too - camera + LEDs - http://www.spycameracctv.com/spycamera/birdbox-nestbox-day-night-hidden-camera



« Last Edit: November 08, 2010, 09:46:08 AM by geep »
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BritBrat

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Re: web cams
« Reply #9 on: November 08, 2010, 10:12:42 AM »

Wow, you're good.

The reason I was asking about extra LED's was to have some flexibilty on lighting if a camera did not provide enough, sort of forward thinking :)

I am leaning towards a hardrive CCTV recorder with network connections to router then I can either stream the video or have it avialable over LAN and aslo be able to record with motion detection.

That seems the best route at the moment as I did look at a NAS attached device but I would then have to use wirless or USB cameras or get convertors and that seems to be more trouble than it is worth.

But I am still flexible in decision making.

Just noticed that link you gave is near me:

Open 24 Seven Ltd
Unit 4,
Netham View Industrial Estate
Redfield
Bristol
BS5 9PQ

Now I have seen this:
http://www.spycameracctv.com/spycamera/dvr-with-motion-detection-pinhole-camera
« Last Edit: November 08, 2010, 10:31:01 AM by BritBrat »
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silversurfer44

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Re: web cams
« Reply #10 on: November 08, 2010, 10:33:28 AM »

I might have not made myself clear, but I think we are on the same lines geep. I have included a very rough diagram of what I meant.

[attachment deleted by admin]
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geep

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Re: web cams
« Reply #11 on: November 08, 2010, 11:09:23 AM »

@silversurfer44 - yes - that's OK.
I thought you just meant:
Code: [Select]
    +-->|--+
     |      |
--R--+      +---
     |      |
     +-->|--+

@BritBrat - I did wonder why you want to use 9v.
As you can see you're wasting power in the resistor. If using a PSU, doesn't matter much (apart from global warming :) ).
But if on battery it would be better to use some cells in series and reduce the volts drop across the resistor.
Alkaline or Zinc-carbon cells are nominally 1.5V, and NiCd/NiMh nominally 1.2V.

Cheers,
Peter
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