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| Electronic Projects Design/Ideas/Reviews Are you building an electronic project or want to? Maybe you need some assistance? Come and submit your electronic questions here and let our experienced members find a solution. |
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Thread Tools | Display Modes |
9th February 2008, 08:19 PM
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Experienced Member
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Using a 2N7000 LED indicator, will it not load down an input?
Application is multi use input PIC input / output pin
Supports all PIC pin modes, digital I/O & analog input. I wonder if it will load the pin in analog mode?  |
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9th February 2008, 08:52 PM
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Experienced Member
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I'm not sure what you're asking.
Providing you're only switching the LED at low frequencies there shouldn't b any problems. Just make sure the PIC is powered from a high enough voltage to turn the MOSFET on enough to light the LED. |
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9th February 2008, 09:01 PM
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Super Moderator
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Since input impedance of MOSFET is quite high, it won't load the analog source. But the LED will turn on and off unexpectedly depending on voltage at the PIC's pin.
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"There is no way to peace, peace is the way!" |
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9th February 2008, 09:07 PM
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Experienced Member
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Well I'm trying to put I/O indicators on the Ladybug for the digital I/O modes. I was using a 2N3904 and may return to that.
http://www.blueroomelectronics.com/u...February08.htm |
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9th February 2008, 09:12 PM
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Experienced Member
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The BS170 and 2N7000 Mosfets have the same ratings. Their threshold voltage can be as high as 3V for a current of 1mA.
They are not Logic Level Mosfets so their minimum current will vary a lot with a gate voltage of only 5V.
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Uncle $crooge |
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9th February 2008, 09:32 PM
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Experienced Member
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With a Vgs of 4.5V the 2N7000 will pass 75mA with a drain-source voltage drop of only 400mV so I think it'll power a 10mA LED with no trouble.
http://www.fairchildsemi.com/ds/2N%2F2N7000.pdf |
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9th February 2008, 10:33 PM
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Experienced Member
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Hi Hero,
I am glad that you found more complete spec's for the Mosfet than I did.
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Uncle $crooge |
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10th February 2008, 03:26 AM
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Experienced Member
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One annoying thing with using 2N7000s on pic outputs is that they float when set to input. This is not normally a problem but when developing the product the LEDs will glow during the program download. This becomes more of a problem if your driving something like relays as the FET ends up with around half the supply voltage across it (=hot). For this reason, it is a good idea to temporarily add a pull down on the gates.
Mike. |
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10th February 2008, 07:50 AM
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Experienced Member
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generally use of MOSFET is with a termination so that gate will not be in charged condition. As such bill might try it with say 100K pull down. It may not effect the port pins but will work as termination for the gate.
I remember some CD4xxx gates which resemble like a pair of N-mos and P-mos complimentary pairs.either we could use one off them and it will serve like 2n7000 but far cheaper,perhaps.
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Regards, Sarma. Last edited by mvs sarma; 10th February 2008 at 08:01 AM. |
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10th February 2008, 10:00 AM
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Experienced Member
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Don't PIC pins have enough guts to power LEDs on their own?
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10th February 2008, 10:13 AM
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Experienced Member
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Quote:
you are leading us near solution. Mr Bill has managed Opto out element with just diode of the Opto-isolator and 330E in series. If this could work , perhaps an efficient LED also can be driven around the same 330E or at 470E for say RED color.
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Regards, Sarma. |
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10th February 2008, 03:07 PM
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Experienced Member
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Well I've moved the LED on the Relay section
 The relay I/O section  The Opto I/O section Last edited by blueroomelectronics; 10th February 2008 at 03:29 PM. |
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10th February 2008, 03:39 PM
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Experienced Member
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Sir, why not on the same collector? Anyway reverse diode will protect
the LED can be in normal way and with 330E in series to the Vcc.
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Regards, Sarma. |
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10th February 2008, 03:50 PM
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Experienced Member
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Which diode do you mean?
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10th February 2008, 03:53 PM
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Experienced Member
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Quote:
these can be on the collector of the same transistor parallel to relay. the Back emf diode helps these components even from failure.
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Regards, Sarma. |
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