Using a 2N7000 LED indicator, will it not load down an input?

[blueroomelectronics]

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?
**broken link removed**

 

[Hero999]

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.

 

[kinjalgp]

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.

 

[blueroomelectronics]

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.
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[audioguru]

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.

 

[Hero999]

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.
https://www.electro-tech-online.com/custompdfs/2008/02/2N2F2N7000.pdf

 

[audioguru]

Hi Hero,
I am glad that you found more complete spec's for the Mosfet than I did.

 

[Pommie]

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.

 

[mvs sarma]

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.

 

[Hero999]

Don't PIC pins have enough guts to power LEDs on their own?

 

[mvs sarma]

Don't PIC pins have enough guts to power LEDs on their own?

Wah!
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.

 

[blueroomelectronics]

Well I've moved the LED on the Relay section
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The relay I/O section

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The Opto I/O section

 

[mvs sarma]

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.

 

[blueroomelectronics]

Which diode do you mean?

 

[mvs sarma]

Which diode do you mean?

I meant Rd1 and LEDa4

these can be on the collector of the same transistor parallel to relay. the Back emf diode helps these components even from failure.

 

[Pommie]

So, where exactly did that 2N7000 go?

Mike.

 

[blueroomelectronics]

I pulled the 2N7000 as I was not clear how it would affect the Analog input mode when not using a relay (not shown above). You could alway put a 2N7000 where the 2N3904 is.

I put the LED directly to the I/O pin to hopefully act as a pulldown (when the I/O is an output ie RESET) as well as an indicator in relay mode.

This schematic is one behind the current, I'll update the image ASAP
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It's the whole schematic (and I've added an XBEE)!

 

[blueroomelectronics]

Here's a closeup with the LED back on RLY power, and the 100K pulldown.
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[mvs sarma]

Here's a closeup with the LED back on RLY power, and the 100K pulldown.

Thanks Bill, for considering the suggestion. Perhaps the 100K may not be needed as you are using a bipolar, I feel. You have not used on other pins anyway.

Nice protection of relay contact. A single transient suppressor from the common connection to the return wire of the circuit on the relay contact would do. and you can save a device while still protecting the contact.

 

[blueroomelectronics]

Many of the resistors prefrom double duty. Rb1 is also used in the analog mode and as a pulldown in direct I/O mode.