Turning LEDs on/off

[rsisk]

When I buy the FET, is there anything specific I need to look for? Probably going to Radio Shack to find one tomorrow. If not available then will probably go to mouser.

[EricVanWyk]

Originally Posted by rsisk

When I buy the FET, is there anything specific I need to look for? Probably going to Radio Shack to find one tomorrow. If not available then will probably go to mouser.

Your application is really light load and low voltage, so most FETs will work fine. Here are the general things you look for:

Type: Your options are "N" or "P" channel. Either can be made to work, but Ether drew the schematic with an N channel. N is generally preferred because they have better electron mobility.

Vds: Maximum voltage from Drain to Source, or how much voltage you can block. You need at least 5V, but I've never seen a normal discrete FET with a Vds rating less than 5V.

Vgs: Maximum voltage from Gate to Source, or how much signal voltage you can apply. Again, most FETs can handle 5V just fine.

Vgs_thresh: Minimum voltage from Gate to Source to guarantee turning on the FET. Most FETs will be in the 1 to 3 V range, so you'll be fine.

Rds: Resistance from Drain to Source while on. Subtract this from the 82 ohm series resistor you calculated. It'll be relatively small, you can likely ignore it.

Ids: Maximum current from Drain to Source while on. It needs to be more than 20mA. Just about any FET that is big enough to be in its own package can handle this.

[rsisk]

OK, just got back from Radio Shack trying to get the right components. The transistors were hard to match up, so I got a couple.

One is a MOSFET N-channel transistor and the other is a bag of NPN-Type transistors. I've included pictures below. My guess is the MOSFET is the closest match and where I am starting to build the circuit.

Am I correct in assuming the following:

Source - The +5V/82ohm/LED side of Ether's diagram
Drain - The ground side of the diagram
Gate - The DIO Signal/100Kohm side of the diagram

Here are the components I purchased, 100K-ohm resistors, MOSFET transistor, and NPN transistors



And the back of the MOSFET transistor and NPN transistor bag

[Ether]

Originally Posted by rsisk

Am I correct in assuming the following:

Source - The +5V/82ohm/LED side of Ether's diagram
Drain - The ground side of the diagram
Gate - The DIO Signal/100Kohm side of the diagram

No. For an N-channel MOSFET, the source is connected to ground, and the drain is connected to the load, which is connected to the power.

The gate is connected to the signal, as you said.

So in the sketch I posted earlier, the source is on the bottom of the FET, and the drain is at the top. See annotated sketch attached.

[rsisk]

Thanks for that timely information, just starting to put this thing together.

[Ether]

Originally Posted by rsisk

Here are the components I purchased, 100K-ohm resistors, MOSFET transistor, and NPN transistors



And the back of the MOSFET transistor and NPN transistor bag

Maybe it's just my connection, but my DNS can't resolve rsisk.com:

[EricVanWyk]

It resolves for me, that FET will work fine. It is a beefy fellow.

An easy way to figure out which way to orient a FET is to look at the arrow in its schematic symbol, as it (sort of) represents the FET's bonus parallel diode. You want this diode to oppose normal current flow, or else it will always allow it and you won't be able to turn the flow off.

In your symbol it points from the source, so you want normal current flow to go the other direction (from the drain).

Sometimes they draw the diode more explicitly, I've seen it several ways.

[Ether]

Originally Posted by EricVanWyk

It resolves for me...

I just ran tracert from a web server and it found it:

216.86.200.206

I'll try that...

edit: got it, using http://216.86.200.206/electronics1s.jpg weird.

[Ether]

Originally Posted by rsisk

http://www.rsisk.com/npn.jpg

Rick, just a heads-up: be careful with the part number 276-1617 package of 15 NPN transistors.

According the the reviews here (scroll down), the package may contain PNPs instead of NPNs. And if they are NPNs, the pin labeling on the package may be wrong. Check the datasheets.

... and let us know when you get that MOSFET working.