I've been googling around, putting together a schematic

I forgot to mention the transistors are going to be regular generic 2n2222's (or whatever the common part is)

>>1186333
R1=R2=R3=10kΩ, no need to adjust anything. R4 depends on supply voltage and LED voltage/current.

that sounds fairly consistent with what I'm finding in google. there's a lot of math involved and lengthy forum discussions that I think is overkill for what I'm doing -- I just need to turn on an LED from far away with a magnetic reed switch, nothing too precise here.

I only have 20k resistors as far as anything in that range, I'll just put them in pairs, in parallel for the test version of the circuit then.

thanks, I appreciate your help.

I'm building right now, so I'll post back with results.

>>1186345
You can do this with half the parts... ill post in a sec.

File: pic.jpg (29KB, 1024x768px) Image search: [Google]

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>>1186345

>>1186349
Seconding this. Far too many parts in OP pic.

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>>1186347
>>1186349

You did this while I was working on OP's pic

>>1186356
Sorry for my shit paintbrush trackpad version.

File: magnet led switch thing.jpg (353KB, 800x524px) Image search: [Google]

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it works exactly as needed! --but the true test will be when I put the magnet switch 300 feet out. it would be great if I can get away with powering the whole thing with only 5 volts.

>>1186349
>>1186352
>>1186356
I opted for the second transistor to invert the output -- wouldn't a single transistor version of the circuit as you've shown here have the LED turn on only when the switch is closed? I need the led to be on when the switch is open -- indicating the magnet is not near the switch (the door is open, or the object is not in place, etc...)

I still greatly appreciate the help.

>>1186362
No - look at the circuit. Current through R1 (10K) to the base of Q1 will turn on Q1, thus turning on the LED. When the switch is closed, all current through R1 will go to ground, leaving no current for the base of Q1, turning LED off. An NPN transistor already "inverts".

>>1186365
I'm almost done building the single transistor version right now. with all that I've learned, transistor logic still escapes me. It's frustrating.

I wonder why there were so many instances of the two transistor version then...

I'll post back with results once completed.

>>1186362
While there is nothing wrong with your circuit, it just uses way more stuff than you need to. More stuff = more to go wrong. There is a place for inverters like this but for a simple binary switch it isn't necessary.

It would use more power, but a simpler circuit would be to just power the LED and put the switch across the LED.

>>1186371
I think his mag switches are N.C...

>>1186374
>I think his mag switches are N.C...


Doesn't matter what the normal condition is - if the closed switch is across the led the power is shunted around the LED when the switch is closed.

When the switch opens the power goes through the LED.

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>>1186374

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>>1186374
it's a normally open magnet switch. I've never seen a normally closed one, would love to get some-- it'd make some circuits simpler.

but hey! it works!!
I don't understand it... will have to study what's going on here.

at first, I was like "...nothing's happening", but it turns out I had the switch wired wrong, and actually, everything was happening as it should, because wired wrong, the switch part of the circuit was in the closed position, so the LED would be off anyway. fixed, it works as needed.

this is great guys.

>>1186362
>wouldn't a single transistor version of the circuit as you've shown here have the LED turn on only when the switch is closed?

No. Only on when its open.

>>1186383
No magic smoke is always a good sign. Can't put it back once its out.

File: spdt magnetic reed switch.png (67KB, 628x403px) Image search: [Google]

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>>1186383
>I've never seen a normally closed one, would love to get some-- it'd make some circuits simpler.

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>>1186383
>I've never seen a normally closed one, would love to get some

We used to sell them at work in a plastic housing with three screw terminals.

>>1186389
oh, right. now that I think about it, I might have seen those after all.

>>1186382
that looks like it would work too. but while the switch is closed, that'd just be wasting energy as heat through the resistor..

with the transistor, while the LED is off, wouldn't the current draw be much less?

>>1186356
>>1186349
wait.. what if I was stuck with a higher voltage power supply, say around 12 volts, wouldn't I need a resistor a the base of the transistor to keep it happy? the resistor on the LED would need to be bigger too, but I'm not worried about the LED so much.

>>1186461
>with the transistor, while the LED is off, wouldn't the current draw be much less?
Yes, by around 10 fold. 10K vs 1K

>wouldn't I need a resistor a the base of the transistor to keep it happy?
No, not to ground.

There's kinda 2 separate circuits. They act similar to a relay coil and contacts

1) The base-emitter current circuit (similar to a relay coil)
-The current path is Vcc-10K-base-emitter-ground.
-With the switch closed, the current path is Vcc-10K-ground. The transistor base has no current.
-The base-emitter current switches the other circuit(collector-emitter).

2) The collector-emitter circuit (similar to relay contacts)
-The current path is Vcc-1K-LED-collector-emitter-ground.
-When the base-emitter circuit has current flowing, the collector-emitter circuit is switched on.

This is simplification, but explains basic switching.

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>>1186330

>>1186383
5v reedrelays have an NO and NC connection
and still react to magnets