Proximity switch

[zachtheterrible]

since its gonna b a wile till i get my LEDs to make my LED flashlight, and im bored, i figured id start on a new simple project. i want to make this proximity switch. two questions:

1. can i use IRF510 instead of IRF511? funny thing, i got mine from radioshack, and on the package it is labeled IFR510 :lol:

2. what can i use for a 100M resistor???

thanx :lol:

 

[TheOne]

Here is a similar circuit. Mains induced "hum" from a finger will output as square waves from the 4093 gate and trigger a re-triggerable monostable to give a output as shown. Duration that the output will stay high is determined by the RC constant (R6,C1) on the reset pin. Sensitivity depends on plate size.

 

[zachtheterrible]

hi theone, forgot that i posted this post :lol:

i would rather make my circuit because 1. its simpler and 2. i have most of the parts alredy.

one question tho, i alredy firgured the others out, how would i connect just a regular 100k resistor instead of the POT? i dont have any POTS and want to assemble the circuit on my breadboard real soon. thanx :lol:

 

[TheOne]

You could join 5 x 22k resistors in series for experimenting, and find a suitable tap point with a small crocodile clip.

 

[zachtheterrible]

?? i dont know wut u mean by finding a suitable tap point.

 

[TheOne]

If you want better resolution use more lower value resistors to make up the 100k

 

[zachtheterrible]

OHHHHHH, DUH!!!

i was thinking u wer talking about 22M resistors to make the 100M resistor :roll: . i didnt realize u sed 22k, not 22M

stupid stupid stupid stupid :x

 

[zachtheterrible]

well, i made the circuit on my breadboard and terrible's law proved true once again: it never works the first time. my buzzer just stays on no matter wut.

i dont really understand how this circuit works. if you could give me an explanation on how it works, i think that i could probably be able to fix it myself, thnax :lol:

 

[TheOne]

It is a very primitive circuit, so I am not sure how well it will work. If the buzzer stays on then the gate must stay charged when you remove the AC "hum" induced current from a nearby hand or finger. What worries me is that there will always be some current coming through the 100M from the +ve supply to charge the gate, so what discharges the gate once the induction stops?

You can short the gate-source to see if it will switch off. I don't know what the 39pF cap is doing (perhaps to prevent the device from oscillating) :?

 

[TheOne]

Zach,

I wondered if the circuit was copied correctly by the article? Does not make sense.

You can try my modification, it sort of worked on my simulator. The cap seems to make the buzzer sound in a pulsating way when a small AC is present on the gate but who knows. Also I think it will work better with 0V tied to physical ground (cold water pipe)

BTW their circuit stayed on!

 

[gerty]

Zach, what are you using for a buzzer? If it's a piezo, they will operate on a few mils . You could have enough leakage current thru your switch to cause it to work..If it is a piezo, try biasing the neg side of it to the positive supply with a 4.7k resistor and see if it'll shut off..

 

[TheOne]

Here is another version I feel will switch more "positively". There is NO mistake in the circuit :lol:

 

[zachtheterrible]

um . . . i dont know if this is a mistake, but where does the emitter of the 2n3904 supposed to go? :lol:

actually, i guess i shouldve mentioned this, but im using an LED in place of the buzzer, just so i dont have to listen to an annoying beeeeeeeeeeeep. i gues it probly doesnt make any difference?

i tried shorting the gate source and it did make the LED shut off.

 

[TheOne]

um . . . i dont know if this is a mistake, but where does the emitter of the 2n3904 supposed to go? :lol:

Nowhere

 

[zachtheterrible]

:lol: :lol: :lol: :lol: :lol: :lol:
so its pretty much just a diode?

wen i get this circuit working im going to need an explanation of how this thing works :lol:

 

[TheOne]

Not just a diode, a VERY special (Santa) diode! And I mean that in more ways than one.

 

[zachtheterrible]

hi theone. i assembled ur circuit and nuttin! the only way i can get the buzzer/LED to turn on is to short the gate and the drain.

i am using a 9v batery, is that ok? could my FET be fried? ive had it 4 a long time, long enuf before i knew about grounding myself before touching it.

 

[TheOne]

Like I said, I only simulated it. I will dig out a FET and see how close the simulation was. It is a bit difficult simulating a hand or finger inducing 60Hz "hum" What happens when you touch the sensor plate?

 

[zachtheterrible]

Not a thing

would i be alright using a walwart supply? because i know that that produces a 60hz hum, which would probably cause radiation of energy off the circuit . . .

 

[TheOne]

How did you make up the 100Meg resistor?