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Light LED When Switch Opened

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691175002

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Is it possible to make a battery powered circuit that lights an LED when a switch is opened?

Something that can go for a few months when the switch stays closed?
 

691175002

New Member
Something that draws very little current when the LED is off (Switch closed). It will light only for very brief periods so if it could last a while on a coin cell or AAA battery that would be nice.
 

KeepItSimpleStupid

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If your talking a single LED that draws about 10 mA, i think you can do it with a J176 JFET, a reisistor (about 1 M ohm) and your switch.
http://datasheetcatalog.com/datasheets_pdf/J/1/7/4/J174.shtml I'd use 2 CR2032 coin batteries in series.

The JFET (D) would go to the (-) of the battery. The (+) of the battery would go to the LED (+). LED (-) would go to the (S)

The switch would connect the resistor to the gate and the (+) side of the battery.

Only with this circuit a series resistor would not be required for the LED. Hopefully, I got my polarities right.

I think it would work. The LED would draw ~19 mA when on and I assumed a 1.2 V voltage drop.

Why not just get a Normally Closed switch?
 

691175002

New Member
Thanks for the help, that looks good.

I am trying to build a touch probe for a CNC mill:
http://fadedbits.com/2011/02/touchprobe/

They can only be built in such a way that a touch opens the circuit. Normally you just use it like a switch and plug it into your control board, but I have an automatic toolchanger which means the tool cannot be connected by wires.

This means that I either need to build some kind of slip ring type setup to make electrical contact with the probe when it is in the spindle, or find some other way of getting the signal to the machine.

I was considering an LED on the probe with a phototransistor to detect when a touch is made.

Here is an example of a commercial system:
http://www.renishaw.com/en/omp40-2-compact-touch-probe--7992
It uses an AA battery and a bunch of IR LEDs to transmit to the receiver.

Some kind of slip ring might be easier in the end though.
 
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KeepItSimpleStupid

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Interesting.

Could yo incorporate a small tactile switch into the design that would totally turn off the device when it was in the tool holder?

Somebody makes a very nice supercap (Linear tech http://www.electro-tech-online.com/...urnal-V21N4-03-df-LTC4425-GeorgeBarbehenn.pdf) charging circuit that uses two super caps. Could you use PC board probes (look up pogo probes) to connect with the gizmo while it's in the tool holder. What can then happen is the tool would charge the caps while in the base.

Here is a possible spring loaded spring probe: http://www.alliedelec.com/search/productdetail.aspx?SKU=70009131

Ideally, before using a battery powered tool, you would need a battery check button. Another option is a rechargeable LiPo battery.

If the orientation of the sensor and the detector on the quill would stay the same, then the IR approach should work.

In the case of the LiPo battery, you could keep the IR LED on all of the time. For the cap case, the on when off makes sense.

To add a little wierdness (more complicated), you could use a processor such as an ARM or PICAXE. Some of the ARM processors can basically shut down totally, but allow certain input pins to be used for an interrupt. I'm not sure where the idea fits, but sometimes it's worth throwing it out.

Bear with me. These ideas are just stubs for the basis of a design.

As for you slip-ring idea. I used pogo type probes as sliding contacts on a PC board tracks for a model gantry crane that I did for somebody.

An interesting problem.

I'm kinda picturing an IR remote control being carried by the tool. Not exactly, but you get the idea.
 

691175002

New Member
I'm kinda picturing an IR remote control being carried by the tool. Not exactly, but you get the idea.
Now that you point it out, it seems like the perfect way to describe the problem.

It is possible to charge the probe it in the tool holder. A super-cap would be cool.

The probe will rotate in the spindle so it will need 360 coverage with the IR leds.


The supercap idea would make things fairly straightforward since power consumption is no longer an issue, I'll probably go with that.

I have some spare parts from a laser tag project a while back. I'll just have to see if I can get enough IR out of the probe once I build it.

In the commercial systems I have seen, the detectors are a meter or two away from the probe to stay clean.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I have another wierd idea. How about creating a magnetic field. Bassically a torroid around the tool and use a magnetic position type sensor such as this: http://www.melexis.com/mlx90333

It would only work if you can get a sensor close to the rotating part.

It would be kinda neat if the gizmo could "test" the "switch" before using it. Something like 2-way Bluetooth communication

You could still possibly use the IR remote type detectors and receivers for the project. Turning on a modulated carrier might be better than just detecting light.
 

dr peppers

New Member
I used to maintain CNC millers for cross's, they used commercial type probes, communication was done back to the terminal via radio.
You can get very small am 433mhz transmitter/receiver modules designed to transmit serial data, if you tied the data input of the transmitter high then power ing up the transmitter will make the receivers output go high when its on, and low when the transmitter is off, the modules I used from maplin electronics did this perfectly well for a security project.
This might be a better solution for your needs, you could connect the transmitter across the led and use the fet as mentioned further up.
 
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691175002

New Member
You can get very small am 433mhz transmitter/receiver modules designed to transmit serial data, if you tied the data input of the transmitter high then power ing up the transmitter will make the receivers output go high when its on, and low when the transmitter is off, the modules I used from maplin electronics did this perfectly well for a security project.
Looks good, this should make the project very easy:
http://www.sparkfun.com/products/10534

If I power it with supercaps it should be maintenance free and fairly reliable.

Thanks for all the great help so far.
 
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