Small Wireless Contact Switch

deanp · Post by **deanp** » Thu Jul 27, 2006 2:57 pm

I'm hoping someone can help me solve what might be a challenging electronics problem.

I need a short range (3ft), wireless, battery-powered contact switch in as small a form factor as possible. What I'm looking for (ideally) is some kind of transmitter I can wear inconspicuously on my ring finger that I could press with my thumb transmit a signal to a receiver on my belt that would generate a contact closure. The receiver on my belt should be small too, but could be as large as a pager or cellphone.

Off the shelf would be preferable. Several channels would be nice too, but a one bit signal would be sufficient. Non line of sight would be preferably, but isn't entirely required.

Any suggestions?

Thanks!

--Dean

philba · Post by **philba** » Thu Jul 27, 2006 3:03 pm

check here: http://www.rentron.com/ lots of stuff. many of which can do exactly what you ask with a small amount of effort.

Bob Scott · Post by **Bob Scott** » Thu Jul 27, 2006 3:58 pm

Chilling video offers
step-by-step suicide vest instructions

http://www.msnbc.msn.com/id/6746756/

Bob

HighFrequency · Post by **HighFrequency** » Thu Jul 27, 2006 4:51 pm

Yeah, your request is borderline suspicious. Though it's likely some benign "gag" pproject, it sounds like it could be more.

What is your project?

jwax · Post by **jwax** » Thu Jul 27, 2006 5:09 pm

I doubt we have a request for initiating a suicide bomb here.
It depends on your circuit design aptitude. You could design a simple single transistor oscillator at 100KHz or so, and the receiver would have a 567 chip- a PLL tone detector that will activate only when 100 KHz is sensed.
We are curious about the application, however!

And welcome!

deanp · Post by **deanp** » Thu Jul 27, 2006 5:38 pm

Thanks everyone for the warm welcome!

I certainly don't have any nefarious intent in mind with my application.

I'm into magic, and I'd like to be able to trigger a remote event (e.g. a sound generator at the back of the room) with the most subtle of hand cues.

I'm not at all adept at circuit design, but I have a PhD in computer science, so there is a (remote) chance I could build a simple electronics kit if the parts list and instructions were pretty complete.

.

Thanks!

--Dean

jwax · Post by **jwax** » Thu Jul 27, 2006 7:19 pm

Hmm. Confusion now. You want a ring-finger device to activate contact closure on a belt-worn device, to set off some event across the room?
Need some clarification here, Doc!

stevech · Post by **stevech** » Thu Jul 27, 2006 7:30 pm

a year or so ago, I bought at Target some of those $8 teeny radio controlled cars. 3 different brands.
I extracted the receiver from the car and analyzed it.
Some are 27MHz; others are higher freq. All are size of postage stamp.
All had a large capacitor to store a charge to run the drive and steering.
The boards have two transistors or some such to do forward/reverse on the drive and (most) left vs. center on steering.
Radio runs on 1.75 V or so.

The handheld remote's transmitter, when extracted from its case had push buttons for each action.

So I fiddled. Put 6 ft. antenna on the 27MHz receiver. Hooked it to a battery. Hooked an LED where the motor would go. Longer antenna on transmitter.
Got 50 ft or so line of sight.

Read websites about these radios. People talk about mods to undo the range-restricter that the manufacturers added. This was to deal with interference where there are too many cars on the same channel too close. Just snip out a resistor as I recall.

Anyway, these radios are dirt cheap and give you 4 or so on/off bits with 10s of ft. range.

My application was a transmitter from a motorcycle to a rider's helmet. Headlight on, then helmet-mounted LED tail light comes on automatically. Build a few for folks. Receiver draws very little current.

If you are cheap like me.

I see these for $5 or less now and then.

Bob Scott · Post by **Bob Scott** » Thu Jul 27, 2006 7:51 pm

"stevech" old buddy!

I see you live in the same place as "deanp"! They work when you are on motorcycles too? If you click a switch on your hand they could activate something in the Mercedes next to you?

Rgards,
Bob

deanp · Post by **deanp** » Thu Jul 27, 2006 7:52 pm

First, if I can go directly from a small transmitter I can trigger from my finger to the other side of the room (25ft away) that would be great. I just figured it would be more tractable (lower power, smaller components) from my finger to my belt. I know I can easily get a signal from my belt across the room. Sorry for the confusion.

Funny you should mention those little radio controlled cars. I actually had the same idea yesterday, and ordered three of what are probably similar ones to those you describe. Sounds like the seller has a bunch if anyone else is interested. Below is the link to the ebay auction. I have no affiliation with this seller, and in fact have not yet received the merchandise, since I bought it only yesterday:

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 6066533959

I was hoping once I take the shell of the pen off, the transmitter will be small enough.

The receiver (in the tiny 1.25in car) will certainly be small enough. The range sounds like it won't be a problem either. But I'm still a bit doubtful the transmitter will be small enough to be inconspicuous on my hand.

--Dean

philba · Post by **philba** » Thu Jul 27, 2006 8:11 pm

do you wear gloves? you could do a lot with that.

another possibility is to sew the switch into your jacket/shirt. with a little delay on the start of the noise, you can avoid the obvious connection to the motion.

stevech · Post by **stevech** » Thu Jul 27, 2006 9:58 pm

These are crude radios so don't wire the receiver to anything mission critical!

Some photos of the receivers and transmitters from my archives...

I just cut off the wires to the car's motors and power capacitor. To the left of the black blob (the all in one IC), note the vertical row of transitors (the black rectangles). These are the motor drivers. The 1st photo is largest

Transmitter PCB was from a palm-sized controller...

stevech · Post by **stevech** » Thu Jul 27, 2006 10:41 pm

this is a sub-page of a website with lots of info.
On this page they talk about the range-cripler.
Applies to certain vintage.

Click to go on up to their top level web page. More radio links there.

http://tinyrc.com/qfm/racing/mod_02.htm#05

jwax · Post by **jwax** » Fri Jul 28, 2006 3:47 am

deanp- That Ebay page describes an infrared-controlled "RC car". It is not on a radio frequency, rather on a line of sight infrared beam.

deanp · Post by **deanp** » Fri Jul 28, 2006 6:34 am

Thanks for the additional information on the tiny RC cars. Yes, they do appear to be radio frequency, and hence none line of sight. I'm looking forward to receiving the ones I ordered so I can play with them.

--Dean

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