Electronics project help needed!

[Blaineoliver]

If one causes the other to light up, then you dont need it both ways do you?

[DanceswithUnix]

The only cheap/compact way I can think of doing this would be actually quite a complex design project using a very cheap microcontroller and electromagnet resonant coils for communication.

The circuit would be a wire coil with capacitor to make a resonant circuit, an LED, and a PIC controller to oversee things.

Once a second the PIC controller wakes up, looks for a voltage recovered from the coil, then it would send a signal at the resonant frequency into the coil to transmit to the other side.

A diode+capacitor from the coil would convert anything the coil picks up into a simple voltage which the controller can convert using an analogue to digital converter input. A transistor across the sampling capacitor allows the controller to discharge the capacitor before going to sleep ready for the next reading. Use of a resonant circuit and sleeping the controller for most of the time keeps current consumption low.

Turning on an LED with a controller output is easy to do, some controllers even have special output pins to drive an LED directly.

Sounds like quiet an amusing final year university electronics project

Off the shelf version? Perhaps you can embed a couple of old mobile phones, write a java app that monitors bluetooth connection signal strength and turns the backlight/keyboard LEDs on? Lots of assumptions there, I don't know if you can get that sort of control.

[Rob_B]

Waaaay to technical for me guys I'm afraid!

If one causes the other to light up, then you dont need it both ways do you?

I need them both to light up, I'd need to have a recieve/send on each item.

for example, talking about a kids wrist band thingy I have sitting around somewhere...

You set the receiver to a certain setting which relates to distance from the childs wristband, if they move outside that area an alarm goes off, as soon as it comes back into range the alarm stops. I would need to reverse this (so it goes off within range) then have two parts per item, one 'wristband' and one 'receiver'
They are only cheap things & I've not looked inside but they don't look like there are many internals so hopefully they could be stripped down.

Just went to maplin, not got anything I could see would give me what I was after.
On a related note though I love the ready to build kits for bugs/doorbells/etc - they're pretty funky & about my level

[switchmode]

Quiet interesting to think about.

What about ultrasonics to transmit, like the early tv remote controls.

I would think that should not be to difficult, but not sure of the range or if its need a clear line of sight.

[DanceswithUnix]

Quiet interesting to think about.

What about ultrasonics to transmit, like the early tv remote controls.

I would think that should not be to difficult, but not sure of the range or if its need a clear line of sight.

Range is hard, you need line of sight and you need to time how long the pulse takes to travel. Then you need to know when the other transmitter started transmitting in order to know when to start timing. Then you need to work out if you got a signal from another device or a reflection off a wall from your transmitter.

Magnets are still the easiest way I can think of but still quite an advanced project.

[switchmode]

Range is hard, you need line of sight and you need to time how long the pulse takes to travel. Then you need to know when the other transmitter started transmitting in order to know when to start timing. Then you need to work out if you got a signal from another device or a reflection off a wall from your transmitter.

Magnets are still the easiest way I can think of but still quite an advanced project.

I was just thinking of a simple frequency you could detect when in range to switch your LED on.

Are you thinking of low frequency radio with a tuned circuit ?

[Kumagoro]

Hmm how about just AM transmitters and receivers they are pretty simple, you would just have
them both tuned to their opposites transmitting frequency. Then you could tweak the power of
the transmitter easily.