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Thread: Electronics project help needed!

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    ALT0153™ Rob_B's Avatar
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    Electronics project help needed!

    I've got an idea for a little project and need some help on the electronics side of things, I've not done it since school so even something like this has stumped me!

    I'm after small setups that, when within ~ 50cm of each other will light an LED…simple eh?!

    I'm thinking they should be as small as possible & run off large watch batteries (if possible)

    I guess I need some sort of range detecting thingy, I imagine the rest would be easy after that (optimism or what!?)

    So can anyone offer advice or have any suggestions for where to buy the parts from? I'm obviously aiming for as cheap as possible!

    Thanks guys!

    Rob

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    Moderator DavidM's Avatar
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    Re: Electronics project help needed!

    In the past I'd have recommended Tandy

    Maplins is an obvious one ... think they used to have some basic boards with i/o options on - CPC catalogue used to have some good parts.. and is Farnell still going?

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    ALT0153™ Rob_B's Avatar
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    Re: Electronics project help needed!

    No idea it's something I've never been into.

    Just realised there is a maplin up here, might have to give it a try as thy're open until 8PM
    Still doesn't mean I know what I'm looking for though!

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    Moderator DavidM's Avatar
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    Re: Electronics project help needed!

    Been a long time since I did anything like that - but I still had the electronic timer circuit I made at school up to only a few years ago (and yes, that was a few years old )

    Google brings up this btw...

    http://www.maplin.co.uk/Module.aspx?ModuleNo=219998

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    ALT0153™ Rob_B's Avatar
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    Re: Electronics project help needed!

    I'd not seen that, googled for 'range' and maplin came up with a parking sensor, not exactly what I'm after though.
    Not sure if two ranged devices that can interact with each other (even if only to light an LED) will be expensive though :-/

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    Re: Electronics project help needed!

    Some kind of low powered radio transmitter and detector?

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    Senior Member oolon's Avatar
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    Re: Electronics project help needed!

    This is easy, you need a battery, LED and resistor. Depending on if you connect the LEDs in parallel or series, will decided the Watt rating of the resistor. So lets say the battery is 1.5V and the LED need .2A. LEDs are strange and need a voltage drop rather than being resistive. They are normally drop .65 to .7V

    V=IR
    1.5-.7 = .2 * R

    That means you need a 4 Ohm resisitor rated at .2 Watts.

    2 leds in series.

    1.5- (.7*2) = 0.2 R

    Means you need a .5 Ohm resistor. rated at 0.2 A

    2 in parallel.

    1.5 - .7 = .2*2 R

    Means you need a 2 Ohm resistor rated at .4 A

    Resistor rating are normally in watts so don't forget

    W=VA

    I LED W = .8*.2 = .16W
    2 LED series = .1*.4 = .04W
    2 LED parallel = .8*.4 = .32W

    You will notice that 2 LED is series "wastes" the least amount of power. However a .1V drop is getting a little on the tight side.

    Hear is a web site to do the calcs.

    Your need the specs for your LEDs and correct.
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    Re: Electronics project help needed!

    Quote Originally Posted by Blaineoliver View Post
    Some kind of low powered radio transmitter and detector?
    I'd need it both ways though, I'd assume that would increase the size to a point where it might be too big as well as expense ?
    I've got a kids alarm wrist band thing at home we never used with our little un, you can set the distance before it goes off, might have to take that apart to see what makes it tick!

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    Senior Member oolon's Avatar
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    Re: Electronics project help needed!

    Your choice of device depends on what you need it for, if you want it trigged within a range then your need someone radio. If you want to check for a barrier being broken you need something light related or a mechanical switch. The electronics gets more complex if you want to make it more difficult to fool, A light and a photo cell would do, A photo cell is a light sensitive variable resisitor. However as soon as the light goes away it will go back to the safe setting so you need some electronics to trigger then keep on working until reset. Adapting something you already have is great idea, you could always use the voltage from the speaker to trigger another circuit for the "alarm".
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    Re: Electronics project help needed!

    Yeah I'm wanting it range activated, I'm imagining it polling every second or two, if it finds the other device in range then it light the LED (this works both ways of course)

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    Re: Electronics project help needed!

    Probably illegal however you could make a crystal radio, that listens on a frequency and get a transmitter to blast noise on that frequency. When the noise goes over a level the device is "triggered", which an be done by a transistor.
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    Re: Electronics project help needed!

    What exactly is it you are trying to achieve?
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    Re: Electronics project help needed!

    The "computer" way would be to get a blue tooth device and scan for when it is in range. Bluetooth is a nice adhoc network.
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    Re: Electronics project help needed!

    Quote Originally Posted by peterb View Post
    What exactly is it you are trying to achieve?
    I intend to put them into clothes or items with a design of some sort in front, the electronics would sit inside a pouch & when the two things get within range a logo or design would light up on both of them. I've seen kids toys that do it but only one way.

    Hence why they need to be fairly small/lightweight & not too complicated.
    Last edited by Rob_B; 10-08-2010 at 11:37 AM.

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    Re: Electronics project help needed!

    try rapid electronics for most of the items you will need, havent used them in a long time and i got school discount for my A levels so id say check the prices between maplins and RE and get the cheaper one. Maplins do an IR reciever and transmitter, i had to setup a trip wire security system but i couldnt help you now .
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    Re: Electronics project help needed!

    One way to do it is with a short range radio transceiver "short range device (SRD)" .

    Transmit a constant frequency sinewave at 2K Hz for example, the receiving part of the transceiver has a notch filter (only lets a narrow frequency through) on its output tuned to the frequency you send. You would use the output from the notch filter to drive a transistor to power your LED etc.

    You might want a schmitt trigger circuit before the transistor to give a clean switching point. without you would get the LED turning on gradually.

    The devices would need to be exactly the same and calibrated to work at the same levels which would not be that easy but possible I would think.

    I have no idea what transmitters you are allowed to use license free in the UK but I think you can get something. The power consumption would not be good for a small battery though.


    Its a lot of trouble to build and design just for a one off.

    I used this technique of sending an ac signal to a notch filter for an electronic lock system so that a straight voltage applied did not let the lock open. It makes your device selective rather than sensitive to everything.

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