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Set of circuits for new project - Guidance would be great!

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Newbie level 4
Aug 5, 2008
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I am a product designer by trade an occasionally take on a project with electronic aspects, this project is by far the most complex I've attempted.

In short, I am trying to wirelessly charge (inductive) a flashlight that will contain NiMH batteries, that will power 2 sets of the LEDs. The user of the flashlight will change the "mode" of the sets of LEDs via a magnetic switch (an array of hall effect sensors that control the LEDs). The flashlight, when seated in its inductive charging station needs to send a status of its charge to the base station from where the station will display a red LED for charging and a green LED for charged.

I have designed the hall effect/LED circuit and it would be ideal to power this with 4 1.2V AA NiMH batteries.

It looks like using a MAX712 IC to charge the batteries would be a good pick. The IC has provisions to turn on an LED when it is receiving power and when it has changed to trickle charge. I need to take these "signals" and wirelessly send them back to the charging station to where it will light LEDs and display the charge "status".

An idea on a low cost, easy to grasp wireless technology I could implement? Sparkfun has some real cheap RF trasmitters and receivers but I believe I will need a MCU to read and output these signals to an LED.

ANY input at all would be GREATLY appreciated! Thanks in advance!

Re: Set of circuits for new project - Guidance would be grea

This can easyily be done, Looks like you have all the baiscs covered for the project, By using the MAX712 which uses LED'S what you could do is have a TX&RX unit and 2 12F675 pics to take care of it, Now what you have to take in to consideration is the range of the TX&RX unit and the surrounding of the buildings,

The cheaper TX&RX units may suffer in range, This you would need to carry out the tests to see has most RF modules work upto 250 meters in line of site I.E open space the more noise and metal the less the range you get. There are some RF modules that can go up 10KM but these cost mega bux.

What range do you require ?

Re: Set of circuits for new project - Guidance would be grea

I require a whopping 4 inches of range only. The flashlight only has to transmit its charging status while seated in its charging station, once it is removed it doesn't matter.

I have no experience programming PICs, I have only ever programmed an Atmel AVR Butterfly. Im assuming I can use 2 ATMega8s (or similiar) to do the same thing?

Any chance I will have interference with a 434MHz TX/RX circuit and the inductive charging circuit?

Re: Set of circuits for new project - Guidance would be grea

Sorry idtkid I just think is more poular has I'm the oppoiste never used AVR.
Yes you could do the same thing with AVR, If it's only 4 inches could you just use longer wire ? Unless it's an external LED you need in a other room.

I will have a look at the MAX712 data sheet this should be straight forward. has for the AVR/PIC you can do it a few ways the simplw way is have a pull down resistor on the input pin and watch the port for it to go high then send the signal you could say send a value of 147 to say charging and a value of 198 to say charged, I know how to do it for PIC chips but not AVR. Let me look into data sheet then I can see about doing a drawing for you. but it will use a PIC but just replace with AVR

Re: Set of circuits for new project - Guidance would be grea

The "charging status signal" has to be sent wirelessly, no wires can be used to transmit this signal to the corresponding LEDs, the signal only has to travel about 4 inches max.

Any form of diagram you'd be willing to create for me would be great!

Did you consider, that you have already built a link between the two devices by tranferring energy? Modulating the energy demand somehow seems to me the obvious way to transmit back to the charger without involving much new hardware. You may want to see the operation of passive RFID, that's also based on load modulation as an analogon.

That was my initial thought as well however executing that to make a successful prototype seems to be way beyond my means at this point :/

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