minimum current/voltage for diode?

Question

Been working on an idea for the Bike project. (a detector that will open or close a relay if the peddler quits peddling.

I tried placing a strong magnet on the rim of the bike and can get a multi-meter (analog) to make a very small jump as the magnet passes by a coil I've got positioned close to the rim. Plan is to use the small amount of generated current to charge a capacitor that would then have enough current to pull down a relay (or transistor switch). A resister would be in place drawing current away from the capacitor so that the rider had to keep charging the capacitor faster then the resister was discharging it or it will complete/disconnect a circuit alerting the rider that he has quit peddling. Kind of a nag alarm really, (gosh, I hate to get nagged, but Hey!, I'm not the one who is going to have to ride this exercise bike)

My thoughts are that this would produce an A/C pulse so naturally I grabbed a diode and put it inline with the coil to convert it to pulsed DC. For some reason I get no signal at all now that a diode is in-line. Makes no difference which way the diode is oriented or what direction the wheel is turning.

Is the signal so low from the coil that the diode can't handle it? I don't know what sort of diode I was using, I just pulled a couple of different ones from a old board and gave it a try.

Do I need a special diode? Do I need a full wave rectifier for this to work?

Thanks in advance, Lawrence

ehsjr · Accepted Answer

It's not AC. The voltage is likely too low for the diodeto conduct.  Use a reed switch instead of a coil:               Reed              SwitchBattery+ ---[SWITCH]------+----+--->TO YOUR CIRCUIT                           |    |                         [CAP] [R1]                           |    |Battery- -----------------+----+Each time the magnet goes by, the reed switch closes,and the capacitor charges to the battery voltage.Here's a complete circuit if you need it:             +-------------[RELAY]------------+             |                                |             |                                c             |                               /Battery+ ---+---[SWITCH]---+----+---[R2]---b   NPN                            |    |   470     \                          [CAP] [R1]          e                            |    |            |Battery- ------------------+----+------------+R2 limits the current to the base of the transistor. Thevalues of the CAP and R1 will depend primarily on...

mike · Answer

What great lengths we go to because we have no discipline.If I quit pedaling, I know that I quit pedaling.  I don't need no damn nag to tell me that. If I lack the discipline to achieve my goals, I'mgonna need a lot more than a nag.Since it's a stationary bike, use a reed switch and a magnet or a led and detector shining thru a hole.  Build an oscillator with frequencyvariable around the desired wheel rpm. Use a frequency detector to sensewhen the wheel is turning slower than the set frequency.When it's lower, use a car spark coil to apply 40KV pulses to the seat.PUt a towel under the bike, cause there's gonna be a wet spot.OR, we could just have a little discipline!!!mike-- Return address is VALID but some sites block emailswith links.  Delete this sig when replying..Wanted, PCMCIA SCSI Card for HP m820 CDRW.FS 500MHz Tek DSOscilloscope TDS540 Make OfferWanted 12" LCD for Compaq Armada 7770MT.Bunch of stuff For Sale and Wanted at the link below.MAKE THE OBVIOUS CHANGES TO THE LINK

kell · Answer

Is this a consumer product you are designing that will go into production, or a one-off hobbyist project? If the former realize that a reed switch after many thousands of cycles may get magnetized and stick closed. Consider using a hall effect sensor.

John Fields · Answer

---And what would be the mechanism which would cause that to happen,exactly?-- John FieldsProfessional Circuit Designer

ehsjr · Answer

It's for kids on an exrecise bike, as I understand it. But it (mag/reed switch) is a common commercial design. Supposedly, they are good for 10^6 to 10^9 operations:

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As a side note, I have over 700 miles on my bicycle odometer which works on that principle - reed sw/magnet. The wheel is

26" - that's over 50 thousand operations, and still working. So I have to ride another ~ 13,300 miles to achieve the bottom end of reed switch failure life. I get tired just thinking about it! :-)

Ed

minimum current/voltage for diode?

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