minimum current/voltage for diode?

T

Tristar500

Guest
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
 
Tristar500 wrote:
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
Click to expand...
It's not AC. The voltage is likely too low for the diode
to conduct. Use a reed switch instead of a coil:

Reed
Switch
Battery+ ---[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. The
values of the CAP and R1 will depend primarily on the
speed of the wheel (number of rotations per second), but
also on the gain of the transistor and the resistance of
the relay coil as well as its drop out voltage. Higher
values of R1 and the CAP result in the relay staying
energized longer. You'll need to experiment to find good
values. If you make R1 variable you can require the rider
to go at different minimum speeds to keep the relay
energized.

Ed
 
Tristar500 wrote:
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
Click to expand...
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'm
gonna 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 frequency
variable around the desired wheel rpm. Use a frequency detector to sense
when 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 emails
with links. Delete this sig when replying.
..
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FS 500MHz Tek DSOscilloscope TDS540 Make Offer
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MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
 
ehsjr wrote:
Tristar500 wrote:
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


It's not AC. The voltage is likely too low for the diode
to conduct. Use a reed switch instead of a coil:

Reed
Switch
Battery+ ---[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. The
values of the CAP and R1 will depend primarily on the
speed of the wheel (number of rotations per second), but
also on the gain of the transistor and the resistance of
the relay coil as well as its drop out voltage. Higher
values of R1 and the CAP result in the relay staying
energized longer. You'll need to experiment to find good
values. If you make R1 variable you can require the rider
to go at different minimum speeds to keep the relay
energized.

Ed
Click to expand...
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.
 
kell wrote:
ehsjr wrote:

Tristar500 wrote:

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


It's not AC. The voltage is likely too low for the diode
to conduct. Use a reed switch instead of a coil:

Reed
Switch
Battery+ ---[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. The
values of the CAP and R1 will depend primarily on the
speed of the wheel (number of rotations per second), but
also on the gain of the transistor and the resistance of
the relay coil as well as its drop out voltage. Higher
values of R1 and the CAP result in the relay staying
energized longer. You'll need to experiment to find good
values. If you make R1 variable you can require the rider
to go at different minimum speeds to keep the relay
energized.

Ed


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.
Click to expand...
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:
http://www.madisonco.com/reference/electrical.htm

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
 
On 14 Jul 2005 19:57:11 -0700, "kell" <kellrobinson@billburg.com>
wrote:


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.
Click to expand...
---
And what would be the mechanism which would cause that to happen,
exactly?

--
John Fields
Professional Circuit Designer
 
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