Inductive Pickup Circuit

[thomas walter x5955]

> A good idea!  But my dictated goal was to demonstrate "transformer"
> action to the H.S. kids, and the inductive pickup illustrates how one
> can "magically" induce currents into the pickup.  Life would be muck
> easier otherwise.

> Is the SCR approach the "best" method?

Bruce,

For an EFI system, I would use the tach port.  

Since you want to couple the pulse, as a demo, I would have a little
more fun "playing with what works" type circuit. :-)

I hate ASCII drawings, but:


                         VCC   ------------------
                               |                |
                               _            ----------
                               ^  1N914     |        |
                               |            |        |
Vin ---/\/\/\--------------------/\/\/\-----|        | --->|- LED
       20K     |                |   10K     |        |     |
               |                _           |        |     |
               _                ^ 1N914     |  '14   |     |
               _ 10nfd          |           ----------     |
               |                |                |         |
Gnd ---------------------------------------------------------- GND



A loop of wire, next to the High tension lead should be enough to
pickup any radiated energy.  Lots of voltage, low current.

So a "pulse forming" cicruit can be easily made by using a low
pass filter (20K & 10nfd) then with DIODE protection to Vcc and
Gnd (two diodes, NORMALLY not conducting! Just dumps the excess
energy), another 10K resistor, then into a 74HC14 scmitt trigger
(I think it is '14). Output is now a clean 0V to Vcc pulse.

[Note the IC has two ESD diodes built into it, while these diodes
help shunt the ESD spikes from damaging the input gates of the 
device, the 10K input resistor just limits the energy going
into the device to start with. The external diodes do the same
thing, but may not turn on as quick as the internal ones...
hence the extra resistor].

Fun part about this, is letting the students 'play' with the 
orientation, size, and number of turns, of the loop. What
happens when they make a figure "8" out of the coil? ;-)

For the SCR... if you want to drive a flash tube, then that is
the device to use. If you want to drive just a LED (might need a 
divide by 10, or 100 circuit) the brightness will give an idea
of the rpm involved). Little easier to them to move the pick up
coil and notice the difference. (need to look at a databook,
but with just one output driving the LED, the IC may be around
20ma... enough to drive a LED to brightness, and not using
the normal transistor and R in the traditional LED drive
circuit. 

Just random thoughts.

Cheers,

Tom Walter