Inductive Pickup Circuit

[Steven Buchholz]

Tom Walter <walter at roadster.sps.mot.com> wrote (excerpted):
> I hate ASCII drawings, but:

You did do an admirable job of expressing the circuit though!
> 
> 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. 
> 
It turns out that having the gate drive the LED to ground would
probably result in blowing up the LED and possibly the gate.
If you were using the standard logic family (i.e. SN7414) you
will find that the sink current is limited by the gate.  I have
used this successfully a number of times myself, but if you want 
to be safe, use a resistor to limit the LED current to 20mA assum-
ing about a 3 volt drop (i.e. 150 ohms).  The other benefit to 
this modification is that the LED would flash ON when the signal 
was coupled in, rather than the existing circuit that will turn 
OFF the LED.  It will probably not be possible to even see the 
intensity of the LED change until the pulse rate got quite high.

> 
>                                                   150 (A/R)
>                          VCC   -------------------/\/\/\--
>                                |                |         |
>                                _            ----------    V
>                                ^  1N914     |        |    - LED
>                                |            |        |    |
> Vin ---/\/\/\--------------------/\/\/\-----|        | --- 
>        20K     |                |   10K     |        |     
>                |                _           |        |     
>                _                ^ 1N914     |  '14   |     
>                _ 10nfd          |           ----------     
>                |                |                |         
> Gnd ---------------------------------------------------------- GND

... apologies for drifting ever so slightly away from the subject :)
I don't have the address of the original requestor for this thread.

Steve Buchholz
s_buchho at kla.com
San Jose, CA (USA)