[sdiy] Body capacitance problem?

[Dave Manley]

Dave Kendall wrote:
> Hi all.
> 
> Possibly a dumb newbie question, I don't know....
> I'm working on an experimental circuit which uses a dual monostable 
> (CD4538). The R/C combination is 1M and 100nF which gives a measured 
> pulse of just under 100ms.
> There is a flying lead about 2 1/2 inches long connecting a trigger 
> source to the positive going input of the monostable. All unused inputs 
> are terminated as per the datasheets.
> I notice that touching this lead with a finger will cause the pulse to 
> be emitted, when no trigger pulse is being sent to the input. Leaving 
> your finger there produces no further pulses, but removing and touching 
> it again will trigger it again, every time.
> 

The input is high impedance.  When you touch it, you must be supplying 
enough voltage to cause a trigger.  What happens if you ground yourself 
before touching the trigger input?  You could try adding a pull-down 
resistor on the input (perhaps a series resistor as well).

If you have a scope, it is an interesting experiment to touch the end of 
the scope probe with one hand (don't touch the ground), while moving 
your other hand around near various things that emit fields - 
transformers, florescent lights, power cords, etc.

-Dave