[sdiy] Body capacitance problem?

[Magnus Danielson]

Dave Manley skrev:
> 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.

You are a big antenna... we do not naturally transmitt a steady 50 Hz 
distorted sine...

Touching high-impedance stuff like that and you should expect it to go 
off. Lowering the impedance by shifting to a lower resistor would 
significantly reduce the impact if you need it to be tolerant.

You can learn a few lessons by poking around. :)

Just don't touch the high voltage stuff.

Cheers,
Magnus