[sdiy] Body capacitance problem?

[Dave Kendall]

Thanks Paul - it worked first time :-)

cheers,
Dave


On Mar 6, 2009, at 22:08, Paul Perry wrote:

> yes, I think so.
> Maybe a 1 meg to ground on the trig input would fix it. (or 1 meg to  
> +, depnding which way around it is)
>
> best, paul perry
> ----- Original Message ----- From: "Dave Kendall"  
> <davekendall at ntlworld.com>
> To: "synth DIY" <synth-diy at dropmix.xs4all.nl>
> Sent: Saturday, March 07, 2009 7:40 AM
> Subject: [sdiy] Body capacitance problem?
>
>
>> 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.
>>
>> Is this body capacitance?
>>
>> I hope to eventually have a very short trace connecting to the trigger
>> input (rather than a long wire), but am slightly concerned that this
>> might still be a problem.
>>
>> Anyone come across this before?
>>
>> Any advice gratefully received. :-)
>>
>> cheers,
>> Dave
>>
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>
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