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monostable timer

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You must have a clamp diode across the relay coil.
The leads to the [2ft] switch should be twisted or screened.

Could u explain a bit more pse :confused:
 
You must have a clamp diode across the relay coil.
The leads to the [2ft] switch should be twisted or screened.

Could u explain a bit more pse :confused:


hi,
The relay coil when switched off will generate a 'back emf' [high voltage] which can damage the 555.

The long wires to the switch can act like an aerial picking up external electrical switching noise,
twist the two wires together about 3 twists/inch.

Also add the caps as shown in this dwg.
These caps should always be fitted to a 555 timer [ or other logic]
they are to suppress electrical noise on the +V/0V lines.

When fitted they MAY fix your switch ON problem.:) Lets know
 

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tried the caps, same prob:confused:

hi,
This should do it.

Without the R/C on pin4, the mono triggers every power up.

With RESET R/C its OK, only triggers with uSw.

I have use 100K and 100nf, if required the cap [can be 220nF] and it never triggers on power up.

Lets know how you go.:)
 

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Hi Eric, done what u sugested in the last post and it WORKS, I had to use a 470uf as the smaller ones only had a 50% hit rate the larger cap is about 90% hit rate, can i change the res to improve it ? Dont know what size to go to ?

thanks eddy :D
 
Hi Eric, done what u sugested in the last post and it WORKS, I had to use a 470uf as the smaller ones only had a 50% hit rate the larger cap is about 90% hit rate, can i change the res to improve it ? Dont know what size to go to ?

thanks eddy :D

hi,
Are using the CMOS or TTL version of the 555.?


EDIT:
If you increase the value of the cap, I would add the discharge diode across the resistor.
This will quickly discharge the cap, ready for the next power ON time.

IIRC the CMOS version of the 555 should be OK upto about 470K or so.?
 

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