I sure one or two of you will know the answer, but as I don't do SMT very often ( mainly because I can't see it ) I'm hoping it'll be something simple..
I have three Anemometers I need to repair... One was easy.. Crack in PCB (sorted )... The second had a mechanical issue on the chopper wheel ( glued and sorted )... The third has a component fault... I'm pretty sure that a 555 has done a bunk, but as I said.. Tight circuit and SMT.... The chopper ( IR led / receiver combo ) is working... The buffer opamp is also working.. This op amp is supposed to fire the 555 monostable via a small 1nf cap.. But!! On the scope at this point there is the most feeble pulse that cannot drive the trigger... But the trigger has a flyback diode to 2.5v( sorry that's the positive..Its actually Vcc - 2,5v.... ie.. 21.5 ~ 24 v ) Anywho.. There is also a 47k pullup and another decoupling cap that I'm sure is to the 2.5v... The problem is the 555 isn't triggering... If I trigger it manually ie.. short pin1 and pin 2... It works as expected... So I don't fancy taking the tiny 555 off and replacing unless absolutely necessary..
My gut feeling is there is something up with the impedance on the trigger pin, which is keeping it high... I cant see anything else causing it.. I have removed the diode and the caps ( tough going ), tested them out of circuit...
Anyone else think its the 555 IC???
I have three Anemometers I need to repair... One was easy.. Crack in PCB (sorted )... The second had a mechanical issue on the chopper wheel ( glued and sorted )... The third has a component fault... I'm pretty sure that a 555 has done a bunk, but as I said.. Tight circuit and SMT.... The chopper ( IR led / receiver combo ) is working... The buffer opamp is also working.. This op amp is supposed to fire the 555 monostable via a small 1nf cap.. But!! On the scope at this point there is the most feeble pulse that cannot drive the trigger... But the trigger has a flyback diode to 2.5v( sorry that's the positive..Its actually Vcc - 2,5v.... ie.. 21.5 ~ 24 v ) Anywho.. There is also a 47k pullup and another decoupling cap that I'm sure is to the 2.5v... The problem is the 555 isn't triggering... If I trigger it manually ie.. short pin1 and pin 2... It works as expected... So I don't fancy taking the tiny 555 off and replacing unless absolutely necessary..
My gut feeling is there is something up with the impedance on the trigger pin, which is keeping it high... I cant see anything else causing it.. I have removed the diode and the caps ( tough going ), tested them out of circuit...
Anyone else think its the 555 IC???
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