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 Post subject: i need some help with a 741 op amp.
PostPosted: Fri Aug 04, 2017 11:42 am 
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Joined: Fri Feb 08, 2002 1:01 am
Posts: 4249
Location: USA / North Carolina / Fayetteville
i have been playing with these for sometime now, mainly for light / dark relays for solar lights i build.
i was having a issue with relay chatter right as it got dark, but i fixed that with a resistor ( hysteresis control )
but i am looking to add a bit more to it,, i am looking for a way to add in some delay, so that if it a passing cloud or such does not trigger it to come on, then back off right away..or any other shadow, have had a few times just before dark and a passing cloud would make light come on, then go back off after a few seconds..
any ideas? i'd like something to give me about 15 to 30 seconds delay before transitions.. is this something that can me added to the 741 circuit or will this require another separate circuit ?

this is all 12vdc operated circuit, the circuit drives a relay.


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 Post subject: Re: i need some help with a 741 op amp.
PostPosted: Fri Aug 04, 2017 12:04 pm 
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Joined: Thu Oct 26, 2006 7:11 pm
Posts: 1293
Location: Maryland
How about a large value, low leakage capacitor, directly across the sensor to slow down the swings from the light changes? :idea:
Or add a filter to the sensor lens to tame it down a bit.
No, I don't think there is an optical capacitor :sad:



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 Post subject: Re: i need some help with a 741 op amp.
PostPosted: Sat Aug 05, 2017 5:25 am 
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Joined: Mon Feb 09, 2004 1:01 am
Posts: 2132
Location: NY
Seems like ideal application for a 555 timer. The sensor, instead of triggering the relay, would start the timer, and any new burst of light would keep resetting the timer. Then, if enough time passes without light, the timer would trigger the relay.


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 Post subject: Re: i need some help with a 741 op amp.
PostPosted: Sat Aug 05, 2017 8:04 pm 
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Joined: Fri Feb 08, 2002 1:01 am
Posts: 4249
Location: USA / North Carolina / Fayetteville
i'll try the cap on sensor...

i am trying to see if i can keep it down to just 1 chip.. space constraints


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 Post subject: Re: i need some help with a 741 op amp.
PostPosted: Thu Aug 10, 2017 12:07 pm 
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Joined: Thu Aug 01, 2002 12:01 am
Posts: 2801
Location: San Jose CA
you want comparator and timer function in one chip, then an 8 bit uController with at least one comparator or voltage detect built in might do the job, maybe don't need a comparator, just an A/D should be enough. Just passively connect the sensor to the A/D input, monitor voltage and execute code with delays included to decide what to do with the I/O pin connected to the relay. Microchip ATtiny can do most of that in a 6-8 pin package. I don't think you can do this with one analog chip, I don't know one with comparator and timer in one package but there might be one, maybe you can find a way with a dual or quad op amp device


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