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Set a light sensitive switch in the water tank

Post Options Post Options   Likes (0) Likes(0)   Quote Janesllin Quote  Post ReplyReply Direct Link To This Post Topic: Set a light sensitive switch in the water tank
    Posted: 28 Nov 2017 at 4:50pm
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Hello I planned a light sensitive one pulse activated switch to know when water tank is completely filled and to turn the water pump off using a relay until I reset the circuit using its N/C Switch.


NOTE: There is an external circuit to detect if water starts to overflow through overflow pipe and turns a Red LED(http://www.kynix.com/Product/Cate/545.html) on to make this light sensor circuit detect it.


Light sensitive part is working fine, but the problem is when I supply power to the circuit, Relay 2(RL 2) I mentioned in this circuit diagram keeps turning on and off continuously and I hear its extremely fast continuous switching sound like a damaged speaker. Relay 2(RL 2) will die if it turns on and off in such speed. (The purpose of RL 2 here is giving continuous current to the RL 1 to act as a one pulse activated switch.) I want to keep Relay 1(RL 1) turned on even light turns off and only to be turned off when I reset the circuit.


Here is the circuit diagram:

What changes can I do for this circuit to keep the Relay 1 (which activates by a light falling on LDR) on until I reset circuit using N/C Push Switch?


Thanks in Advance.

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Post Options Post Options   Likes (0) Likes(0)   Quote krow Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 2017 at 8:50pm
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No electrical engineer but I'd try a resister in the points contacts RL2 as to me it looks like RL2 would loose current once bypassed causing trip on and off. 
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Post Options Post Options   Likes (0) Likes(0)   Quote Tagit Quote  Post ReplyReply Direct Link To This Post Posted: 29 Nov 2017 at 10:31am
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Moderator - Black Belt
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Krow is on to it. RL2 seems to be 'shorting' itself when the contacts close (relay operates) so releases until the contacts open and then cycles again.It is also putting 12v hard across that diode which may be stressed?
A simple solution (depending on the setting of "VR" might be to use the contacts of relay2 to bypass (short circuit) the LDR (instead of where they are now). That will leave the 2 transistors and RL1 powered up until you push the button. Might have issues with heating VR depending on its setting and power rating.
A better option might be to remove D and take the +12v feed to RL2 from the pump side (assume it is switching +12v?) of the RL1 contacts via S1 (remove all other wiring from that terminal on RL2). That means RL2 operates when RL1 operates but can still be released by pushing S1. Then use the RL2 contacts just as you have them drawn now. Big difference is that the +12v side of RL2 is no longer connected to the 0V side of RL1 but only to the RL1 contacts (pump side not +12v side).
Due to the inductive loading of the pump I would also look at putting a C/R spark suppressor across the RL1 contacts. Might be a bit of overkill but for the few cents cost of an extra capacitor and a resistor it is just a good design feature.
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