Light Sensitive One Pulse Activated Switch

Question

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 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.

Answer 1

When the circuit is powered up, even if the LDR is dark and the transistors are cut off, there is still a current path through RL1 and then through RL2. When RL2 turns on, its normally open contacts close, which shorts out the RL2 coil. This causes the NO contacts to open again, and the cycle repeats forever. This is why RL2 is buzzing.

Are there any extra contacts on RL1? If so, you can use some NO contacts from the T1 collector to ground, with the NC pushbutton in series. Eliminate RL2. Once the relay is activated, the NO contacts will keep it activated until the button is pushed.

Answer 2

Relay 2(RL 2) I mentioned in this circuit diagram keeps turning on and off continuously

once RL2 is energized, its contacts shorts the coil, de-energizing it, which opens up the contacts, and RL2 is ready to be re-energized again -> it goes on and off forever.

You can use the contact from RL1 to control the RL2's coil. Or change the logic and just use RL1.

Answer 3

The problem is due to an unregulated voltage across LDR/R ratio. When the relay turns off , I presume the 12V rises enough to turn on the transistor.

Also you have 2 relays in series instead of a single relay with DPDT contacts. (where you use NO on one and NC on the other for some odd reason)

If you add a filter cap or regulate the voltage to the LDR +side relative to the emitter ground , so it does not change, then you won't have this negative feedback Relaxation Oscillator effect.

Change the Relay to DPDT rated for your Pump Current and voltage. and then you can have NO and NC on each Pole. One for AC and the other for DC.