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. Light Sensitive One Pulse Activated Switch

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.

  • \$\begingroup\$ In my own words: "When the circuit is in a quiescent state, the pump motor relay in open so that the motor is OFF. When you push a push-button, the circuit changes state and engages the relay so that the motor is ON. Overflow causes a red LED to turn on, which the LDR detects, which should return the circuit to its quiescent state." (I assume a red LED lit up would hold the circuit in the quiescent state even if you push the button.) Is that about it? \$\endgroup\$
    – jonk
    Commented Jan 28, 2017 at 7:15
  • \$\begingroup\$ No, its not. All circuits and motor are connected to 230V mains socket with power supplies. Quiescent state of this circuit is the overflow, not motor on and off. \$\endgroup\$
    – StartX596
    Commented Jan 28, 2017 at 7:25
  • \$\begingroup\$ RL2 is wired wrong. But besides that, isn't the circuit wrong? When LDR is dark T1 and T2 are off and RL1 is open. How will the pump motor work if RL1 is open? \$\endgroup\$ Commented Jan 28, 2017 at 7:36
  • \$\begingroup\$ Why not use probes to detect the water level? You can check in the change of conductivity. \$\endgroup\$
    – Bradman175
    Commented Jan 28, 2017 at 7:49
  • \$\begingroup\$ because live wire of motor AC Supply connects through RL1's COM and NC pins. \$\endgroup\$
    – StartX596
    Commented Jan 28, 2017 at 7:50

3 Answers 3


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.

  • \$\begingroup\$ "some NO contacts from the T1 collector to ground" Please explain this more. \$\endgroup\$
    – StartX596
    Commented Jan 28, 2017 at 6:59
  • \$\begingroup\$ What do you mean as ground? Is it the negative terminal shown as 0V? \$\endgroup\$
    – StartX596
    Commented Jan 28, 2017 at 7:08
  • \$\begingroup\$ RL1 directly switches motor on and off, so there are no any contacts with its NO and NC pins with circuit, please show me what is the current path (As you mean like "there is still a current path through RL1 and then through RL2") that causes buzzing. \$\endgroup\$
    – StartX596
    Commented Jan 28, 2017 at 7:32

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.

  • \$\begingroup\$ No, I can't use contacts from RL1 to control RL2, because its contacs used to directly switch motor on or off. Isn't there any other way to stop buzzing? \$\endgroup\$
    – StartX596
    Commented Jan 28, 2017 at 13:32
  • \$\begingroup\$ but buzzing is what your circuit is designed to do... I don't understand... either you want the buzzing, or you've designed the wrong circuit \$\endgroup\$ Commented Mar 16, 2018 at 21:30

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.


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