How to make the alarm work continuously ON when laser barrier is breached on the LDR?
I have ldr light sensor and laser, when laser barrier is breached the alarm is activated but as soon the laser is back the alarm stops but i want the alarm to continue to work till I turn it off.
Please note on the attached photo, there is alarm on the place of the LED and there is separate laser beam on the LDR.
Thanks.

Update:
Is it this latching relay circuit good to connect it to the previous laser beam trigger circuit so the alarm can be continuous?

 

Does the circuit have a Latch facility jumper, if not you will have to wire the Ldr in series with a set of NC contacts.

 

You will need an external latch and the most straight forward way is to make a latching relay:

 

You will need a latching circuit. You can configure an additional relay as DCELL posted, do as Dogydave suggested or think about just using your existing circuit relay to trigger a SCR circuit. There is a number of ways you can go about it. Just adding a DPDT relay as DCELL covered would be a simple solution where one set of contacts on the added relay will latch it and the second set drives your alarm. Pushing the normally closed push button switch will open the contacts and reset the system.

Ron

 

You can do it with two methods, the first one is using relay to make a self-latch control circuit and reset circuit or using a CD4013 to do the latch job, if you modify the circuit from the relay controller then it can be cut from the output of comparator(maybe LM393) and the circuit with bjt to drive relay, and the bjt input with resistor will be connected to the output of D Flip-Flop(CD4013), and the clock input or or set input (D Flip-Flop) will be connected to the output of comparator, something similar like that, and how to design a right trigger phase to trigger the click of D Flip-Flop is the important issue.

The following thread was the same with you to asked for a latch circuit to hold the relay until you press the reset switch - Latch relay 2 by short pulse relay 1.

The product you bought.
XH-M131-Light-Control-Switch-DC-5V-Relay-Photoresistor-Module-Detection-Sensor.
There is a good top view in the Chinese version and it was used the pdf file.

 

Actually an scr is a great idea as long as you are prepared to lose at least 1V.

 

Sorry 0.4mA

 

Actually an scr is a great idea as long as you are prepared to lose at least 1V.
View attachment 153435

Normally I will in parallel a resistor with GK pins to keep the SCR in the OFF state.

 

You will need an external latch and the most straight forward way is to make a latching relay:View attachment 153413

What about this circuit?

 

The circuit in post #3 is a classic latching circuit. But it can be done with the circuit shown in post #9 with just a couple of changes. Probably a diode would be needed also, I am thinking. As it stands, it only would energize if the NO pushbutton were held closed. So that would not be a very useful alarm circuit.

 

The circuit in post #3 is a classic latching circuit. But it can be done with the circuit shown in post #9 with just a couple of changes. Probably a diode would be needed also, I am thinking. As it stands, it only would energize if the NO pushbutton were held closed. So that would not be a very useful alarm circuit.

any other changes except the diode?
Where to put diode?
Thanks

 

I will presume that the item labeled "momentary pushbutton" is the momentary sensor contact, only closed when the beam is broken. Then the connection from "86" must be connected to the 12 volt "Minus" connection, the same as the alarm device. That is to say that the alarm device must be connected between 86 and 87. When the alarm is triggered and 12 volts is applied to 85, current will flow both through the coil to 86 and the alarm, energizing the relay and connecting 30 with 87, providing a constant feed of 12 volts to the alarm sounder if the "switch" is closed. If this circuit has not worked then either the sensor is not as shown or else some part is inoperative. AND, the posted pictures don't provide me with any useful information. One more thing is that the text in the initial post seems to be missing a lot.