Laser alarm project - Page 1
Forum section: electronics / projects

author: fixit7
author_rank: Regular Contributor
timestamp: May 02, 2019, 07:55:29 pm
content: I am working on this. I have the laser."https://blog.prayogindia.in/make-laser-light-security-alarm/"I emailed the author but have no heard back."https://www.dropbox.com/s/cy7sy1itwzqcg7t/circuit_of_laser_security_alarm-engineeringprayog.com_-768x432.png?dl=0"Is the 2n7000 the BC547 transistor?And there is a 6.8K resistor that is not in his parts list?Thanks for your help.

author: ajb
author_rank: Super Contributor
timestamp: May 02, 2019, 08:34:43 pm
content: A 2N7000 is an N-channel MOSFET, a BC547 is an NPN bipolar transistor; they are not the same thing at all. The information on that page is very confused; the schematic shows an NPN transistor but with MOSFET labeling (G,D,S instead of B,C,E). Either one could be used in this circuit, but the value of the 6k8 resistor will need to be adapted depending on the resistance of the LDR.

author: fixit7
author_rank: Regular Contributor
timestamp: May 03, 2019, 01:14:17 am
content: I agree that the page is confusing.Would you be able to help me with the detector circuit?

author: fixit7
author_rank: Regular Contributor
timestamp: May 03, 2019, 01:32:27 am
content: I found this.https://makezine.com/projects/laser-tripwire-alarm/Does this look like it may work?It has fewer parts than the other project.

author: fixit7
author_rank: Regular Contributor
timestamp: May 04, 2019, 07:55:04 pm
content: Could use some help here?Thanks.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 04, 2019, 10:20:43 pm
content: the easiest solution would be to use the LDR in a potential divider circuit. you can use 2 LDRs to negate the effects of the ambient light as well. then use a voltage comparator which allows you to have sensitivity adjustment as well. connect the output of the voltage comparator to a mosfet, which will drive the buzzer (make sure your buzzer works with DC, otherwise you need an oscillator circuit to drive it which is a bit more complicated)+ the LED

author: ajb
author_rank: Super Contributor
timestamp: May 04, 2019, 10:50:13 pm
content: What specifically do you need help with?

author: fixit7
author_rank: Regular Contributor
timestamp: May 05, 2019, 04:38:07 am
content: The circuit diagram here is not real clear how to hook up the single pole double throw switch.It shows 3 wires, one goes to the speaker, one to the 555 chip(but not which position), and not clear where the 3rd goes.Since speaker has a positive and negative, does it matter which it goes to?https://makezine.com/projects/laser-tripwire-alarm/To turn off the alarm and reset the system, a (single pole double throw) switch disconnects the speaker and sends the LOW signal from the output pin 3 to the trigger pin 2. The system is now deactivated. To reactivate it, flip the switch back to the original position. The alarm will remain off until the next time that the light beam is interrupted.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 05, 2019, 12:42:52 pm
content: just use a buzzer that works with DC (makes a sound when you connect it to a battery). 555 is used to make oscillations for the buzzer like I mentioned before.this circuit only needs 3 parts: 1 resistor, one LDR and one transistor (or mosfet, both will do just fine)

author: fixit7
author_rank: Regular Contributor
timestamp: May 06, 2019, 02:14:02 am
content: Quote from: OM222O on May 05, 2019, 12:42:52 pmThat's good, but I will need some more details.I made another circuit which was lacking in impt details and ended up with a smoking BC 547.

author: fixit7
author_rank: Regular Contributor
timestamp: May 13, 2019, 08:10:52 pm
content: I put this together on my breadboard. The author said that R1 was probably not needed, so I did not use it.https://makezine.com/projects/laser-tripwire-alarm/I do not have a potentiometer, but tried a 110 and 530 ohn resistor for R2.With no power to the circuit, laser shining on the LDR, the reading was around 195 ohms.I shined laser at the LDR and used the switch in both positions.Piezo buzzer did not sound.What can I try next?

author: ebastler
author_rank: Super Contributor
timestamp: May 13, 2019, 08:24:45 pm
content: Could you post a photo of your breadboard setup? One picture showing the breadboard large enough to see all the wiring, and one showing the complete setup (including the power supply, switch, and speaker) would be best.

author: fixit7
author_rank: Regular Contributor
timestamp: May 13, 2019, 09:33:30 pm
content: https://www.dropbox.com/s/edy9xs8p5rfm15q/Project1.jpg?dl=0https://www.dropbox.com/s/leak8ty4dsmw88e/Project2.jpg?dl=0I found out that the power rail is broken up into 2 separate sections.When I put 2 jumpers between the sections, the piezo buzzer started chirping.It did so no matter if the laser hit the LDR or not.I think the problem is in determining what to use for R2.Specs for my LDRFeatures:Maximum voltage rating @ 25°C: 350VDCMaximum allowable power dissipation @ 25°C: 400mWResistance (Dark): 1MΩResistance (Light): 12kΩLead length: 1.44"Operating temperature: -30°C to +70°CAble to withstand soldering at 230°C for 3 seconds