Laser trip wire help. - Page 1
Forum section: electronics / beginners

author: BettyMorlock
author_rank: Newbie
timestamp: June 11, 2015, 10:24:53 am
content: Hello I am new to the forums and pretty new to electronics, but I am building a laser trip wire based on a 555 timer as a beginner project and need a little help getting it to work just like I want it to. First off I have the main circuit down I got it fromhttp://www.instructables.com/id/Laser-Tripwire-Alarm/?ALLSTEPSNow the issue is that I want it to turn off the laser if it is tripped. That way it is less detectable and can only be set off once before having to be reset.So far I have tried to use a discrete t-flipflop at the output of the main circuit but after thinking about it realized that toggle flipflops don't work that way.Is there a way using discrete logic to achieve this goal.All I have in parts are the ones I used to make the trip wire plus some npn transistors and a large assortment of resistors and caps. I am not against buying more parts but It will take me a little while to get anything else.~Sry ahead of time if it is a stupid question I am very new to this.~

author: sean0118
author_rank: Frequent Contributor
timestamp: June 11, 2015, 02:02:57 pm
content: You could use a relay to disconnect the laser from its power source? Or did I misunderstand the question?Stay safe though, disassembling lasers requires some safety considerations.

author: BettyMorlock
author_rank: Newbie
timestamp: June 12, 2015, 10:26:40 am
content: Thanks That sounds like it should work that way. I can still hook up an led if to the output if I want to later. So I will try to hook up the relay where the buzzer is on the diagram and hook up the NC line through to the laser and I should be good that way right,? and if I wanted to hook up an LED to the circuit I could just use the NO pin to tell me that it's been tripped and the power just didn't go out or something.Here is what I drew up in GIMP (I know weird). Does this look like it would work? Also I Just realized this after making the image, that I have to different grounds so the grounds after the relay are for the 4.5 v DC source.

author: McBryce
author_rank: Super Contributor
timestamp: June 12, 2015, 10:48:06 am
content: I'd add a diode across the relay coil to avoid killing the 555 too if I were you.Bryce.

author: LukeW
author_rank: Frequent Contributor
timestamp: June 12, 2015, 11:06:27 am
content: The 555 output can both sink and source current.Neither your diode laser or your alarm buzzer are particularly high-current devices (but, generally, in a situation like this, you should be asking yourself specifically what their current needs are), so how about driving the laser from the output when it's high, sourcing current through the laser, and sinking current through the alarm buzzer into the 555 when the output is low?http://www.555-timer-circuits.com/images/sinking-and-sourcing.gif

author: BettyMorlock
author_rank: Newbie
timestamp: June 15, 2015, 12:06:23 pm
content: Sorry for the late reply work has had me tied up.I put the schematic in Yenka and just started messing with it to see if I could get better results from my last schematic I posted and Found that I could hook up the switch on pin 3 to the laser directly and push it to ground instead of the positive rail and It would reverse the output from "trip on" -to- "trip off". So when the photo-resistor has light, it will leave the laser on and when it goes dark it will shut the laser off.Both the resistors coming off of the positive end of the 9v battery are 680 ohmsThe voltage coming out of pin 3 (before the 200 ohm resistor) is 8.75 at 19.9 mA.So do you think this would be reliable and actually work on a bread board and be ok? The only reason I ask is because "Yenka" can be really weird sometimes and there is only one 555 timer I can use, and I don't know what the properties of it are. I have a NE555 timer if that helps, and a standard 4.5v cheap laser diode that runs off of 3 cell batteries.Also the only reason I use "yenka" is because I don't know enough about what I am doing to use LTSpice for this. I tried to use it and got really confused.Thanks again.