I made a $2 dollarstore laser into a run off the wall science fun laser - Page 1
Forum section: electronics / beginners

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 10, 2021, 04:13:39 am
content: I am not sure how long it will last. I had a small transformer that says 6v, but reads 9. I used 2 150 0hm resistors and branched from the middle to make like a 4 volt 250 ma source to replace the 3 LM44 batteries that used to be there. I bypassed the push switch.

author: bob91343
author_rank: Super Contributor
timestamp: January 10, 2021, 05:02:41 am
content: How can you draw 250 mA through such high resistance? You don't have enough voltage.

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 10, 2021, 05:19:17 am
content: I started with 9V 600ma so the amperage to the pointer part was probably less than 250ma. With ohms law I got 300ma = 9 volts/300 ohms. I thought the max output of a LM44 would probably be around 200ma. So I was shooting for 3LMs in series so 4.5 volts, 200ma.

author: ledtester
author_rank: Super Contributor
timestamp: January 10, 2021, 05:34:17 am
content: What bob91343 is saying is that there's no way you can get 250mA through a 150R resistor with only 9V. Ohm's law says you can only get 60mA (= 9V/150R).Two other things...1. Where is your switch in the circuit? -- it looks like you are always draining current even when the switch is off.2. From anecdotal evidence on the web, the cheap laser pointers should only be run on about 20 mA of current. If you have 3 volts worth of batteries you can measure the current draw with a multimeter -- it's worth doing to see what the normal current draw for the device is. Then you can size your resistor appropriately.What's likely a better setup is:Code:9V ---- /\/\/ ---- /\/\/ --- switch --- laser --- GND150 150Measure the drop across the resistors and you can determine how much current is going into the laser pointer.

author: kjr18
author_rank: Regular Contributor
timestamp: January 10, 2021, 05:38:44 am
content: And 9V will be your open circuit voltage of that transformer, under load it will fall a bit. Se even less that 60mA.

author: coppercone2
author_rank: Super Contributor
timestamp: January 10, 2021, 05:42:18 am
content: if you want that to be a good lab source for experiments I recommend under powering it, then you can put it in a box and be fairly confident it will last a long time

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 10, 2021, 05:45:12 am
content: There is no switch it is just on when you plug in the transformer. I still used the pointer's resistor in circuit, so I was just trying to mimic the 3 LM44 batteries. Anyway it works and looks the same as when the batteries were in, so so far I would call it a success. It is not like I am going to leave it plugged in for hours.

author: coppercone2
author_rank: Super Contributor
timestamp: January 10, 2021, 05:47:42 am
content: ehh lasers are really sensitive, when you over power them it might not get too bright before burning out (saturation behavior). Try blowing out a LED, you will see its not really impressive before it burns out.they are not the cool scifi devices that give you some kinda massive boost on over load like after burners in a plane

author: ledtester
author_rank: Super Contributor
timestamp: January 10, 2021, 05:57:04 am
content: QuoteWell - it appears there is a switch on the laser pointer you could use.

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 10, 2021, 07:45:12 am
content: I didn't want to have to press it. I wanted to do things like transmit through fluids or prisms. The connection is made after the switch.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 10, 2021, 05:08:35 pm
content: Looks a cool project.Plugging it in and out to turn it on/off is OK... Simplicity is a good thing!Can you measure the actual current in the circuit with your meter?

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 10, 2021, 07:06:27 pm
content: Well not any more it is potted. But I was getting weird readings for the transformer like it said 600ma, but I was getting like 1.2A from the voltmeter. I was a bit confused. I did check the voltage between the resistors said it was 4.1 volts. I ran it for like 1 hour cumulative and it works fine still. Last night I tried to make a double slit interference pattern, but failed to do so. It is weird there are like little speckles from where the light is and coming even out the back that you move and the speckles move.

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2021, 01:46:21 pm
content: Quote from: msuffidy on January 10, 2021, 07:06:27 pmHow did you take those current measurements?A common mistake newcomers make is connecting their meter, set to current, directly accross the power supply, which would measure the short circuit current and is a bad idea, because it could blow a fuse, or distroy the power supply. The correct way to measure current, is to break the circuit and put the meter in series with the load. If you're unsure of the current, always start with the highest current setting, on he meter first.

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 11, 2021, 06:50:08 pm
content: Thanks I totally did not know that I just went across it.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 11, 2021, 06:59:05 pm
content: Quote from: msuffidy on January 10, 2021, 07:06:27 pmNot weird - that's the scattering of coherent radiation (i.e., from a laser). It's one way you can tell just by looking if something is being illuminated by a laser or a non-coherent light source. You are, in effect, seeing a holographic diffraction pattern.TL;DRIf the object scattering the light moves, then the speckles will change. This is used in scientific instruments to measure the movement of nanoparticles in liquids in order to estimate their size or electrical charge.

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 15, 2022, 01:43:01 am
content: I got another of the pointers and got to the thinking the first one didn't work quite right because it has a really wide beam. I decided to do it again, and I found out at least the new ones had adjustable distance lenses that you twist. So when I tried to use the other setup it didn't work and I concluded maybe they changed the specs. I made this diagram and did my first real resistor network calculations. It is working great, just hope it does not burn out. The things with the tilde are approximations I made like a bit more than the needed, or I don't have quite that resistor. You were right I don't know how I got those values the last time. The 121 is the on board laser resistor. The pointer had 4.5 of batteries and I am using a 9v transformer. I was keeping with the idea that I was mimicking the original supply, but maybe you could have just used one resistor instead of two.

author: Gyro
author_rank: Super Contributor
timestamp: January 15, 2022, 10:32:27 am
content: Quote from: JohnnyMalaria on January 11, 2021, 06:59:05 pmIrrc, the direction in which the speckles move when you move your head can be used to indicate whether you are short or long sighed. I can't remember the details, but certainly, when I remove my glasses, the speckles move in the opposite direction. Presumably, if your sight is perfect, they just move at the same speed as your head.

author: tggzzz
author_rank: Super Contributor
timestamp: January 15, 2022, 11:58:54 am
content: Quote from: msuffidy on January 10, 2021, 04:13:39 amWhat is the optical output power? Get that wrong and someone could beinstantly and permanentlyblinded- possibly by a specular reflection off a shiny surface.If you don'tknowthat the device cannotby designemit more than 5mW, then you should not be using it.Yes, I know many people "repurpose" semiconductor lasers so they can be used on a 3D printer. That does not imply they are safe; it does imply the perpetrators are dangerously ignorant. Ditto many cheap laser pointers that can be bought.Understand and act on the old poster in laser labs: "do not look at laser with remaining eye".