USB power for a laser pointer

[jackssmirkingrevenge]

as per title, will it work?

Even more ambitious, is there enough current available to run my sentry gun controller and servos?

[Techie]

im not sure about the servos, but usb ports provide 5v's. All the laser pointers i have used run off 3v so youll need some resistors.

[jackssmirkingrevenge]

The one I have has three 1.5V L1131 button cells in series, that's 4.5V...

The logic in this case is that I have no choice but to have the camera and servo controller connected to the PC, so if I can run everything off it it will save me having to charge batteries.

[ramses]

most likely, IIRC, USB allows 500mA. Your controller, etc. may require more than 2.5W.

[mobile chernobyl]

You have 100 mA with USB 2.0, and if your on top of whats new - 150mA with usb 3.0.

That's per port.

Off the motherboard socket you can get 500mA on 2.0 and 900mA on 3.0.

Some laptops have a power USB option where the USB port is stacked on a power port, and with that you can get from 5V to 24V depending on what series of connections it makes. @5V you get up to 6A, and the current available lowers proportional to the voltage increase.

Your best bet is probably to have a Lipo on board your device and have it charge off the USB when needed, charging circuits are cheap nowadays.

[Technician1002]

Info on servo current draw is here.
http://www.societyofrobots.com/actuators_servos.shtml

Just a quick note on solid state lasers, some use a current limiting resistor. They don't like higher voltage. Some use an electronic current regulator. Those are more forgiving of voltage variations such as 5V instead of 4.5 on 3 cells.

[ramses]

I don't think he wants to power the servos off USB, just the controller/interface.

[Technician1002]

Data and laser pointer should be fine. Servo motors, probably not.

[jimmy101]

Some use an electronic current regulator. Those are more forgiving of voltage variations such as 5V instead of 4.5 on 3 cells.

But batteries actual voltage is often a fair bit different than there spec. Not just the obvious "voltage drops as the battery goes dead" but also a fresh battery generally has a higher voltage than it's spec. 1.5 volt alkalines are often 1.6V, 1.2V rechargeables are often 1.3V or even 1.4V ... three 1.5V button batteries may well be darn close to 5V when brand new.

Back to the OP. I wouldn't suggest trying to pull anymore than 100mA (5V) from a USB port regardless of it's spec level. Computer manufacturers haven't always followed the spec closely¹ and exactly how things change when more than one USB device is plugged into the computer is not always well defined. Still, you might be able to get away with two USB connections. Use one for the laser and one for the servo(s). It would definitely be a good idea to look up the power spec on the servos. 100mA is usually a pretty small motor but that might be all you are using. Of course, you may have more than one servo...

For a cheapo laser pointer it is easy enough to measure the voltage drop and current drop with a basic DVM. From that info you can pick a suitable current limiting resistor for the USB's 5V. (You probably don't need any finer control than just current limiting just like a generic LED.)

A quick google search didn't turn up the current capabilities of L1131 batteries. I did find mAH ratings of about 40 to 50. Figuring a generic battery can source about 10X its mAH rating (but only for few minutes) means the batteries could be supplying as much as 500mA. Or, figure the laser will run for an hour on it's batteries so nominally it is drawing about 50mA at 3.5V. To get 3.5V at 50mA from the 5V USB you need a (5-3.5V)/0.05A=30 ohm resistor. (The current really should be measured though, if the laser is actually only drawing 20mA that 30 ohm resistor will be supplying too high a voltage.)

-----------------------
1. When the iPods came first came out there were fairly often getting fried by out-of-spec USB ports on computers. The "5V at 100mA" wasn't. It cost Apple a fair amount of money replacing iPods. Eventually they strongly recommended iPods not be used on particular manufacturers systems.

[chinnerz]

without reading any of the above....

YES, but as other have stated, the power out is in correct. So you will need to make a pwm circuit.... or use a voltage regulator (less efficient but smaller i guess)

Most diodes run of 3V (well most where i live and i have the option between a 1mw and a 5mw in red or a 10mw green) so the ones in laser pointers already have a a circuit to change the voltage i think

[jackssmirkingrevenge]

A quick google search didn't turn up the current capabilities of L1131 batteries. I did find mAH ratings of about 40 to 50. Figuring a generic battery can source about 10X its mAH rating (but only for few minutes) means the batteries could be supplying as much as 500mA. Or, figure the laser will run for an hour on it's batteries so nominally it is drawing about 50mA at 3.5V. To get 3.5V at 50mA from the 5V USB you need a (5-3.5V)/0.05A=30 ohm resistor. (The current really should be measured though, if the laser is actually only drawing 20mA that 30 ohm resistor will be supplying too high a voltage.)

So USB might work only for the laser?

For everything else I took some sensible advice and ordered a 6V 1.5A AC adapter

[Technician1002]

USB may work for the laser. you may need a resistor if the diode does not have active current regulation.

Laser modules are not super effecient at convering electric to light, so a 0.5mw laser pointer may draw 350 mw to power it. The modules I have investigated for red laser pointers show typical draws of 60-150mA. I have not measured any green or high power lasers.

Most do not make a laser beam on only 20 ma, but will light up nicely.

[jackssmirkingrevenge]

I have a couple of cheap-o lasers that came with cheap-o airsoft pistols, I suppose I can afford to burn one out if there's too much power

[Insomniac]

Remember, button cells have a bit of internal resistance, which allows you to run things such as LEDs directly without a resistor (it's not the healthiest thing for the LED or laser, but it'll work). If you were to put a LED on a power supply with the same voltage as the button cells, with no current limiting resistor, it would likely blow (I assume the same for lasers).

Note that some cheap lasers have a tiny SMD resistor soldered onto the laser board, so even though it may look like you're connecting it directly, some have an in-built resistor. Better ones have proper regulation circuitry.


Don't power your servos from the USB port. HXT900 servos (a well known, small, very popular 9g servo) can peak at just under half an amp of current draw under load. This of course rapidly drops to somewhat less, but with two of your rather larger servos I don't think it's a good idea to run off USB power.

[Ragnarok]

I have a couple of cheap-o lasers that came with cheap-o airsoft pistols, I suppose I can afford to burn one out if there's too much power

Can I point out the not impossible alternative that you might damage the computer? That is a bit more of a costly mistake.