Picaxe 40x2 running off usb 5v?

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James Ward

New Member
Hi, I was just wondering if anyone could tell me whether it is possible to run a 40x2 + 08m2 from a standard pc usb socket. I am having floating ground issues currently with standard psu's so I was thinking of switching to usb due to it being so readily available. any ideas?

P.S. I finally got my LED cube project working ;) see it on youtube at: https://www.youtube.com/watch?v=EgouCx3QN8Q
feel free to post ideas for what to put on it when I go to the UK national science and engineering competition!
 
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nick12ab

Senior Member
Yes, however it is recommended that you use some protection circuitry due to the risk of damage to the PC if a short or higher voltage is applied to the power rails of the circuit.

Some computers actually have the USB ports connected directly to the 5V power rails of the PSU so it's also important that 'alternative supplies' for your circuit, if used, won't provide power to the USB plug.
 
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nick12ab

Senior Member
James Ward said:
What kind of protection circuitry?
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The Arduino system can be USB-powered and it is open source so its protection circuit is freely available and contains a good example of such a circuit. The schematic for the Arduino UNO shows a fuse (actually a polyfuse) and a MOSFET to disconnect the +V terminal of the USB socket from the rest of the circuit.
 
Technical

Technical

Technical Support
Staff member
Powering a non-USB communicating device device from a USB port is naughty, because the dumb device does not 'talk' to the computer and negotiate the power configuration etc. correctly. It should certainly take no more than 100mA if you plan to do this. The best solution is to probably fix your circuit rather than look for an alternate supply.
 
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Goeytex

Senior Member
I am having floating ground issues currently with standard psu's so I was thinking of switching to usb due to it being so readily available.
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As Technical indicated, the best solution would be to solve the "floating ground" problem rather use the USB V+ as a power source. I do not see how using the USB supply will solve a floating ground problem. It is just another power source.

Perhaps if you tell us why you think the ground is floating and what steps you have taken to solve the problem, then we can be of more assistance. Also if you post a schematic, then the problem might be quickly identified.
 
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AllyCat

Senior Member
Hi,

Bizarre indeed; some ebay listings name literally an hundred or more "excluded" countries. Occasionally I amuse myself trying to work out where they will send the item. :)

James Ward said:
I am having floating ground issues currently with standard psu's so I was thinking of switching to usb due to it being so readily available.
Click to expand...
Technical said:
The best solution is to probably fix your circuit rather than look for an alternate supply.
Click to expand...
+1. Also note that the items being suggested here deliver "RS232 TTL" which is inverted relative to the signals that a PICaxe needs for programming. Not a fundamental problem but maybe a case of "out of the frying pan into the fire".

Cheers, Alan.

PS: I missed Goeytex's post on page 2, so it's now +2 !
 
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nekomatic

Member
USB seems the obvious choice for powering any computer-connected PICAXE project to me. The solution to the inverted signal problem is to use an FTDI based adapter and program the RX and TX signals to be inverted - you can do this yourself with the utility you can download from FTDI, or Proto-pic now sell an adapter specifically for PICAXE that's already set up like this.
 
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