flashing helmet idea

[shokjok]

I responded to a local ad for a designer to create a flashing sport helmet. The advertiser attached several hundred LEDs to a helmet, laid out to require 45 volts per string.
His idea was to have a pocket-sized 9V-powered flasher with five extra 9V batteries for the output circuit. I know this could best be done with MOSFETs, a clock generator and any CMOS counter IC, but wouldn't the case get warm and uncomfortable? The five batteries would be worn on the waist.

 

[Sceadwian]

Setting this up to run at 45 volts using a string of 9 volts in series like that is an absolutely horrible idea! Aside from the fact that 9 volts have low energy density, they also have high series resistance, so even with the low amount of energy available in the cells you'll be wasting a lot of it in the cells themselves.

I know this could best be done with MOSFETs, a clock generator and any CMOS counter IC, but wouldn't the case get warm and uncomfortable?

I'm not sure why you think it would get warm, modern Mosfets are very low on resistance and would waste a minuscule amount of power, a 555 timer running a fet would use hardly any power hence hardly any heat.

Personally I'd use a low end micro controller to drive a logic level fet from a 2 series Lithium pack or a 5 cell NiMH pack with a voltage regulator. Regular AA alkaline would work pretty well too.

 

[transistor495]

Personally I'd extend this idea(uC and NiMH cells) with solar cells designed over the helmet surface so that it can be charged over daytime.

 

[duffy]

Aside from the fact that 9 volts have low energy density, they also have high series resistance

LOL - that may be all that's stopping them from burning out. More than once I've gotten something from an art department that had a string of LEDs driven directly off a battery. Then they burned them out because they couldn't be bothered with a resistor, or to learn how LEDs work, or anything - they just figure us contemptible "tool users" put the resistors in to confuse people. After all, they saw one work without a resistor!

And the burned-out LEDs are permanently buried in plastic resin.

And they want you to make it light up again.

 

[Sceadwian]

Transistor, you would never wear a helmet like that in the daytime it'd look ridiculous.

As far as practicality goes, I don't see any problem with putting a lithium cell directly in the helmet. No wires or waste pack required. All you have to do is carve out some of the foam for it and make sure it's thermally okay.