Optical Tachometer

[sternwake]

I can make this toy work, and there is no indication, yet, that  the seller is going to not replace it.

I did open it up.

The LED, (not  a laser, what a surprise!) does have a crappy solder joint, but I also put my voltmeter on the led's  legs, one of which has a + sign next to it, It read - 2.73 volts. NEgative.
I'd expect if it were fed 5v dc+ it would light up fairly brightly.  I think the LED's driving circuit is faulty.

I related this to seller, ball is in their court.

I don't see any point in reflowing the solder if the LED is getting fed -2.73v.
There are certainly e bay items which appear exactly the same, for twice the price.

  I guess the internals could be made to different spec's/quality, then  stuffed into the same casing.  I've seen that plenty with LED dimmers. which now have all been phased out in favor of voltage buckers instead, even the ones with what appear to be burlier components.