About a year ago I bought 20 TLC555 CMOS timer chips via Ebay from
a Hong Kong (China) seller.
Last week I finally got around to use one of them in a Clark zapper
circuit - but when I looked at the output signal on my oscilloscope,
I saw no waveform. I replaced the IC with a 7555 (which I knew
worked) - and I could see a nice 28KHz square wave.
I then tested the other 19 chips, and found that none of them worked.
If you look at the manufacturer's data sheet, you see that three
100,000 ohm resistors are connected in series (for a total of 300,000
ohms, measurable from pins 1 and 8. Using an ohmmeter, I found
infinite resistance from the two pins on all 20 ICs.
So the moral of the story is to buy electronic parts from a reliable
distributor such as Mouser or Digikey.
David
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