Stupid Transistor Question?

Posted: 7/19/2005 11:34:01 PM

From: New York City

Joined: 7/16/2005

I am trying to build an optical (pseudo?)theremin and just had a transistor in an oscillator circuit burn out, complete with acrid smoke. How normal is this? What kinds of things fry transistors? I had combined two very similar oscillator modules (this dramatically increased the volume), one with 5x10kOhm resistors and the other with 1x10kOhm resistor combined with a photosensitive resistor to control pitch (the overall pitch wouldn't drop below the frequency of the fixed oscillator, hence the extra resistors). The speaker I was using was an 8-Ohm 0.5 watt speaker I raided from an old clock radio. All this was powered by 4-AAA 1.5-volt batteries. Is any of this suspicious?
Posted: 7/20/2005 5:16:17 AM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

Hello Dean,

“I just love the smell of thermal runaway in the morning”

Not knowing your circuit design I tried “Remote Viewing” your construction table but the clutter and smoke was hurting my eyes!

If all else fails you might consider this design

Good Luck,

Posted: 7/20/2005 7:54:33 AM

From: New York City

Joined: 7/16/2005

Thanks for the tip- I did see your design and it's obviously far more elegant than mine. I'm a total novice and I need to pull something together very quickly so I aimed for a design that's a bit simpler the first time around (at my level of expertise, I'm intimidated by IC's for some reason- when I'm ready to face that demon, I'll hopefully be ready to build a "proper" theremin). The final product will go in a public place and is likely to be unattended, so I can't really have it catching on fire...

Obviously, it's not really possible to comment on my problem without seeing my design, but just in case: is it possible that my makeshift amplifier could have overheated the transistor? I know enough to know that there was more resistance on one side of the circuit.

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