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Could some one explain to me how my circuit works? I accidentally hooked this up while trying to work out an amplifier. I'm thinking there's something like RC resonance going on, but I'm not sure. Please help? Thanks.enter image description here

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I remember seeing a similar configuration (an oscillator where the transistor's base isn't connected) a while ago, there, and would also like to understand how it works. – Renan Mar 14 '12 at 2:31
At what frequency is the tone? – tyblu Mar 14 '12 at 3:04
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Maybe draw the base lead and mark it 'NC' for not connected. – JustJeff Mar 14 '12 at 3:13
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It's a friendly transistor. That's why it waves. – David Kessner Mar 14 '12 at 18:16

2 Answers

up vote 2 down vote

Maybe your circuit is rather amplifing than generating an oscillation.

This can happen if the "unconnected" base is picking up some stray fields (e.g. radio signal or fields generated by a switching power supply, etc.)

Since you are using a solderless breadboard, the base is probably connected to a line than can act as an antenna.

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More information needed.
What is this built on?
Plug in solderless breadboard, or strip/matrix/vector board with solder or ...?
What is the base connected to? (Copper strip or plug in or ...)

Given what we know, here is a COULD happen.

Base leakage charging base stray capacitance could turn transistor on - how hard depends on leakage.

(1) Low collector may provide a discharge path or the speaker indictor and capacitor resonance may drive collector under ground so that CB diode forward biases and discharges base capacitance and allows cycle to restart.

(2). If there is CE leakage this is a classic brounded base amplifier without the base grounded :-). Emitter is input. Collector is output. Positive in phase feedback occurs c to e of transistor. Capacitance between strips on plug in solder board MAY be enough. Base would need a whisper of fwd bias - depends on construction. Leakage alone MAY be enough.

What transistor is used?
What construction is used?
What frequency of oscillation - audio low/high/..., RF ...?

This is possibly somewhat like an informal Colpitts Oscillator.
Not quite any of these - hence "informal"

Many examples here

enter image description here enter image description here enter image description here

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solderless breadboard – user8210 Mar 14 '12 at 4:19
@user8210 - OK. That's 1 out of 4, Please answer 1 : . What is the base connected to? (Copper strip or plug in or ...) [[ie could be floating]]. / | What transistor is used? What construction is used? = SBB / What frequency of oscillation - audio low/high/..., RF ...? – Russell McMahon Mar 14 '12 at 5:28

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