6
\$\begingroup\$

Would someone be so kind as to help me figure out why my circuit is not working?

I am having difficulties with Experiment 11, Steps 2 & 3 in Charles Platt's book. The experiment is designed to teach me how I can use transistors to amplify a signal, and to audibly hear that amplified signal through a 8 ohm speaker.

Here's the schematic:

enter image description here

Here's a picture of the circuit I built:

enter image description here

I am able to build the oscillator with the LED on the top, no problem, but I'm not getting any sound to come out of the speaker. My multimeter tells me that current is going through the transistor. When I scratch the leads of the multimeter against the emitter and collector leads of the transistor, I do get some noise from the speaker, so I know the speaker is not broken.

Any ideas?

\$\endgroup\$
8
  • 1
    \$\begingroup\$ You can use Fritzing to see if what you have matches the schematic. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jun 27, 2013 at 3:32
  • \$\begingroup\$ The bottom 2N2222 is white but the top is black. Maybe you put 2N6027 instead of 2N2222? \$\endgroup\$
    – Vasiliy
    Jun 27, 2013 at 7:52
  • \$\begingroup\$ @Vasiliy: That whiteness might only be the light reflecting off the shiny top surface? I agree it's a good idea to double check the components though. I suspect this is one circumstance where a cheap toy oscilloscope† might actually be useful! :-) († Velleman, DSOnano, etc) \$\endgroup\$
    – RedGrittyBrick
    Jun 27, 2013 at 8:39
  • \$\begingroup\$ @RedGrittyBrick, yes, now I realize it :) \$\endgroup\$
    – Vasiliy
    Jun 27, 2013 at 8:42
  • \$\begingroup\$ @starblue, Yes, I didn't look carefully. I removed the comment. \$\endgroup\$
    – mj6174
    Jun 28, 2013 at 14:01

3 Answers 3

Reset to default
1
\$\begingroup\$

The base of your output transistor is shorted to DC- (aka GND) causing that transistor to be off all the time. See the upper yellow wire on the right? It's not supposed to be there.

\$\endgroup\$
0
\$\begingroup\$

I'd do the following for debug purpose:

  1. Verify that you use correct components (see my comment to your question) EDIT: the comment is to LOL about, nevertheless verify components.
  2. Verify (using multimeter) that there are no open-circuits
  3. Verify that all active devices (transistors) are at correct working points
  4. Verify that there is current through the speaker and its magnitude is enough to make the speaker work
  5. Here you'll need a scope - verify that the frequency of the current through the speaker is in audible spectre
  6. Replace the speaker
  7. None of the above worked? I have no clue what your issue is :)

PS: If you verified items 1-4, but have no scope to measure frequency, I'd suggest to measure (or change empirically) the capacitance at the input of bottom 2M6027. Breadbords have disadvantage of adding parasitic capacitance between leads of the components. Your oscillator is very capacitance dependent, therefore this parasitic capacitance could make it drift out of audible frequencies IMHO.

\$\endgroup\$
0
0
\$\begingroup\$

Make sure your breadboard power rails are actually continuously connected. By this I mean some breadboards have the ground and power rails "broken" up between the pins on the breadboard so you actually have to run jumper wires to the next section of VCC or Ground holes on the breadboard (all the holes on the red and blue lines).

This can be verified with a multimeter, first rule when something goes wrong is probe for proper voltages on all components and power supply.

Also is the resistor on the First 2N2222 collector Brown Black Brown? It looks that way on the picture while on the schematic it suppose to be 2.2K Ohms

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.