Voltage Controlled Oscillator with LM358

Alec_t

Joined Sep 17, 2013
12,162
Post another pic or two, sharply focussed and taken from different angles and distances, so that we can see all the wiring to the breadboard and all the component markings. The problem may be just a bad connection in the breadboard, or a mis-wire.
 

AnalogKid

Joined Aug 1, 2013
9,296
What about it? That schematic is using a TL082 that was breadboarded and confirmed with the components listed.
The LM358 input common mode range includes and exceeds the negative rail (ground in the original schematic) and the transistor switches inverting input to ground. So while the input is grounded, the input stage still is in its linear active region. The TL082 input common mode range extends only down to Vee+3V. More importantly, when the transistor switches on it is not pulling the inverting input down to the negative rail. I think that 12 (or 9) V difference is important to the circuit's operation.

When the output is at its most negative value (approx. -10.5 V), that exceeds the reverse voltage rating of the 3904 transistor base-emitter junction.

ak

Edit: fixed - sign.
 
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sghioto

Joined Dec 31, 2017
2,373
When the output is at its most negative value (approx. +10.5 V), that exceeds the reverse voltage rating of the 3904 transistor base-emitter junction.

ak[/QUOTE]
The actual negative voltage at the base of the 2N3904 during the negative swing is less then -3 volts using the 10K base resistor. Using a 47K will reduce the voltage to just under -1volt. Either way the transistor is operating well within specs.
Steve G
 

sghioto

Joined Dec 31, 2017
2,373
Interesting results actually using a LM358. Wired as in the original schematic from post #1 using a .001uf cap,2N3904 transistor and single ended 12 volt power source. Max output frequency with 12vdc input is 4.4KHz. Square wave output is not symmetrical, slow rise times on the slopes. Triangular wave OK. Recommend a rail to rail op amp. Much better results using a MCP602 at 5 volts. Output frequency 6.7KHz at 5 volt Vin, frequency stable and much less dependent on supply voltage when Vin is sourced through V+.
Steve G
 
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