I am to trying breadboard a square and triangle generator similar to this one.

Triangular-wave generator

While there were a couple of differences (such as a potentiometer) in my original schematic, this simplified one still simulates well. (Positioning of op-amps reversed.)

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I've now built this circuit on a breadboard about 10 times. I've changed resistors and capacitor values, breadboard, jumpers, several TL072s & a TL074. I've checked there's 24V across the op-amp and that ground is in the centre.

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At one point the circuit DID work as intended (a wonderful moment.) Unfortunately I'm unsure what circumstances changed (if any,) but it didn't last long. I was even able to change R10 and C1 to get the frequency down to 0.25 Hz which is about where I want it, but when I came back to it, no luck.

At the moment, I just get -10.7V (to ground) from the integrator and +11.3V from the square wave generator.

You might say there's just a bad connection somewhere but I've built this thing enough times to start doubting that. Is there anything that jumps out or might be less obvious? I noticed once when I switched power on, I got a split second triangle. Do I need to kick it into oscillation somehow?

EDIT: circuit working with R1 at 47k and 22uF polarised cap.enter image description here


1 Answer 1


Change that 100k resistor between the opamps (R1 in the top diagram,) the input to the Schmitt trigger, to 47k. Also the electrolytic should be a non-polarised capacitor because the voltage across it reverses.

You could add a second capacitor in series with the first but place them back to back (opposite polarity) to circumvent the unipolar problem.

  • \$\begingroup\$ And that's why they pay you the big bucks! Reducing R1 worked a treat. Thank you! My experience doesn't agree with the capacitor comments though. I'll add a photo to the OP of the circuit working with a 22uF polarised cap. Very welcome to hear why/if this is a bad idea, but it's working. There is even oscillation with the polarity reversed (to schematic) but the square is...not square. \$\endgroup\$ Jul 21, 2022 at 6:21
  • \$\begingroup\$ @JordanLane It's not good practice to apply reverse voltage to an electrolytic even though it will still charge with the same time constant. In your circuit the charging current is limited to a very low value by R10, the 100k resistor, minimising any detrimental effects. If you were to apply a fixed reversed voltage across the capacitor then the current can increase in a less limited way and destroy the capacitor. If the output saturation limits of the Schmitt trigger aren't symmetrical about ground then the "square" wave will not be square because there will be asymmetrical charge..... \$\endgroup\$
    – user173271
    Jul 21, 2022 at 6:45
  • \$\begingroup\$ ..... currents through R10, the resistor through which the capacitor charges and discharges. You could get around this by using a single voltage supply (as in the first diagram) and adjusting the reference voltage. Incidentally, to increase the amplitude of the triangle wave, just increase the value of that 47k resistor but not too much or the circuit will stop oscillating again \$\endgroup\$
    – user173271
    Jul 21, 2022 at 6:50

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