How to drag a wave form so it starts at 0 volts?

[djacob7]

I put together a triangular waveform generator and it works fine except it starts at 1.5V (to about 5V).
How do I drag the waveform down to zero?

 

[kubeek]

I think tweaking the the ratio of the 10k resistors should do the trick. The opamps are supposed to be able to swing their outputs very close to zero, so unless I missed someting it could work.

 

[Tony Stewart]

You can insert a 1.5V offset into the Pin5 by a separate resistor divider for pin5.
Disconnect Pin 5 V+/2=4.5V

Use 2V Ref on Pin 5 thus 4Vpp will be 0 to 4V

But this will not track V+ when battery drops.

A better way is to choose OP Amp that can swing full scale output ( rail to Rail. Out and In ) and then you can change R ratio gain to get full swing or change bias at Pin 5 to adjust.

 

[alec_t]

You can reduce the value of the lower 10k resistor to 4k; but the triangle wave will then be asymmetrical. Or you can use a third opamp to level-shift the triangle.

 

[Mosaic]

A.C decoupled and DC biased operating point. Blue wave becomes green wave.

 

[djacob7]

Thanks guys. I will try the easiest first.
Will report results.

 

[audioguru]

The LM324 and LM358 produce crossover distortion unless there is a fairly low load resistance to ground.

 

[djacob7]

Alec, would the output from pin 7 be capacitor-coupled to the next amp, or direct?
Would level shifting be done with a divider input to the negative input of the additional amp?

 

[audioguru]

C5 is the coupling capacitor to the level-shifting resistors R10 and R11. An additional amp is not needed but if it is wanted then it should have a high impedance input and not be inverting.

 

[djacob7]

Mosaic: That output scheme worked well. Thanks.
Right, guru!
Was using this circuit to drive a servo.
I hope this video link works...

 

[alec_t]

Alec, would the output from pin 7 be capacitor-coupled to the next amp, or direct?

Direct. But Mosaic's method would be better as the third opamp wouldn't then be needed.

 

[Tony Stewart]

You can insert a 1.5V offset into the Pin5 by a separate resistor divider for pin5.
Disconnect Pin 5 V+/2=4.5V

Use 2V Ref on Pin 5 thus 4Vpp will be 0 to 4V

But this will not track V+ when battery drops.

A better way is to choose OP Amp that can swing full scale output ( rail to Rail. Out and In ) and then you can change R ratio gain to get full swing or change bias at Pin 5 to adjust.

AS indicated, above quick and dirty solution. cut and add 2 R's
With no distortion to triangle and no large Cap required affected by loading.

 

[Tony Stewart]

For a more elegant solution a different design is required. or like TV sync tips. use an active square wave edge trigger to short with a FET switch a series cap to ground for a few microseconds but then floating from 0 to Vmax regardless of Vp-p amplitude or frequency adjustments or battery drainage from 9.5 to 6V.

 

[Mosaic]

Changing the duty cycle of the Square wave with the 77K & the 22K changes the the triangle wave form to a sawtooth.

 

[Tony Stewart]

When I had to generate a 100 KHz sawtooth with <50nS return ramp for Doppler mixing I just used a CMOS switch to choose the inverted output and then used a CMOS switch for the sample and hold phase detector using the incoming sine wave with a created zero crossing pulse to sample the sawtooth. Thus every beat cycle was equivalent to the rocket's distance travelled using the standing wavelength per cycle.

a null pot was used for the OCXO 1e-12. To match onboard and ground station.
Thus after a 2KM flight accumulated distance could be measured in meters and direction from two antenna with RF phase shift on the subcarrier for recovery. The USA TRACs system meanwhile,took two RF techs at a cost of $250K/yr annual cost up in Churchill. my fixed cost was ~1k$.
Velocity was the high pass or derivative on the beat sawtooth. Circa 1976.

 

[djacob7]

Tony thanks. It looks like an easy solution.

 

[Tony Stewart]

Do you need a sawtooth or just a triangle wave?
What range of f , V and load.

 

[djacob7]

Your bias solution worked great, Tony.
Ideally I want a sawtooth with a slow rise or an asymetric triangle.
I'm using this to drive a servo on a RC bird that flaps its wings (not an ornithopter).
I assume that the upstroke of a bird's wing is faster than the downstroke, that's why the asymmetry.
Range is 0-6V. Load estimated to be around 200mA (a tiny DC motor in the servo).
Here's a clip of a similar bird using a small gearmotor as the wing drive...

 

[Tony Stewart]

The motor current will be proportion to the voltage slope. and thus may burden your batteries if too fast because the gears have more load unlike a geared compressed spring release like that of a birds folded wing.

But then the servo will be slew rate limited.
WE could even emulate the flared pause before fast wing retract.

Is 0V to servo the low or high position?

try change 11 k to 4.7k if I guess right.

 

[Tony Stewart]

If polarity of ramp is correct.....

This simulation has 3 pots with sawtooth fine flap per second 2:1 range , coarse and mid position bias. with extra R's that you can tune yourself.

It will not go full scale without a slight gain change from 4Vpp to 6.

This non inverted motor for V vs wing position

Slow rise fast down