555 sawtooth and triangular wave generator: from theory to practice

Thread Starter

Baio

Joined Feb 17, 2020
7
Hi, I was looking since some days for a schematic to build a sawtooth and triangular wave generator, then I found this schematic on an online simulator: https://www.falstad.com/circuit/e-555saw.html (I report the original link, obviously, to give credits to the person which has made this schematic), which seems logic and reliable to me. Then I edit this schematic by adding a resistive 47k potentiometer between R4 (10k resistor) and the PIN 7 of the 555 IC, and by the simulation, by adjusting the potentiometer, the circuit is able to give a sawtooth and a triangular wave as reported in these images:

Sawtooth Generator 2 - Sawtooth wave.png
Sawtooth wave

Sawtooth Generator 1 - Triangle wave.png
Triangular wave


I would ask: is this configuration Ok? For the transistor Q1 i will use a BD140 PNP (just because I have a handful of them).
I also used it in past on a square wave generator to increase the output current.

Can I also expect to measure in this circuit a gain in the output current, or such transistor would be excessive for the circuit? And in this case, would I have to change the values of resistors?
The need of this circuit which I will assemble is just didactic for me, there is not specific purpose.

Thanks.
 

Ramussons

Joined May 3, 2013
822
Capacitor discharge through a resistor will not be linear but exponential.
Capacitor charging is also not strictly linear. Replace the 27K resistor with a zener diode and replace the 39K with a suitable resistor for the 5.6 mA current for capacitor charging. The discharge setup will require a setup similar to the charge setup.
An OP amp integrator will be a much better option.
 

Thread Starter

Baio

Joined Feb 17, 2020
7
An OP amp integrator will be a much better option.
Firstly, thank you for your advanced suggestions; however I aim to build such circuit with the components which I have already available.
Anyway: did you tried to edit the simulation with your suggestions? I ask this because would be useful for other people which will see and follow this discussion.

The circuit seems OK, just do not over load the output, maybe 4 micro A.
And....don't forget to buffer the resulting triangle wave.
These two replies are certainly related.
Since I have - as I've said - a bunch of BD140 transistors, would be OK to implement such transistor to buffer the triangle wave output? I have various components and an oscilloscope: I could and would to - as example - load the output with an 800 mA load:
How You would implement the second BD140 (for the buffering purpose) in the schematic? There are various methods. Would be interesting to see the pro and cons of every methods, for everyone.

Many thanks.
 

BobTPH

Joined Jun 5, 2013
2,390
The load on your timing capacitor should be on the order of microamps. You will need, at least, a darlington to get anything like 800 mA out.

I would start with an op amp with a push pull darlington class AB output stage.

Bob
 

crutschow

Joined Mar 14, 2008
24,963
That circuit, with the constant-current drive transistor Q2, gives a linear rise but a exponential fall (curve) in the triangle waveform (as can be seen in the simulation plot.
Is that acceptable for your purposes?
 

Thread Starter

Baio

Joined Feb 17, 2020
7
That circuit, with the constant-current drive transistor Q2, gives a linear rise but a exponential fall (curve) in the triangle waveform (as can be seen in the simulation plot.
Is that acceptable for your purposes?
Sure, will be acceptable. Schematic example?
 

atferrari

Joined Jan 6, 2004
3,962
Don't you have two opamps available?

If so, integrating square pulses fits the bill. Ramusson already suggested that IIRC.
 
Last edited:

Thread Starter

Baio

Joined Feb 17, 2020
7
I was referring to the circuit you posted.
Sorry, I misunderstood because in the schematic there is one transistor referred as Q1 and you talked about Q2.
No problem.

Don't you have two opamps available?
I could obviously buy any components and attemps to realize any kind of more advanced circuit, but I'd like to make it with components which I have already available; so I also aim to increase the output of the triangle waveform using the BD140 (so now I'm asking for the better way to implement it in the schematic): as I've said is just for didactic and fun, and also for testing a second hand oscilloscope which I bought one month ago: a Philips PM3243.
 

crutschow

Joined Mar 14, 2008
24,963
Below is the LTspice simulation of a 555 astable oscillator with two PNP transistors, similar to your BD140's, used as a Darlington stage to buffer the sawtooth waveform:
1582013627605.png
 

Thread Starter

Baio

Joined Feb 17, 2020
7
Below is the LTspice simulation of a 555 astable oscillator with two PNP transistors, similar to your BD140's, used as a Darlington stage to buffer the sawtooth waveform
You have been very kind and friendly, in devoting your time to design and share this schematic: many, many thanks!
I will study it, then I will assemble it and testing!
 

Thread Starter

Baio

Joined Feb 17, 2020
7
You have been very kind and friendly, in devoting your time to design and share this schematic: many, many thanks!
I will study it, then I will assemble it and testing!
I also tried to simulate it and it's ok. I guess that to control the output frequency I have to act on the R1 by also adding a potentiometer, am I right?
 

atferrari

Joined Jan 6, 2004
3,962
No simulation but real hardware.

Just 2 hours to pick up parts, do the laundry, several trips to toilette, assemble and test.

Real triangular waves as in your title. No sawtooth. From a previous experience with this circuit and confirmed now, very sensitive, requires careful decoupling.

Dual op amp TL 072 and lot of tremor hand (cannot avoid).

Buena suerte.

20200218_100343.jpg20200218_114336.jpg

20200218_114322.jpg
 

crutschow

Joined Mar 14, 2008
24,963
to control the output frequency I have to act on the R1 by also adding a potentiometer, am I right?
In my circuit both R1 and R2 affect the frequency and the duty-cycle of the output.
But my circuit was to illustrate the operation of the Darlington buffer, and is not a good circuit if you want to change the frequency and/or duty cycle of the output.

Below is a better circuit for that:
Pot U2 controls the duty-cycle and pot U3 controls the frequency.

1582043415254.png
 
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