Triangle

40106 Oscillator Continued

Sam
40106 Oscillator Schematic

I left my last post (40106 Triangle Waves) on a bit of a cliff hanger. I had shown how to pull a triangle wave from the circuit and identified some issues with the oscillator as it stood. Today I’d like to go over those issues and how I corrected them to get this oscillator up and running.

I do want to mention the oscillator design above is still in a somewhat rudimentary form. I expect there will be quite a bit of optimization that can be done on it as time goes on.

Buffering

Since we are pulling our outputs directly from the loop which sets the frequency, any elements we add to the circuit which draw current will affect the frequency. This is not ideal. What we need to do is isolate the oscillator circuitry from any further additions to the circuit. For the square wave this is extremely straight forward. Since the signal is binary (digital) we can simply send the output through a second inverter on the 40106 chip. Take note: this will invert the signal (when the original oscillation was high the buffered oscillation will be low) however since this is a repeating signal it won’t cause any impact.

The triangle is slightly more difficult to buffer since we need to concern ourselves with a range of analog values. I accomplished this by feeding the signal through an LM324 op-amp set up as a voltage follower with an additional 10uF capacitor on the output.

Amplification

After the buffering stage I was still left with dramatically different amplitudes for the two wave forms. The square wave after the buffer sat at almost 9V while the triangle was only 1.4V (The output voltage of the LM324). There’s a number of ways you can approach this inconsistency however I found myself somewhat limited by the parts I had on hand and by my decision to run this oscillator on a single 9V battery supply.

How I ended up overcoming this was by using voltage dividers to lower both signals to about 1V peak to peak. From here the selected input is sent into a very basic LM386 power amplifier.

This solution does introduce a large amount of noise into the square wave signal so you may choose to bypass the amplifier with the square wave and only use it on the triangle.

Set up this way I got both signals to reliably output approximately 5Vpp.

Decoupling

One issue I ran into a lot with this circuit, especially building on breadboards was noise. The frequency would bounce around and the wave-forms would not be crisp. This can be largely overcome by adding decoupling capacitors between the negative and positive supply lines. An excellent overview of decoupling (and many other common capacitor uses) can be found over at SparkFun.

40106 Triangle Waves

Sam

Recently I completed a post discussing the 40106 Inverter and a simple square wave oscillator. I wanted to build on that post a little more today and look at how we can modify this same oscillator to output triangle waves along with square. These triangle waves will sound profoundly different than square waves in the audible range, giving our oscillator 2 distinct voices. Additionally, we can use the oscillator at a low frequency to drive a voltage controlled amplifier or filter. We can get much more variance from the rising and falling triangle wave in these applications than we would with the simple switching of a square wave.

So Where Are These Triangles?

40106 Oscillator with Triangle Output

If you read my previous post you may remember that this oscillator works by charging and discharging a capacitor. As the capacitor charges and discharges it allows the voltage to rise and fall at the input. This rise and fall causes the output of the inverter to turn on and off producing the square wave. What we can also do is take an output at the input of the inverter where the voltage is rising and falling to produce a triangle wave!

An Amplitude Problem

If you try building this circuit as is you’ll very quickly notice an issue with the design. Connecting a speaker to the output of the square wave sounds great but the triangle is barely audible! If you look at the two waveforms above you’ll see that the square wave has a peak to peak voltage of 6.32V. The triangle however is only registering a fraction of that at 1.22V. This is because the output of the inverter (where we draw the square) always outputs a full digital signal. Meanwhile the voltage at the input only rises until it reaches a threshold voltage. At that point the inverter changes state and the capacitor begins discharging again. In this case (running the 40106 on 9V) that threshold appears to be 1.22V.

So What Now?

In my next post I’ll be exploring the use of op-amps to buffer these outputs and equalize them to a usable voltage. Once we’ve accomplished this we’ll be able to start using this oscillator in all kinds of awesome projects. See you all soon!