Current Controlled Oscillator

synaesthesia

Yesterday elektrouwe, Christoph from roboternetz.de, and me had a great time during a spontaneous Lunetta meeting organized by Uwe. Thanks Uwe! We didn't build anything, but we had a lot to discuss. Christoph showed me a circuit that would divide down a pulse train and that sparked an idea. The original circuit used a 4066 analog switch, a pot, and a diode in series. Instead of that I though about using a resistor network and a transistor in series and omit the diode. It actually worked right away.

I call this thing a pulse controlled oscillator because it needs a high frequency pulse or square wave to work. These pulses are sent to the transistor which opens for short times and allows a current from the resistor network to load a capacitor. Once the capacitor voltage reaches the trigger level of the 40106 Schmitt-Trigger inverter, the output changes to low and the capacitor is immediately discharged. Then this process repeats, resulting in a series of short negative pulses at the output of the inverter. The frequency is determined by the current that is switched onto the capacitor. By using a resistor network of three resistors plus a pull-up resistor, I can generate eight different tones by connecting the resistor to logic levels from other CMOS chips. Five oscillators could be built this way from the inverters of a single 40106 chip, sharing a high frequency oscillator made from the sixth inverter.

Below is a circuit that shows how the oscillator could be used. A low frequency oscillator from one inverter feeds a 4040 counter to get a series of low frequency clocks. These can be patched to the inputs of two pulse controlled oscillators. Actually, you could build two more and still would need only one 40106 chip. If you try that I recommend that you use two 680nF capacitors for the additional oscillators.

By connecting the resistor network to different outputs of the 4040 counter, you get different sequences of eight tones. In addition to the resistor network, I added three diodes connected to the base of each transistor. Two or four diodes would be possible as well. These work as a wire-AND gate together with the 100K base resistor. By connecting the diodes to the 4040 counter outputs you will get different rhythm patterns for the melody by disabling the oscillator every time any of the diodes is connected to ground level. You may leave any of the inputs open, there is no need to connect each of them.

In this circuit the output frequencies from the oscillators are finally added via 47K resistors. Instead of using resistors to mix the outputs, they could also be combined by a logic gate and then fed the result to a counter or flip-flop to get a square wave again. That sounds much nicer, but I wanted to keep the circuit simple.

In the recording I start with a nice pattern and after a while begin patching the inputs randomly to other outputs. At about 3:00 min I increase the speed of the 4040 clock for a while. The sound of the circuit is pretty raw. If this is too raw for you, try adding a delay of 0.2 to 0.4 seconds to it and it will become really interesting.

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glot · Joined: Jan 25, 2015 Posts: 15 Location: boston

cool circuit.

could a saw wave be taped from the capacitor?

synaesthesia · Posted: Sat May 02, 2015 11:56 am Post subject:

Nah, this circuit wasn't ready for posting yet. There are a few things I need to add. First, the high frequency switching does have an effect: it does divide the pulse rate down a bit. However, the oscillator works fine without it as well. To make things simpler I removed it. The capacitor needs to be increased a bit then. Also, a small resistor in series with the diode in the feedback path makes the pulses a bit wider. That makes it actually a current-controlled oscillator. I changed the topic text for that reason. And finally, two diodes for the wired-AND are enough. I haven't used all three of them so far. That makes a couple of changes and thus here are two more pictures and a new recording.

I am still not perfectly happy with the circuit yet. Even with two identical capacitor values (well, face-values), there are a lot of dissonances. The capacitor and resistor values will never be so well tuned that the frequencies become harmonic. Maybe there is a way to sync the oscillators somehow so that the dissonances go away? Any ideas are welcome.

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blue hell · Posted: Sat May 02, 2015 12:04 pm Post subject:

Oh .. I had thought that would be what the high frequency OSC was for . for syncing ... lol .. maybe it should not be so HF then.

Edit : and have narrow positive pulses.

elektrouwe · Joined: May 27, 2012 Posts: 149 Location: Germany

synaesthesia · Posted: Sun May 03, 2015 3:09 am Post subject:

Thanks elektrouwe for adding the background info. I admit that there isn't much left of that spirit any more. My circuit seems to turn into a completely different direction. I haven't given up on the idea of the melody sequencer yet and the new approach is: if I cannot make the second oscillator harmonic, I make it percussive. The recording shows where I am right now. I guess this is going to become a kind of drum sequencer. Now I need to add a few more oscillators. Stay tuned.

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rico C · Joined: Feb 27, 2014 Posts: 26 Location: Redondo Beach

love it, thanks as always for sharing

Steveg · Joined: Apr 23, 2015 Posts: 184 Location: Perth, Australia

Hello electrouwe, Can you tell us more about how the frequency division works please?
I'm guessing it is set by the R8/R3 and R2R7 ratios (in the diode version). Is this correct?
Thanks.

elektrouwe · Joined: May 27, 2012 Posts: 149 Location: Germany

Steveg · Joined: Apr 23, 2015 Posts: 184 Location: Perth, Australia

Steveg · Joined: Apr 23, 2015 Posts: 184 Location: Perth, Australia

I think I understand how elektrouwe's diode circuit works now.

If we examine R1 and R14 we can see that A3 has an on:off duty cycle of about 1:10.

This causes R8 to have an effective value (R8eff) of 198K averaged over the entire cycle.

The division factor is roughly proportional to the ratio of time constants ( R8eff*C2 ) / (R1* C1 ) = (10n * 198K) / (3n3 * 100K) = 6 and indeed you can see six steps in the Saw1 waveform.

But wait! I hear you cry, elecktrouwe said the circuit divides by 7. Well yes, I you look carefully at the top of the waveform you will see a little spike where the seventh step triggers A1 and causes the wave to collapse back to 0. And if you count the flat tops of the steps which is the correct measure of the number of A3 cycles you can see there are 7. So elekrouwe is correct.

Similarly you can use the R3:R8eff ratio to calculate the duty cycle of A1 and thus the effective value of R2 and then you can calculate the R2eff*C2 / R8eff*C1 ratio and thus the N divisor. I will leave the calculations to the reader as I am out of mental arithmatic for today.

synaesthesia · Posted: Tue May 05, 2015 10:44 am Post subject:

synaesthesia · Posted: Tue May 05, 2015 1:44 pm Post subject:

One more sample with the claves and bass oscillators. I will use the last oscillator for my synth drum circuit. That makes a total of five oscillators: melody, bass, claves, beat and synth. I wonder how they will sound together.

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synaesthesia · Posted: Sat Aug 15, 2015 6:37 am Post subject:

Back to the oscillator. Here is a simple circuit that I patched today. It is using two CCOs driven by the current from a resistor ladder. Three slow oscillators drive the inputs of the transistor ladder. Because I have used LDRs with a pretty low dark resistance instead of pots in the three oscillators, I had to use rather large capacitors for C1,C2,C3. If you replace the LDRs by 100K pots, you can use much smaller caps, e.g. 10uF, 22uF, 47uF. RV1 is used to balance the CCOs. In the recording I started with RV1 fixed and then waved my hand over the three LDRs for a while. From 0:37 to 0:45 I changed the balance at RV1. After a bit more hand waving I turned RV1 back at about 0:58.

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