cd40106be based oscillators not oscillating unless holding potentiometer input terminal

Thread Starter

bignobody

Joined Jan 21, 2020
50
I built 3 voltage controlled oscillators the other week as the first part of a dronebox (https://www.schmitzbits.de/dronebox.html) using a cd40106be IC. They seemed to be functioning correctly.

I've now built out the rest of the circuit (though I am using a cd4051be as a demultiplexer instead of the 74HC151), but then noticed one of the oscillators always seemed "on" rather than oscillating. At first I suspected a loose/broken wire because it would work when I was holding it and stop when I let go.

I figured it would be quickest and easiest to just swap out the potentiometer - cut out the old one and solder in a new one so did that. Got the same result, and noticed a second oscillator exhibiting the same behaviour.

Experimenting a little more, it seems like the oscillators work fine if I physically hold the potentiometers by the input terminal. When I let go, they stop.

I looked over the data sheet again and realized I had not grounded the unused inputs on the IC as it recommends, so I did so to rule that out but the original issue remains.

Does anyone have any suggestions for me? I don't think a loose connection is the issue.
 

dl324

Joined Mar 30, 2015
10,072
I've now built out the rest of the circuit (though I am using a cd4051be as a demultiplexer instead of the 74HC151), but then noticed one of the oscillators always seemed "on" rather than oscillating.
The substitution isn't equivalent. CD4051 is an analog multiplexer. Component values aren't given for the pot on the output, but you're probably loading the oscillator output.

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Thread Starter

bignobody

Joined Jan 21, 2020
50
The substitution isn't equivalent. CD4051 is an analog multiplexer.
Ha, well I certainly chose badly there. You are correct, of course. I just looked at the 74HC151 datasheet again and saw the words "binary decoding", so yeah, poor choice on my part.

you're probably loading the oscillator output.
Would you mind explaining this a little more? I am connecting the oscillator outputs to an LED, which counts as a load, and to an A,B or C input of the demultiplexer (not that my CD4051 is going to work anyway).
 

dl324

Joined Mar 30, 2015
10,072
Would you mind explaining this a little more? I am connecting the oscillator outputs to an LED, which counts as a load, and to an A,B or C input of the demultiplexer (not that my CD4051 is going to work anyway).
Loading the oscillators will change the frequency and/or stop them from oscillating at all.

You can't hang LEDs on signals that you also want to have a valid logic level. CD40106 can't sink/source much current to begin with.

If you're not using the other 3 inverters in the CD40106, you can use them to buffer the oscillator outputs.
 

Thread Starter

bignobody

Joined Jan 21, 2020
50
Loading the oscillators will change the frequency and/or stop them from oscillating at all.

You can't hang LEDs on signals that you also want to have a valid logic level. CD40106 can't sink/source much current to begin with.
Good to know, thanks! Funny how all the simple oscillator tutorials want to show you how to make an LED blink with them, haha.

If you're not using the other 3 inverters in the CD40106, you can use them to buffer the oscillator outputs.
Something for me to research. Thanks!

That puts quite a heavy load on the oscillators. First try disconnecting the LED and see if it fixes the oscillator problem.
I will try this. Thanks!
 
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