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CD40106 Schmitt trigger not working

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Chris Keogh

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I have recently been attempting to delve into the world of electronics and have been trying to build a very basic oscillator using the CD40106 chip. I have set up the circuit as follows:
9v to pin 14
pin 7 to ground
1 microfarad capacitor from pin 1 to ground
100k ohm potentiometer between pin 1 and 2
small speaker from an old stylophone from pin 2.

I can't seem to get any signal through the speaker. I appreciate that I probably require an amp but the strange thing is that I do get a (quite unstable) square wave coming from every other pin on the chip except the one i would expect it to come from. This is the same if I set up the same circuit with all the other schmitt triggers. The pin that I expect the signal to come from is always the only one that doesn't produce a wave.
I was of the understanding that all the independent schmitt triggers were not connected to one another but somehow the the resistor and capacitor connected to one trigger is causing all the other ones to produce a signal.
Does anyone know why this is?

Thanks.
 

JimB

Super Moderator
Most Helpful Member
I can't seem to get any signal through the speaker. I appreciate that I probably require an amp
The speaker probably has a low impedance and will load down the output of the 40106 and kill the oscillation.
Use an amplifier to listen to your oscillator.

but the strange thing is that I do get a (quite unstable) square wave coming from every other pin on the chip
CMOS chips have very high impedance inputs and will pick up any stray electrical fields and do unpredictable things.
The way to stop this is to connect the unused inputs to the 0 volt line (pin 7).

JimB
 

alec_t

Well-Known Member
Most Helpful Member
The way to stop this is to connect the unused inputs to the 0 volt line (pin 7).
By way of emphasis, Jim does mean unused inputs. Don't make any connections to unused outputs.
 

Chris Keogh

New Member
Thanks for your responses everyone. Seems like the logical thing to do is connect it up to an amp and reassess.

Thanks again for your help!
 

Tony Stewart

Well-Known Member
Most Helpful Member
Buffer it , connect 3 to 2 and out 4 to 5 then a speaker between 4&5 will get enough to hear.

Your circuit was shorting the feedback so it wouldn't oscillate. complementary emitter followers would help.
 

dr pepper

Well-Known Member
Most Helpful Member
Try a 1k resistor in series to the speaker, audio will be much reduced but theres a good chance you'll get some sound.
 

Tony Stewart

Well-Known Member
Most Helpful Member
Buffer it , connect 3 to 2 and out 4 to 5 then a speaker between 4&5 will get enough to hear.

Your circuit was shorting the feedback so it wouldn't oscillate. complementary emitter followers would help.
upload_2017-3-27_17-41-16.png


ground unused inputs or tie in parallel
 
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