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Schmitt inverter oscillator

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magester1

New Member
Hello,
I need some help with this oscillator, the one you connect the output to the input through a resistor and a capacitor from ground to input. I decided i would try it out since it looked pretty simple, however, im having some problems with the output.
I'm using a 74HC14, 20kΩ resistor and 100µF capacitor and instead of measuring the output i just connected it to a 10k resistor and an NPN transistor (C4367) with a LED on the emitter.
Now, using a stable 5v supply i thought i would get about 0.5Hz, which i did for just a moment, it started oscillating faster, then slower again, and so on.

I thought there could be some noise or something on the output, maybe the capacitor messing with it, so i connected a second inverter but i still got the same problem.

Can anyone help me with this?
As you might have already guessed i'm new to electronics, so i don't know much about this.

Thanks.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hello,
I need some help with this oscillator, the one you connect the output to the input through a resistor and a capacitor from ground to input. I decided i would try it out since it looked pretty simple, however, im having some problems with the output.
I'm using a 74HC14, 20kΩ resistor and 100µF capacitor and instead of measuring the output i just connected it to a 10k resistor and an NPN transistor (C4367) with a LED on the emitter.
Now, using a stable 5v supply i thought i would get about 0.5Hz, which i did for just a moment, it started oscillating faster, then slower again, and so on.

I thought there could be some noise or something on the output, maybe the capacitor messing with it, so i connected a second inverter but i still got the same problem.

Can anyone help me with this?
As you might have already guessed i'm new to electronics, so i don't know much about this.

Thanks.
hi,
If thats a 10K from the output of the 74HC14 ic, to the transistor base, its too high a value..
A resistor of 470R to 1K5 would improve the base drive, the LED with its series resistor will draw about 10 to 20mA.

Have you grounded the unused inputs of the 74HC14..???
 

Ubergeek63

Well-Known Member
capacitor leakage varies with voltage and temperature and you are likely using a cheapie. you can try an X5R or better capacitor and/or a divider to get the frequency down.
 

audioguru

Well-Known Member
Most Helpful Member
The 74HC14 has a very high input resistance so you can use a small value very low leakage film capacitor with a high value resistor.
a 470nF film capacitor with a 3.9M resistor.

The 5V power supply needs a bypass capacitor.
The 9V battery must be new because its voltage quickly drops to less than 7.2V which is too low for the 7805 regulator. Use a low-dropout 5V regulator with a 9V battery.
 

ericgibbs

Well-Known Member
Most Helpful Member
The 74HC14 has a very high input resistance so you can use a small value very low leakage film capacitor with a high value resistor.
a 470nF film capacitor with a 3.9M resistor.

The 5V power supply needs a bypass capacitor.
The 9V battery must be new because its voltage quickly drops to less than 7.2V which is too low for the 7805 regulator. Use a low-dropout 5V regulator with a 9V battery.
hi agu,
I dont see any post about a 9V bty and 7805 Vreg.?

He says he is using a stable +5V supply.??
 

ericgibbs

Well-Known Member
Most Helpful Member
Sorry,
I was dreaming.
Its those new high powered infra red lenses you are using, you are starting to read the posters thoughts...
I could use some new lenses, mine are getting really crappy.:)
 
Last edited:

magester1

New Member
In reply to Ericgibbs:
I did't ground the unused inputs of the 74HC14, i didn't thought it would make a differences... clearly i'm wrong, but why is it that i need to do it? (So that i can better understand it).
I'll try with a lower value resistor for the transistor's base, although i've used the 10k one with this same transistor many times and didn't have a problem.

To Audioguru:
I tried with ceramic and electrolytic capacitors, the problem seems to be the same. I'll use a film one and see what happens.

Thank you all for your replies.
 

ericgibbs

Well-Known Member
Most Helpful Member
In reply to Ericgibbs:
I did't ground the unused inputs of the 74HC14, i didn't thought it would make a differences... clearly i'm wrong, but why is it that i need to do it? (So that i can better understand it).
I'll try with a lower value resistor for the transistor's base, although i've used the 10k one with this same transistor many times and didn't have a problem.

Thank you all for your replies.
Hi,
The HC in the ic type indicates that its CMOS technology.
The inputs have a high input impedance and should not be left 'floating' as this can cause the ic to give erratic performance.

The 10K from a 5V supply into a transistor base,,, 5V - 0.7V = 4.3V.
So 4.3/10000 = 0.43mA... if your LED and series resistor required say 20mA, this would mean that the transistor would require a gain of 46.

Depending upon the type of transistor used, its possible that the transistor would not saturate.

I would have at least ~ 2mA to 4mA into a base for a 20mA collector current.

OK.?

EDIT: do you have cap decoupling on the 5V supply.?
 
Last edited:

ericgibbs

Well-Known Member
Most Helpful Member

magester1

New Member
I decoupled it but still got the same problem, i decided to change the circuit in the end: with 2 schmitt invertes, a cap and 2 resistors.
That one worked perfectly, by the way, i changed the transistor's base resistor for a 1k one and got the same result, so i guess the 10k resistor didn't make any difference.
 
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