sine to TTL with a SMD transistor

Thread Starter

rompelstilchen

Joined Jul 22, 2014
5
hello ,

i try to convert a sine wave (0.25v peak to peak) to a ttl signal with a transistor

some guy could do it with a bc547 here : https://www.youtube.com/watch?v=eKHKGpOcllA

so i tried a simulation



it seems ok, but in practice i can barely have a tension follower (gain=1)

i use a BC848 that has a gain of 90 to 180
not much but it should be able to at least double the input and i cant make it work

any advice?

thanks
 

bertus

Joined Apr 5, 2008
22,270
Hello,

You are right that the 4093 will not put anything out, as the hysteresys is about 1.5 - 2 volts at 5 V powersupply.
The 0.25 v signal you have available will be to low.

Bertus
 

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ian field

Joined Oct 27, 2012
6,536
Hello,

If you want to turn a sinewave into a real squarewave, a schmitt trigger would be the tool to use.
On this page of the wiki, there is a two transistor schmitt trigger:
http://en.wikipedia.org/wiki/Schmitt_trigger

Bertus
Nowadays you can get single gate packages for various logic families - fair bet there's an integrated Schmitt trigger on the market somewhere, for 1 or 2 input gates; the package is barely any bigger than a SS SMD transistor.
 

MikeML

Joined Oct 2, 2009
5,444
What load impedance is the collector of the BC848 driving (not the 1K pull-up)?

Are you sure you had the EBC connections correct?
 

THE_RB

Joined Feb 11, 2008
5,438
That circuit should work perfectly fine with the right value resistors and a sine of 0.25v p/p.

You need to change the base voltage divider to have less impedance, and better adjustability.

Try a 100k instead of the 330 ohm resistor, and keep your 25k trimpot.

Now you will be able to adjust it with a lot more accuracy, and it won't be loading your sine amplitude down as much.

You can change R1 collector resistor to 10k too, it does not need to be as low as 1k.

To adjust the trimpot to suit TTL output remove the sine signal, and adjust the trimpot until the collector (output) voltage is 2.0v. That is about halfway between the TTL digital threshold voltages.
 

AnalogKid

Joined Aug 1, 2013
10,987
Here is a 1-transistor approach. It needs a low source impedance for the input signal. The average transition level is around 1.8V for TTL and 2.5V for CMOS. Adjust R2 and R3 for the right combination of duty cycle at the transition level and keeping the positive output peaks at 4V or more.

ak
 

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ian field

Joined Oct 27, 2012
6,536
but comparators seem to need a lot of peripheral componants

A comparator needs a voltage reference (including pull up resistor) and a high resistance for positive feedback if you want hysteresis like a Schmitt.

An op-amp needs all those things - and a much higher price tag if you want it to switch as fast and clean as a comparator.
 

THE_RB

Joined Feb 11, 2008
5,438
Here is a 1-transistor approach. It needs a low source impedance for the input signal. The average transition level is around 1.8V for TTL and 2.5V for CMOS. Adjust R2 and R3 for the right combination of duty cycle at the transition level and keeping the positive output peaks at 4V or more.
...
That circuit performs far worse than the original with the parts changes I suggested.

It's better to have a high impedance voltage divider for the base, and bias it with the transistor half on. Then the output vaveform will be more evenly distributed (closer to 50:50 duty) and more square.
 
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