# Level shifter

Discussion in 'General Electronics Discussion' started by Sravanthi, Apr 1, 2015.

1. ### Sravanthi

72
1
Feb 4, 2015
Hi....

I have one small doubt.....

In my design I have zero crossing detector which is giving the output voltage as +12V and -12V.
I want to shift these voltage levels as +12 V as +2.5V and and -12V as 0V.
So can you give any idea how to do this....

Thank you....

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
2,830
Jan 21, 2010
Can you show us your current circuit?

3. ### Sravanthi

72
1
Feb 4, 2015
Hi...

I have Zero crossing detector which gives +12V and -12V.
When ever falling edge comes I want to make flipflop clear.
So I have a doubt that will falling edge of +12V to -12V gives clear input.
Is there any limit of voltage level to be applied at clear input.

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4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Use something like this:

5. ### hevans1944Hop - AC8NS

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Jun 21, 2012
All of the inputs to the dual D-type flip-flop, SN74HC74, must be CMOS level inputs: -0.5 V < Vi < (Vcc+0.5) V

For a 5 VDC supply this generally means all inputs must be between ground and +5V.

The easiest way to guarantee this with ±12 VDC outputs from your UA741 op-amp (which is NOT a comparator) is to apply a "catch" diode for positive outputs and a "clamp" diode for negative outputs.

For the "catch" diode, its cathode is connected to +5 Vcc, its anode is connected to CMOS input terminal, and the output of UA741 is connected through a 10 kΩ resistor to the CMOS input. The "catch" diode will conduct when the UA741 output exceeds +5 V plus the foward voltage drop of the "catch" diode. Also required is a "clamp" diode with its cathode connected to the CMOS input and its anode connected to the SN74HC74 common or ground. The "clamp" diode will conduct to prevent negative UA741 outputs greater than one diode forward voltage drop from being applied to the CMOS input.

This combination of a 10 kΩ current-limiting resistor and two diodes must be applied at every CMOS input subject to ±12 VDC voltage swings. Small-signal diodes are recommended, although general-purpose power diodes will work if the level transitions are not too fast.

The active-low CLEAR (RD) and SET (SD) inputs of the SN74HC74 are level-triggered NOT edge-triggered inputs. Design accordingly.

Edit: @(*steve*) beat me to it. Schottky diodes are good. Wish I had one of those nifty schematic drawing programs. <sigh>

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
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Jan 21, 2010
Only just...

And I use the schematic editor that Digikey provides.

7. ### hevans1944Hop - AC8NS

4,606
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Jun 21, 2012
Just now tried that editor. Very spiffy!
How do you get the background grid to appear with the schematic when exporting the drawing?

8. ### BobK

7,682
1,688
Jan 5, 2010
Looks to me like he did a screen capture.

Bob

(*steve*) likes this.
9. ### hevans1944Hop - AC8NS

4,606
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Jun 21, 2012
I think you must be right. I used to use PrintKey 2000 at work, but after just installing it on my laptop, I can't get the rectangle selection tool to work.

Last edited: Apr 2, 2015