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Step 4: Using a Zener Diode

For unidirectional shifting from 5V to 3.3V, you don't get a much easier and straightforward (as well as cheap) solution as using a zener diode.  Find a zener diode that matches your voltage requirements, two common ones are 1N4733A (5.1V) and 1N5226 (3.3V).  You'll want to match your wattage; these are 0.5W diodes.

By attaching a load resistor inline before the zener diode, and taking the zener to ground, you can ensure that the voltage that gets past the zener is at or slightly below the zener voltage (Vz) of the zener diode.  In the example picture below, you can see that inputting a 5V signal outputs a 3.18V signal. 

In the second example, I'm "regulating" 5.1V after stepping down from 10V.  This isn't to be confused with a constant-current/voltage regulation.  This is best used as a signal with little to no load.
<p>I don't understand what CD4504 does. Does it change scale up the voltage it gets at its input? I am new to electronics please help.</p>
<p>The idea is that the CD4504 is powered at the higher of the two voltages, so it can output it's input voltage, say 5V, on the output, or, it can lower it to a voltage corresponding to the TTL equivalent. So, if a TTL device sends a HIGH signal, the CD4504 intercepts it, and if the select pin is HIGH, then it will ramp up the voltage if needed to make the corresponding value to the CMOS attached to the output of the IC. Grab the datasheet. It will probably explain it better than I can.</p><p>Good luck!</p>
<p>Hi! thankyou for this useful information. I am using a controller <br>which operates on 3V3. Output of controller goes to a buffer 74HC245 <br>(which operates on 5V). The VOH of controller is 2.9V(min) and the VIH <br>of the buffer is 3.5V (min). So there are chances that buffer wont <br>recognize the controller's signal as valid. I checked out the HCT series <br> of buffer that is 74HCT245 (it's VIH is 2V min). So is it right to use <br>HCT series instead of HC series. Can you identify the other measures <br>that can cause problem? or it will work fine? what would be the output <br>of HCT245 buffer when 3V3 signal is on the input?</p><p>Regards</p><p>Mehdi</p>
Thank you for the informative instructable. I am currently taking an electronics class and we are covering TTL and CMOS.
This is OK, I guess. But it's largely a decade or two out of date. You'd be hard-pressed to find either bipolar TTL (ANY of the bipolar ttl families) or 4000 series CMOS in any recently-published project. 74HC logic is pretty much the new standard.<br><br>Also, I think you're confusing two issues. One is that 5V TTL output levels (1 is &gt; 2.7V as you say) are not necessarily compatible with CMOS input levels. This turned out to be rarely a problem, and more with supposedly &quot;ttl compatible&quot; devices that output ~3V than with actual TTL chips (which have an output pretty close to 5V, unless heavily loaded.) The other issue is that CMOS logic families can frequently operate off extended power supply levels., while TTL was strictly a 5V family. And there isn't any way that an HC part running on a 2V supply or a cmos microcontroller running at 1.8V is going to produce logic pulse compatible with other chips running at 5V, regardless of logic family. So there you MUST have level shifting.<br><br>The reason CMOS has taken over is power consumption. A small 74xx IC consumed as much power as a modern cmos microcontroller, and did a whole lot less.<br>
The HEF and CD 4000 series are alive and well, if mouser/digikey/arrow/avnet inventory can say anything about it. Jameco also has a half page of original 7400 series IC's as well as 3/4 page of 4000 series IC's. My hunch is that people are still using them, even in modern designs.<br><br>I appreciate those two issues and they definitely are distinct problems. However, I don't touch on 1.8V ICs except with the maxim IC, which is powered at 5V anyway and intercepts all 1.8V signals. The main issue I discuss here between TTL and CMOS is that by their standards, there is a ~ 1V gap between what they each consider to be HIGH levels, and if a signal falls within that gap it is undefined and behavior is unpredictable.<br><br>

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Bio: Gian is a computational biologist and is the Managing Director at Open Design Strategies, LLC. He holds a BA in Molecular/Cellular Biology and an ... More »
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