Level shift 3 V digital signal to 5 V with fewest possible components

Question

I'm trying to convert a digital signal which, when high, is a bit less than 3 V, to an equivalent signal where the high is 5 V. I have at my disposal the signal wire, a ground wire, and a 5 V wire. This signal runs at around 2.5 MHz.

I know there are a few different ways I could do this. For example, in this question: What's wrong with this single-transistor level-shifter? There is a circuit involving 3 resistors and 2 transistors which I think would work perfectly. However, I'm interested in finding a solution that uses the fewest number of extra components. Ideally, it would be great if I could find a single IC that does what I need. Does such a beast exist?

Answer 1

The schematic you linked to is an "emitter follower", whose output is always 0.7V lower than its input, so it can't be used to produce a 5V signal from 3.3V. The alternative (still using a single transistor) would be a "common-emitter" configuration, which is able to produce 5V and 0V from a 0V and 3.3V input, but it also inverts the logic level, which may not be what you want.

There are many ICs that do what you ask. Here are a couple designed for this very purpose:

• CD4504

• CD40109

The most overlooked option (in my opinion), though, is the use of 74HCT logic ICs. These CMOS devices can be powered from 5V, thereby having outputs which are 0V or 5V logic signals, but their inputs are designed to be compatible with the outputs from older 74LS TTL technology. Interestingly, TTL outputs are guaranteed to be at least 2.4V for logic high, and this is the threshold that the 74HCT devices are designed operate with, which happens to be perfect for 3.3V logic signals too.

In other words, a simple 74HCT08 (4 AND gates) or 74HCT32 (4 OR gates) can do what you need (as could many 74HCT ICs), and have the advantage of being:

1. Cheap
2. Modern
3. Readily available
4. Very fast
5. Also compatible with regular CMOS outputs
6. Usable in any logic circuit powered from 2V to 6V

They are really good jelly-bean parts for your workshop.

Oh, I nearly forgot, if you have noisy or slow slewing inputs, the tool for the job is the 74HCT132. This consists of 4 schmitt trigger input NAND gates. This is one of my favourite devices, and I use them a lot. Even a plumber should have a couple of these in his tool box.

Answer 2

That level shifter won't work for you -- VOUT = VIN-0.7 V.

If you can invert the signal, then use an NPN: E = GND; C = 1k to 5 V; B = 10k to the input. The output is the collector. This is 1 NPN + 2 resistors.

If you can't invert, then use an NPN: B = 10k to +5 V and 8.2k to GND (10k would do); E = input directly; C = 1k to +5 V. This is 1 NPN + 3 resistors.