Circuit Corner - Issue 3

Call it an inspiration streak, I'm on a roll and I keep coming up with new circuits. Today I sat down to play with a Xilinx CPLD, and I needed a few circuits to do it.

Cheap Voltage Regulator

An inexpensive, reasonable performance 3.3V positive voltage regulator.
The first thing I noticed was that the LM3940 3.3V regulator, which I was planning to use, gave me a nice +5V on the output pin when I first fired it up. Expecting that this was just unloaded performance, I attached a load resistor, but to no avail. I checked the datasheet again, and it specified that the regulator requires a relatively large capacitor at the output or it will become unstable and oscillate. Frankly, I don't trust any regulator that lets more than the specified voltage through, ever. The datasheet was very short on actual details, but I suspect that it's actually a charge-pump switching regulator disguised as a linear regulator.

Now, the electronics store is an hour away by transit, and I didn't feel like making the trip for a regulator that may perform equally badly. So I decided I would just use a common-emitter amplifier on a resistive voltage divider instead. A simple, thoroughly tested design.

I ran into a slight snag along the way. When under load, my circuit experienced a voltage sag large enough to bring the operating voltage under par. So I replaced the single transistor with a darlington pair. I used BC548's, but the type of transistor makes little difference as long as they have a substantial gain. I also added a bias resistor to keep the output transistor in the active region, to prevent a slight overvoltage condition while unloaded (poor regulation while unloaded is not serious, but it is annoying).

Simple Logic Probe

A simple, inexpensive, versatile logic probe.
I also wanted to be able to read back a pin or two, so I built a simple logic probe on the board. This is a reduced-cost variation of a probe that I have been using successfully for many years. This one is about as cheap as you can get. But don't let that fool you! It will probe virtually any family of logic, at virtually any voltage level. It is safe to input a wide voltage range into the input pin.

You may find it useful to add a voltage drop of some sort between the input resistor and transistor, like a diode or two, to increase the width of the "off" voltage range. This version considers the start of "on" somewhere slightly above 0.7V. Since I am going to be probing 3.3V CMOS, this version should do fine for me. These are so simple to build, I usually just build the probe directly on a breadboard rather than soldering one up.