Circuit Corner - Issue 7

There are many times when we find ourselves requiring a piece of test equipment that we lack. Rather than doing without, I always found it worthwhile to try to build one. Here are a couple designs that I find useful.

3-State Logic Probe

An improved logic probe. Slightly more complex than the usual bicolor design, this one brings the additional benefit of being able to detect not only 1 or 0, but high-Z.
My standard two-transistor logic probe works very well for most situations, but there are some cases where it's just not enough. For example, it's unable to determine if a connection is out of place, or if a pin is an input or an output.

This is a simple logic probe I designed to deal with that deficiency. A comparison basis point is established with a pair of diodes and a 1K resistor forming a potential divider. This is then connected to a window comparator formed from a PNP and an NPN transistor. A complex biasing and reverse-protection network supplies input to these transistors and draws current from the input circuit. Presense of a source or sink on the input indicates that a logic level is likely present, absense thereof indicates a high-Z state.

The output of the window comparator drives a pair of buffer transistors, which in turn drive a pair of LED's. One LED should be green, and one should be red. When a logic level is present, the appropriate LED will light. When no logic level is present, or when a connected bus is in a high-Z state, then no LEDs will light. When the signal fluctuates rapidly, it will appear as if both LEDs are lit at once.

Ultra Low Voltage Continuity Checker

An improved continuity tester. Since it does not activate semiconductor devices, it is suitable for testing broken traces on PC-boards and similar things.
An email from a reader prompted me to consider the problem of continuity testing with extremely low test voltages. Low voltage continuity tests will not turn on semiconductor devices, so it cannot be used to test diodes, but in some circumstances, that is actually beneficial. For instance, when testing for breaks in printed circuit board traces, turning on semiconductor devices can cause false negatives. In some cases, it can even cause device damage.

The circuit I came up with uses 6 transistors, 2 PNP (I used BC557B) and 4 NPN (I used BC547B). The transistors MUST be the same kind as each other (though they can be your favorite brand of general purpose small-signal transistors), and if you want to beta-match them, all the better. Resist the urge to combine the two diode-connected transistors into one: the resulting circuit will not work (or, if you get lucky, it will work very poorly). Otherwise, this circuit is not too fragile.

I used a 9V battery to power mine. The LED and its limiter resistor can be replaced with virtually any indicator you want, including a sounder, so long as the indicator doesn't use more than about 20mA. If it uses more, the buffer transistor should be replaced with a darlington pair.

The circuit function is a bit subtle. The 33k resistor is used to establish a bias current through the diode-connected transistors, which is then mirrored back through the PNP transistors into the collector of the other transistor. Since the NPN transistors are run at different current densities, this creates a small voltage offset between the two sides of the mirror. Theory predicts an offset of about 26mV or so. In practice, the voltage of mine worked out to about 75mV, which is within expected tolerance when using components that are not beta-matched. It's also worth noting that at this current level, measurement error is nontrivial: my meter may easily have had a low enough input impedance to significantly effect the result.

Because in the default operating condition, the rightmost NPN is operating in the saturation region, all the current from the mirror is drained and the LED is off. When the two terminals are connected, the offset voltage causes the voltage that would normally feed the base of the right transistor to instead flow through the left pair, cutting off the right transistor and allowing the current mirror to feed current into the base of the LED buffer transistor, turning on the LED.

This works out to be one of the simpler ultra-low-voltage continuity testers. It seems pretty reliable from the testing I have done.