# Seg 3: Ways to judgment of the polarity of the nixie tube

1. The judgment of the polarity of the nixie tube

Before giving the method, we ask ourselves a question. Why do we judge the positive and negative polarity of nixie tube? Since the nixie tube is similar to LED, to make LED bright, the polarity must be right. This can make LED switching into conduction.

Figure 6-3 Schematic diagram of Arduino measures the polarity

There are two general methods to judge the polarity of nixietube shown in the following.

First method: use directly the ports on the Arduino (heuristics)

1. According to the schematic diagram in Figure 6-3, we finish the connection of circuits, as shown in Figure 6-4, where, the voltage is provided by the 5V port on the Arduino. In such experiment, the value of resistor is R=220Ω, and the voltage is 5V.
2. Since we don’t know the COM port in nixietube whether anode or cathode, we don’t know how to connect the polarity of power. But, we can know the common pin of nixietube. So, we can let the negative connect to the COM port (i.e., 3 and 8 pin), then the positive can be connect to the redundant pins randomly. At this time, if the segment is bright, then it shows that the polarity is right. That is, the COM port is negative, and the other pins are positive. Then the segments for the nixie tube can be lightened. Moreover, this is a common cathode nixie tube. Therefore, generally speaking, if the segment is not bright, it shows that this is a common anode tube. However, to make sure it is common anode, we can change the order of the connection of negative and positive till some segment is light. Note that, a current-limited resistor must be connected to avoid the damage of LED. In Figure 6-4, the positive (in hand) is connected to the 5V port on the Arduino board, while the GND is connected to the COM port of nixie tube by a mother dupont line. After connection, the 10th segment is light, which in fact is corresponding to the pin 10 of nixie tube. This shows that the polarity is right, and the LED is conducted. Thus, we know that the negative is com, i.e., common cathode.

Figure 6-4 Using Arduino to measure the polarity of nixietube

Second method: Using the multimeter

In addition, we can also use a multimeter to measure the polarity of nixie tube. Its principle is also can be referred to Figure 6-3. In fact, the main principle is that the multimeter can be viewed as a power for the nixie tube, as shown in Figure 6-5.

Figure 6-5 Using multimeter to measure the polarity of nixietube

The measuring method is very simple. After the simple setting of the multimeter, as shown in the special marked label in Figure 6-5, the multimeter must be turned to the diode. The black pen is connected to the COM port (pin 3 or 8) of nixie tube, and the red pen is touched to the other remaining ports randomly. Then, if the segment is bright, it shows that the polarity is right. So, this is a common cathode nixie tube. In other cases, we changing the connection order of the pen. But, in this experiment, since the voltage is not very high, so the brightness is not enough.