![]() In the left photo, where the display is upright and the label is visible, pin 1 is beneath the lower-left corner. When you orient the part such that you can read the label, the effective polarity mark is on the left-hand side. You can sort of make out marks on the back side beneath the epoxy, so it’s tempting to look for orientation hints there, but these types do normally follow the label-orientation rule. That’s a big “+1” for checking the datasheet! While you’d normally expect that the pin to the left of the label would start at pin 1, this device (and apparently some other alphanumeric/7-segment displays) follows a convention where pin 1 is in the upper left of the display– opposite the decimal point. However, the label itselfis a polarity marker and it’s important to remember that.Įdit: We initially had written down that these follow the label-orientation rule, but that turns out not to be the case. LED displays can be pretty baffling, because the pin 1 location is not explicitly marked, and there’s no half-moon shape or other obvious polarity marker. Here’s one that we get asked about quite often: A 17-segment alphanumeric display. There’s a printed bar on the left hand side of this chip to act as a polarity indicator, taking the place of the half-moon shape. This chip also relies on a combination of text orientation and a bevel at the side with pin 1.Ĭareful: That apparent “dot” is not a polarity indicator pin 1 is still at the corner of the chip. Presumably, it has seven pins so that you can’t put it in backwards. It’s a neat little solid-state relay capable of switching small loads on AC line voltage (0.9 A at up to 240 VAC) from a low-voltage digital input. ![]() This is a somewhat unusual seven-yes-seven pin DIP chip. If you look very closely, you’ll see that there is one additional polarity marking feature, in that this chip also has a very slightly beveled front edge. As we discussed earlier, you can rely on the orientation of the text in cases like this, and imagine an effective polarity mark on the left hand side of the chip. The entire front edge of the chip– the edge containing pin 1 –is slightly beveled.Īnd now here is a chip that has less of a “direct” indication of its orientations– no dot or half-moon shape. This 74HC245D “octal bus transceiver” chip from NXP has the half-moon shape on the left hand side, plus a slightly more unusual polarity marking feature. Now, that dot actually looks like it’s closer to pin 2 than to pin 1– Again, the marker often labels the corner where pin 1 lives, not the individual pin. The orientation is given by the half-moon shape on the left hand side and by the dot in the lower left corner. It’s a wide, low-profile plastic package called a 66-pin TSSOP (and a 128M bit DDR SDRAM, if you’re curious). This is a modern higher-density variation on the same design. Each has a molded half-moon shape as well as a more subtle dot by pin 1. These are “ceramic DIP” integrated circuit packages, dated from the end of 1978. Here are some classic and beautiful examples of chips with well-marked polarity. The text orientation is consistent, and for chips of this shape (with pins on two opposite sides), you can reliably assume that the polarity mark goes to the left of the text. But there are also cases where there are *no* direct marks, but you can instead rely on the orientation of the text to understand the numbering. This device has 20 pins, numbered counterclockwise along the two edges from 1 to 20.Īs we’ll see, there are plenty of examples of this, or close variations on it. The polarity marks are a half-moon indentation on the left hand side as well as a dot by pin 1. In this sketch, we’ve drawn an imaginary part number “THX1138D,” manufactured in week 37 of 2013, and it has a mysterious lot or internal code “OHAI” that may or may not be explained in the datasheet. Often pin 1 is in a corner of the chip, and it’s only that corner- not the pin itself –that is marked by the small circle or triangle. Sometimes several of these marks can appear. Another is a small dot by pin 1, or sometimes a small triangle or tab instead. From that polarity mark, move counterclockwise around the chip, and number the pins starting at 1.Ī common polarity marker is a half-moon shape at one end of the chip. Here is a basic rule that applies for mostintegrated circuits: There’s a polarity mark somewhere. And for better or worse, this is the exception, not the rule. In the picture above, pin 1 is clearlymarked on the Allen-Bradley resistor pack. But if it isn’t, or if you’re simply new at this, it’s helpful to know what to look for. But which way does the chip go? Pin 23 is where? If you’re lucky, the orientation is clearly marked, or perhaps diagrammed in the datasheet. You’ve got your components, and your datasheet, and you’re read to start hacking.
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