Howdy! I had a great time assembling the Larson scanner with my kids! It works, but two LEDs won’t illuminate, 4 & 5. We used the extra LED to test that they were correctly oriented, and they are, they just aren’t getting any volts. Most of the LEDs are getting… I think .722 volts when it is their time to shine, but those two middle LEDs are only getting millivolts of power. All of the solder connections look good visually. The chip isn’t getting warm. Any ideas? The resistors aren’t sensitive to orientation, are they?
Sorry to hear about the problem. There isn’t much to go wrong here, so you should — especially with a multimeter handy — be able to find the problem.
Take a look at the schematic (available here) to see how things are wired up. D4 is driven by pin 11 of the chip (which is physically located as the pin on the corner closest to D3), through resistor R4. With power to the circuit off, you can use the multimeter in resistance mode to verify that that pin (11) is connected directly to one end of R4 (with near zero resistance), and it has about 16 ohms total resistance from the pin to one lead of LED D4. Similarly, pin 12 of the chip drives D5 through resistor R5.
A few other notes:
– The resistors are not sensitive to orientation
– You should also check that there is no accidental connection between any neighboring pins near these areas of the board.
– If everything else looks good– the electrical connections are all good and so forth — it is possible that one or both of those LEDs were damaged during installation. That can sometimes happen with excessive heat or bending of the leads. There’s an extra LED there just in case, but we can spot you another if you need it as well– please contact us directly by email. :)