1
\$\begingroup\$

So I have two daisychained 74HC595 Shift Registers driving 16 low current LEDs, and the shift registers are wired to an Arduino Mega.

Initially the ciruit worked perfectly, the test code scanned left to right toggling one LED, a few weeks later it suddenly began to toggle random leds. Also, when you firmly press the cables together it becomes more stable.

I have tried adding more decoupling capacitors (100nF as per datasheet) directly onto the VCC and GND of each Shift Register, I've switched out the cables for a sheilded cable with twisted pairs and the measured current is within the Reccomended specs. Still no change.

I borrowed an osiliscope and the data/clock/latch seem to be fine, but it's a pretty old model that I am not too familar with and I could've made an error.

What could cause this 'noise' and how would you prevent it? Thanks!

EDIT: Quick Schematic schematic[1]

EDIT: After buying a new set of decoupling capacitors, everything worked.

\$\endgroup\$
  • 2
    \$\begingroup\$ "Also, when you firmly press the cables together it becomes more stable." Have you considdered the obvious?; A loose connection.. \$\endgroup\$ – Vinzent Jan 7 '18 at 19:51
  • \$\begingroup\$ It passed the multimeeter's continuity test, and I did replace & re-solder the wires after that. Will recheck. \$\endgroup\$ – Sidney Bruneder Jan 7 '18 at 20:03
  • \$\begingroup\$ You probably have a cold solder joint / loose connection, but what resistors are you using on the LEDs and what is the power supply voltage for the HC595? Multiply the current by the number of LEDs on a single chip and check the max current capacity of the HC595. If you are well over the specified max total current for the chip (check Vcc and Gnd max currents in the datasheet, 70mA for a TI HC595) you will probably have issues with chips dying over time. \$\endgroup\$ – Dean Franks Jan 7 '18 at 20:18
  • 2
    \$\begingroup\$ Without a schematic, no-one can offer realistic help. Please post one with accurate values - the schematic capture tool here is a doddle or you can post a photo of a hand-drawn schematic...but it better be an intelligible one :-) \$\endgroup\$ – TonyM Jan 7 '18 at 20:24
  • \$\begingroup\$ What are the cables used for? The 74HCxx series is CMOS and very sensitive to noise on any input pin. Are ALL unused inputs grounded? Are your cables going to their input pins? If so they should be shielded cables. \$\endgroup\$ – Sparky256 Jan 7 '18 at 22:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.