I soldered a few modules for interfacing with some switches, LEDs and relays.
My problem now is a relay module with 8 relays controlled by one 74HC595 which should be used as a looper for guitar pedals.
When I switch on my box (atmega8, switch module, relay module) there is a loud click like all the relays are active at once but none of the attached status LEDs are active.
The code is not loaded at the time the click is noticable at the same second the power is attached.
Using the code to activate the relays seems to be ok, I can here the clicks, I can see the LEDs light up as desired. This seems to be no coding issue.
When I remove the board and connect only VCC and GND there is most of the time no click. Sometimes Q0 is high which means the first relay is active but not the attached status LED.
When Q0 is not high after VCC is attached I can put 5V to Q0 and the relay is active. Q1-Q7 same thing also the LEDs. Everything as desired.
I checked everything with the continuity tester and it seems to be fine.
At first I thought that maybe the trace for Q0 from the IC to the relay is somewhere touching the VCC trace but it's not.
Can someone give me a hint about whats wrong here?
There was no ULN around so I used BC337 and because I had no high power shift register I chose low power LEDs with the M74HC595B1.
I soldered on perf board and it may not be the cleanest build around.
Relay: FRT5 DC 5V single side stable