I'm working on a project where I'm using 8051 microcontroller as a primary microcontroller which is interfacing with 4 relays. I have tested relays with two of my arduinos but when it comes to AT89S52 it hardly turn on a single relay.
I measured output voltage of Arduino pins with was connected to relays which was 5.12 V and 5.8 V respectively but when it comes it AT89S52 it gives 4.86 V. My relays need 5 VDC as an input which I'm giving externally and grounding nicely everything.
This is my relay board
I'm powering Arduinos by my PC and AT89S52 is powered by 12 V which is converted to 5 V with LM7805.