I have a TLP521 driving a set of relays. The circuit is shown below.
To switch on the relay, I will need to sink the current to complete the P521 circuit, which will trigger the relay.
I am thinking to use a ULN2803A to sink the current as shown in the below diagram. This would mean that when I pull the Qa pin of shift register as HIGH, the 1C point (on ULN2803A) should be zero (grounded) and the circuit should be complete. Is this the correct way to do it? Please note that the common and gnd of ULN2803A are grounded and there is no +Vcc on the IC. Is this OK, or should I connect the COM to +5? (I don't want to source current to 1c by mistake or during initialization)
Another question: Can I get rid of ULN2803 and directly sink the current from 595? I dont think it can sink this current.