Problems driving a relay: ESP-01 via I2C bus controlling PCF8574 to activate a Songle Relay

Question

In the past, I've been able to drive a relay using a 2n2222a via a GPIO pin given the same schematic with PCF removed. However, once I introduce the PCF8574, it doesn't work.

I have read that I should probably change the schematic to use sink instead of source, or a different transistor, or a different I/O expander - but I'm a noob and don't really know how to construct or modify my circuit accordingly i.e. what that specifically changes my circuit.

Do I add a PNP somewhere, or switch to a TIP120 - what's the easiest thing for me to do with the least modifications to this circuit?

I kind of get what is going on, but because I'm not an EE I don't totally understand the theory behind it. I have read every post on SO, and I think the transistor of choice doesn't get fully saturated by the PCF because it cannot source more than say 30 mA so it's not opening fully. I've read about pull-up 1k to 5 V on the outputs of the PCF, but that didn't seem to do anything. I do know the second I ground the collector it activates the relay, so I'm pretty sure the PNP is not fully opening. When I connect an LED, I did notice it was dim and less dim depending on the signal coming from the PCF.

Here is the Songle relay's datasheet.

Here is my current schematic (apologies for the poor layout, first time using Schematic Editor):

EDIT: Forgot VCC/GND connections on PCF.

EDIT 2: Added all other connection and components.

EDIT 3: :facepalm: 1k resistor label..

Answer 1

1kohm at the transistor base is way too high with PCF8574, due to the weak output high drive ability it has (marketing term is quasi-bidirectional IO, which allows a data pin to be an input and an output without a separate data direction register).

It might not work even with a 0 ohm resistor there.

Relay needs 160-180mA, therefore to drive the transistor fully on, the base approximately as a rule of thumb needs in the order of one twentieth to one tenth of that, about 8-18mA.

The PCF can output less than 1 mA, so it can't drive the transistor base with high enough current.

Try a 270 ohm to 470 ohm base resistor to 5V and have the PCF output directly to base. Alternatively switch the BJT to a logic level MOSFET, or use an IO expander with normal outputs, or change to a high side PNP transistor as the output can sink in 20mA current when driving a pin low.