Relay Driving Circuit, Why 3 transistors?

[GeorgeIoak]

I'm looking at a circuit that's driving a small signal relay. I've seen single transistors to drive the relay and I've seen Darlington pairs to drive the relay but this circuit has 2 NPN and 1 PNP. I simulated the circuit and the circuit works but I'm wondering why use this circuit?

Attached are the simulation snap shots of the relay in an OFF state and the relay when it's ON.

The 10Hz signal on the left is just to simulate a signal from the 3.3V MCU. The 3.3V and 5V at the top are power sources and everything else is just probes for the simulation

[daqq]

This circuit achieves two things:
- the relay is it's powered from +5V and the logic is powered from +3.3V, so level shifting (although there are easier ways in this setup to do it)
- it inverts the signal - when the input signal is low, the relay is on and vice versa.

[GeorgeIoak]

Thank you.

So just so my thinking is clear:

• With 0V on Q1, it is OFF and Q2 is ON which then pulls the gate of Q3 low and turns it ON
• With 3.3V on Q1, it is turned on which pull the gate of Q2 low turning it OFF and keeping Q3 OFF

Transistors numbered from left to right