# Relay Driver Specs

## Relay Characteristics

* Model: SRD-05VDC-SL-C
* Internal resistance: 70 Ohm +-10%
* Nominal voltage: 5 V
* Nominal current: 71.4 mA
* Minimum pull-in voltage: 3.75 V

## RP2040 IO Characteristics

* Logic level: 3.3 V
* Max pin current: 
    * 50 mA across all pins (measured here: https://forums.raspberrypi.com/viewtopic.php?t=300735)
    * with 2 relays, ~20 mA per relay

## BJT Requirements

* Saturation Vce < 1 V
* hFE > 3

2N5088: Ic a bit high considering the max is 100 mA
BC549: same as 2N5088
2N3904: seems suitable

## 3N3904 Prototype

With a single relay and 2k base resistor:

* on current (emitter): ~60 mA
* base current: 1.3 mA
* on relay voltage: 4.3 V