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# 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
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