Relay driver design documentation

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diff --git a/relay-driver-bjt.txt b/relay-driver-bjt.txt
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+M 832 320 880 320 0 2.5
+w 832 352 832 320 0
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diff --git a/relay-driver-specs.md b/relay-driver-specs.md
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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
+