An RP2350-Zero PCB

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phil99 Guru Joined: 11/02/2018 Location: Australia Posts: 3319	Posted: 03:12am 30 Mar 2026         Quote   why might it not work? Two possibilities are: 1) Capacitance. With the diode I found junction capacitance was important. A 1N4148 only has about 6pF and will only work with 22pF or more in parallel. Perhaps that applies to transistors also. 2) Reverse gain. MISO needs to be driven high and low so sufficient gain is also needed in the reverse direction (collector positive, emitter negative). The PN200 I used has a forward gain of around 500 and I measured about 30 in reverse. You won't find it in a datasheet so you need to do a breadboard test. It only works with a supply voltage below about 6V as that is the B-E reverse breakdown voltage.