Drive transistor not fully switching off when using different Microcontroller

[ImogenWren]

Symptom:

• Coil remains energised when it should be in "off" state

Cause:

• Microcontroller output has insufficient voltage or current, leading a few hundred mV across the base resistor.

Solution:

• Ensure that voltage output from Microcontroller is equal to supply voltage.

Future Revisions:

• Modify PCB with jumper for 5v or 3v3 selection

[timdrysdale]

A wee nuance that I don't think a 5V/3v3 jumper will fix :- the issue arose for me originally when changing from a 5V arduino variant with a regulator, to a 5V variant without. With the second variant without the regulator, the 5V supply voltage was no longer the same as a high digital output. So, the non-regulated arduino was providing 5.2V to the emitter and 5V to the base of the drive transistor when the coil should be off, but that 200mV difference between the supply and the digital pin voltage was enough to leave the transistor partly on. So ... the boards would be better redesigned to draw power from a digital output, to ensure that regulator presence/absence cannot prevent that transistor being switched off. That would also avoid needing a jumper. Would then want to provide an electrolytic cap and a poly cop as a reservoir for when the coil needs to switch (so that when coil pulses, the power is not exclusively coming from the digital pin). Could you also check whether those encoders can run on 3.3V? I think they might be TTL compatible and need 5V but it's been a while since I last looked at their data sheet.