My understanding is that a MPSA42 is an NPN transistor and the MPSA92 is a PNP. My understanding of how an NPN transistor works is that when the base “P” is turned on the circuit from collector to emitter is shorted. When the base is off the circuit is open. I’m trying to use the decimal point on a Nixie tube rather than an LED for the alarm. So, I have the collector hooked to the tube the emitter hooked to the ground, and the base hooked to 5V signal, I also have a 10K resistor between base and 5V. However, no matter what the decimal point stays lit!! Even when the signal is off. The only time it isn’t lit is if the base is open, if I even touch the base the decimal point lights up!! The transistor has the the imprint A42 B331. It appears to be a knock off and that could be where my problems are coming from. I’ve tried looking for a datasheet, but to no avail. Any help would be appreciated.
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