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Unexpected voltage at base of transistor?

Sean_Miller

I have a situation I hadn't encountered before that is really bringing out my inner "noob".  Below is a circuit I'm developing for one of my 4D Game Engine IoT devices (future Element14 Presents video).

 

The simple question is - would one expect to see a voltage at an open base of a transistor?

 

Here's the details:

The SV1 is a 2x4 female header to plug in a ESP8266.  JP1 receives a 9V battery.  IC1 and IC2 are regulating down to 5Vs and then to 3.3V.

 

With the ESP8266 not installed, I probe pin three (the transistor base) to ground, I get 1.5 volts.  For that pin, if the ESP8266 sees voltage on power on, it will enter its bootloader mode.  In turn, its made the pin not an option to trigger the transistor because the voltage is instantly there when powered on and the circuit will be "bricked".

 

Is it normal to see a voltage on an base?  I rebuilt the board from scratch and breadboarded it just to ensure it wasn't a short, but the problem persisted.  I also added a 10K at the base hoping to "pull it down", but it did nothing for it.

 

image

 

 

My solution, of course, was to use another pin, but I sure would like to have that one for other projects in the future.

 

Thanks,

Sean

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  • Hi Sean,

    You will have to repost your illustration or schematic as it did not come through. Try using the icon for pictures in the header.

    John

  • in reply to jw0752

    That is strange the picture isn't coming through.  I had used the icon and it shows on my end.

     

    However, I believe I figured it out.  I'm mistaking what I'm seeing.  The transistor base is instead acting as a resistance to ground.  So it's like I'm probing a voltage divider circuit.

     

    So, the transistor is pulling the pin down.  The ESP8266 pins need to be pulled high to keep them out of boot mode when powered up.  So, the transistor in this circuit will always give me fits.  One document online talked about using an optocoupler which would simply be driving an LED inside the package which should work in theory.  I'll try that next. 

     

    Never thought I'd spend have >8 hours of probing & researching on one pin!!!  I guess that's what bakes it into the noodle, though.

     

    See ya',

    Sean

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  • in reply to jw0752

    That is strange the picture isn't coming through.  I had used the icon and it shows on my end.

     

    However, I believe I figured it out.  I'm mistaking what I'm seeing.  The transistor base is instead acting as a resistance to ground.  So it's like I'm probing a voltage divider circuit.

     

    So, the transistor is pulling the pin down.  The ESP8266 pins need to be pulled high to keep them out of boot mode when powered up.  So, the transistor in this circuit will always give me fits.  One document online talked about using an optocoupler which would simply be driving an LED inside the package which should work in theory.  I'll try that next. 

     

    Never thought I'd spend have >8 hours of probing & researching on one pin!!!  I guess that's what bakes it into the noodle, though.

     

    See ya',

    Sean

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