Can't keep a pin low

[bbarney]

the one you picked out is a medium power transistor not a switching transistor,that's why there's a big differance in the price of the 2.As for the size sot-23 is the more common one to use and easy to solder.there's probably better and cheaper transistors that will work ok I was just looking for a swithing transistor close to the spec's you gave

[bartman]

I was reading the link that Luciano supplied regarding bipolar transistors. It mentions a second resistor tied to the positive side to ensure there is no negative voltage at the transistor. It also mentions this is not required, but it sounds like good practice.

Now, that link doesn't specifically talk about the transistor being driven by a PIC so I want to know if that second resistor is still good practice in that case or not?

I'm about finished designing my PCB and want to add anything I might need now and not try to fit it in later.

Thanks.

Bart

[Luciano]

Hi,

Upon power up, your PIC I/O is an input high-impedance (tri-state)
and then with your code you make it an output. Without R2 the
base of the transistor will be left "floating" when the I/O is
in high-impedance state.

* * *

Battery operated devices

Hypothetical scenario:
Only the PIC is switched off with a switch and the
emitter of the transistor is directly connected to the battery.

When the PIC is powered down what is the voltage on its I/O?
In this conditions, do you have still some current flow in R1?
If the transitor "switch" is not completely turned off, will the
sound module slowly discharge the battery?

* * *

How to measure the current in R1:

To do this measurement you will need a high-impedance voltmeter.
(Digital multimeters with LCD display are OK).

With the Digital multimeter set for "DC volt" measurements,
measure the voltage dropp across R1.




Example, your Digital Voltmeter measures 1V across R1:
(1 V is dropped across R1 and your R1 is 10000 ohm).

I = V/R


To calculate the current flow through R1:

1V / 10000 Ohm = 0.0001 A (0.1 mA)


Best regards,

Luciano

[bartman]

There is a master on/off switch on the battery to power the sound module and the PIC so when it is "off" it is really "off" to everything. I'm not worried about draining anything.

I just want to ensure when both circuits are active that when I want the pin low it stays low and when I want it high it stays high. I don't want to take the chance that it floats or changes state for no reason.

Since that diagram and article didn't mention connecting it to a PIC output I question that second resistor. I have a feeling it would keep everything too high all the time.

I also would like everything to start up low, but I think the PIC is fast enough that it reads my mute flag and shifts the pin before there is enough time to even notice a delay.

Bart