Need help with a transistor

[fxdif]

I want to switch a transistor (as below) that will supply current via the 100R resistor when power is applied. However, I would like to be able to turn the transistor OFF and ON using a micro. Any ideas?

[tt]

_______________ +5V
| |
| |
_|_ |
| | |
| | |/
| | /¯
¯|¯ |/
|________|
| |_|_ | | | | |
| | _|_
¯|¯ | |
| | | 100R
| | |
| ¯|¯
| |____________ rest of cct
|
_|___________________ GND

[/tt]

 

[MacGyverS2000]

Put another transistor (or FET) between the lower base bias resistor and ground. Control the new transistor/FET with the micro.

 

[fxdif]

[tt]

_______________ +5V
| |
| |
_|_ |
| | |
| | |/
| | /¯
¯|¯ |/
|________|
| |_|_ | | | | |
| | _|_
¯|¯ | |
| | | 100R
| | |
| ¯|¯
\ |
\ | |
\|_______/\______ micro
/| |
|/ | |
/ ¯ |_______ rest of cct
|
|
|_________________ gnd

[/tt]

I think this is what you mean.

The problem with this is that the micro will ultimately control the 100R load. What I need is when power is applied, the PNP transistor is on, regardless of the micro. Then I need to turn the PNP transistor OFF with a micro.

 

[MacGyverS2000]

Using the schematic you just drew, add a pull-up resistor to the second transistor's base. On start-up, the transistor will conduct and turn the whole shebang on. To turn it off by way of the micro, pull the base to ground.