# Help needed with a transistor

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#### fxdif

##### New Member
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?

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

Thanks

#### Nigel Goodwin

##### Super Moderator
Simply disconnect the bottom end of the lower base resistor, and connect it to the micro pin - when it's low the transistor will turn on, when it's high the transistor will turn off.

#### axr

##### New Member
Hello!

I've just been drawn for you draft with 3 versions of coupling CPU output for your application.
For sketch no. A) your CPU have to be capable to drive min. 10 mA of output current. Output is inverted.
On the sketch B) is 2 stage transistor amplifier. CPU port output current is appr. 1mA. Output is non-inverted.
And finally on C) is drawn a solution with MOSFET. CPU's output current is negligible. Output is inverted.

Best wishes
Axr

#### Attachments

• Switrans.png
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#### fxdif

##### New Member
Hi,

thanks for the diagrams. I don't think I explained myself very well.

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.

#### Nigel Goodwin

##### Super Moderator
fxdif said:
Hi,

thanks for the diagrams. I don't think I explained myself very well.

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.

No problem!. Simply have the micro pull the I/O pin low as soon as it powers up - is there any reason (and if so, what?) why you can't simply do that?.

If there is a plug and socket between the micro and the transistor, where it might not be plugged together - you could simply use two resistors in the lower base circuit - like this:

#### Attachments

• transistor.gif
2 KB · Views: 394

#### axr

##### New Member
Yes, this is the right solution for your problem !

73 Axr

#### fxdif

##### New Member
Thanks

Works great, many thanks Nigel.

Regards.

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