Help needed with a transistor - Electronic Circuits Projects Diagrams Free

fxdif · 30th January 2004, 12:06 PM

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** · 30th January 2004, 01:10 PM

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 · 30th January 2004, 01:11 PM

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

fxdif · 30th January 2004, 02:02 PM

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** · 30th January 2004, 02:36 PM

Quote:

Originally Posted by fxdif

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:

axr · 2nd February 2004, 06:32 AM

Yes, this is the right solution for your problem !

73 Axr

fxdif · 2nd February 2004, 09:30 AM

Works great, many thanks Nigel.

Regards.

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30th January 2004, 12:06 PM	(permalink)
fxdif	Help needed with a transistor 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

30th January 2004, 01:10 PM	(permalink)
Nigel Goodwin	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.

30th January 2004, 02:02 PM	(permalink)
fxdif	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.

2nd February 2004, 06:32 AM	(permalink)
axr	Yes, this is the right solution for your problem ! 73 Axr

2nd February 2004, 09:30 AM	(permalink)
fxdif	Thanks Works great, many thanks Nigel. Regards.