From: <Matthias.Klumpp_at_gmx.de>
Date: Sun, 20 Jun 2004 11:41:21 +0200 (MEST)

Hi,

I was just observing a strange behaviour of my JB Studio 20.
Let me explain, what I know and what happened:

As I know the AJB's are operating with 3,3V.
As somebody from this list was telling me once, there is a pull up resistor
(R) inside the JB's remote pin.

With my remote control I'm connected with a open collector transistor (T).
Let me try to make a schematic in ASCII art of that:

         3.3V
          |
         |-|
         | |R
         |-|
          |
    +-----+
T |
  |/C
--|\E
   |
   - GND

T is a PNP BC557 transistor controlled by a Atmel uC.
JB is not connected to power.
When the uC board is powered up, the JB also receives power by a relay
(power source is the Archos power adapter).

What I was observing is the following:
- I'm powering my uC board and so also the JB.
- uC is setting the base of the transistor to high, so collector of PNP
transistor is floating.
- I'm measuring 3.3V at the JB's remote pin, so far so good.

Normally the JB shows now "Charging".

In that above described scenario, it is switching on the green LED, but not
displaying any text. It then accepts no key presses any more. I do have to
disconnect the power from the JB, press STOP and re-connect the power to it.
Then ist also displays "Charging" and I can witch it on by pressing the ON
key.

If I do the same, but holding down the base of the transistor to low level
by the uC (so remote pin is held down to 0V), the JB switches on the green
lamp AND displays "Charging".

Is anybody able to tell me the reason for that ?

Another question is more HW basics related:

Is there any difference in the following transistor circuits ?
What is the difference ?

Open Collector (with PNP transistor):

    +-----to JB remote
T |
  |/C
--|\E
   |
   - GND


Open Emitter (with PNP transistor):

    +-----to JB remote
T |
  |/E
--|\C
   |
   - GND


Many thanks to you

Matthias

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Received on 2004-06-20