Re: Driving two transistors with same signal


"vic" <news@xxxxxxxxxxxxx> schreef in bericht
news:48233453$0$20289$426a74cc@xxxxxxxxxxxxxxx
Hi,

I have two common anode 7-segments displays, and only one wire to drive
them. I need to achieve the following : when the control signal is +5V,
display1 is ON and display2 is OFF. When the signal is 0V, display1 is OFF
and display2 is ON. When the signal is not connected (high impedance),
both displays are OFF.

I tried using a NPN transistor for display1 and a PNP for display2,
connecting their bases together. It works when the driving signal is
present, but when the signal is floating current flows from the base of
the PNP to the base of the NPN and both transistors turn each other on,
resulting in both displays being ON.

The circuit that didn't work :

VCC
+
|
|
___ |<
o---------------------|___|--|
| |\
| VCC |
| + |
| | Display2
| | |
| ___ |/ |
Input---o---|___|--| GND
|>
|
|
Display1
|
|
GND



Is there a way to achieve this ?

Thanks.


Try this circuit. Choose the resistor values to limit the base current into
the transistors, yet this current should be high enough to drive the
transistors into saturation.

+--------+------------------+-------+--Vcc
| | | |
___ |< | | .---.
in----+--|___|---| | | | |
| |\ | .-. | D |
| | | | | | |
| | |< | | '---'
| +------| '-' |
| | |\ | |/
| .-. | +-----|
| | | .---. | |>
| | | | | | |
| '-' | D | ___ |/ |
| | | | +-|___|---| |
| | '---' | |> |
| | | | | |
------)------------+--------+------)-----------+-------+--GND
| |
+----------------------------+


If your displays need GND on one side, then try this. Mind the type of the
transistors.

+--------+------------------+-------+--Vcc
| | | |
___ |< | ___ |/ |
in----+--|___|---| | +--|___|--| |
| |\ | | |> |
| | | | | |
| | |< | | |<
| +------| | +-----|
| | |\ | | |\
| .-. | | .-. |
| | | .---. | | | .---.
| | | | | | | | | |
| '-' | D | | '-' | D |
| | | | | | | |
| | '---' | | '---'
| | | | | |
------)------------+--------+------)-----------+-------+--GND
| |
+----------------------------+
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

petrus bitbyter


.

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