Re: Driving two transistors with same signal

On Sat, 10 May 2008 08:35:11 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Thu, 08 May 2008 19:11:47 +0200, vic <news@xxxxxxxxxxxxx> wrote:

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 ?

---
Yes. View in Courier:



+5V>-------+-----------+-----------+-----------+
| | | |
[100k] [100K] | |
| | | E
+-----------|-----------|---------B 2N3906
| | | C
| | E |
| +--[10K]--B 2N3906 |
[10k] | C |
| | | |
| C |A |A
VIN>-------+--[10k]--B 2N3904 [DISPLAY] [DISPLAY]
E | |
| [150R] [150R]
| | |
GND>-------------------+-----------+-----------+

---
Oops...

Missed the part about when the input signal floats.

JF
.

Relevant Pages

  • Driving two transistors with same signal
    ... 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. ...
    (sci.electronics.basics)
  • Re: Driving two transistors with same signal
    ... I tried using a NPN transistor for display1 and a PNP for display2, ... 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. ...
    (sci.electronics.basics)
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