Re: Driving two transistors with same signal

Dan Coby wrote:
"ehsjr" <e.h.s.j.r.removethespampunctuation@xxxxxxxxxxxxxxxx> wrote in message news:cxHUj.28009$zw.4986@xxxxxxxxxxx
vic 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 ?

Thanks.
Add diodes to isolate the bases, and resistors
to bias the transistors off when the desired
on signal is not present.

Ed

see below



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

The revised circuit does not solve the problem. Both diodes
will be conducting when the input is floating. As a result both
displays will still be on.

Would it work if, instead of diodes, I used Zener diodes of voltage around Vcc/2 ?
.

Relevant Pages

  • Re: Driving two transistors with same signal
    ... 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, ... Add diodes to isolate the bases, ... locations of the added resistors, ...
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  • Re: Driving two transistors with same signal
    ... 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 revised circuit does not solve the problem. ...
    (sci.electronics.basics)
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