Transistor switch to switch negative voltage?

[Martin33]

Hi,

Can I use a transistor switch to switch between 5V(Switch off) and -12V(On)? I am using outputs from a demultiplexer to drive the base of the transistor through a base resistor. I have -12V on the emitter and 5V on the collector(through a pull-up resistor). I have tested this on a bench and it appears to work fine but am I in danger of damaging the demux or the transistor? Perhaps a 12V zener in series with the base resistor would prevent harmful voltages reaching the demux o/p's? Any help on this would be gratefully accepted.

I need to generate these voltages to test an RS232 to TTL convertor and vice versa, and am using a timer/counter circuit to feed the demux cct. in order to automate this test and feed output sequencing led's. Hope I have given enough information here!

Thanks

 

[nbucska]

Use PNP e.g. 2N2907:
R1 = 5K to 20K
R2 = 1K to 5.1 K
R3 = 10k to 20K


Assuming 5V TTL:

Em -- +5V
Col -- R1.1-- RS232out
R1.2 -- -12V
Base -- R3.1
R3.2 -- TTLinput-- R2.1
R2.2 -- +5V


<nbucska@pcperipherals.com>