Need: Power switch between my circuit and PC parallel port

Re: Need: Power switch between my circuit and PC parallel po

Hello all,

I want to attached a schematic to the PC parallel port. My schematic is powered by 12v battery. I designed software that use one DATA pin of the parallel port to switch on/off my circuit in order to safe power of the battery.

So, my question is:

What is the best idea for a power switch between my circuit and the parallel port?
This switch will be driven from the software by means of DATA pin as I described above.

Added after 46 minutes:


I forgot to mention that the current drawn by my circuit is not more than 1mA!

Re: Need: Power switch between my circuit and PC parallel po

Ok i dont know exactly what you are asking, you could post that schematic online. if it is the data pin that controls your circuit you can try to use N- Channel power mosfet and connect the data pin to the gate of your mosfet and the ground of that data pin to the mosfet's source. make sure you check mosfet's data sheets .

Added after 2 minutes:

Your best bet is to draw schematic on p spice and upload it, more people could help you this way

Re: Need: Power switch between my circuit and PC parallel po

hello kukurigu,

If you are using the parallel port, make sure that the current that is going into or going out from any of the pins of the parallel port must be less than 12 mA. Some ports are rated even less than 12 mA. Above this rating the internal registers may bust due to over current. If you are not sure how much sink current is going into the port (in the case you are using the Status pins of the port) or how source current goint out from the Port ( in the case you are using the data pins of the port) It would be wise to protect the port from overcurrent by connecting optoisolators or buffer to the pins you used. since you are using +12V rail on your external circuitry, you can optoisolators. Remember the pins of the port will accept and produce only TTL voltage levels.

To increase Source current from the data pins you can use BJT or MOSFET as a switch.

hope this help

Re: Need: Power switch between my circuit and PC parallel po

if the current is so miniscule that your switching i beleve a 244 buffer chip has enough drive on its own well above i think around 150 ma
far in excess of your needs of 1ma

so
it is enough to just use a current limiter resistor followed by a switching diode


the only thing is that the diode will steal .6 v from the power rail

if this is an issue then you can play a trick on your external circuit by adding another diode to the negative rail to restore the dc level

however i think it will still be aok... to just use the diode instead

if you need to switch a 12volt supply you need to use 74hct4066
and wire it up as a switch powering it from 5volts raail from your switch ..pin

Re: Need: Power switch between my circuit and PC parallel po

I guess you want to switch on/off battery power what means that the switch will be located between a battery and a load ..
In this case you may consider an optocoupler and P-channel mosfet ..
See attached file for an example of a P-MOS switch ..
Regards,
IanP