suitable transistor

johnlong

Senior Member
Hi All
try to interface the sparkfun uSD data logger to write a once a day file
I have it running and writing but if I remove the card with the power connected
it goes into a hissy fit with both leds flashing alternatively
So I figured why not place a transistor on the V+ as a switch turned on via the picaxes pin so when its
not required to write the card can be removed and the unit does a complete
reset on power up.
can any one recomend a suitable transistor that will not lower the 5v on the power rail to much
as to stop the unit from functioning correctly
as the C33725 I have on hand is reducing the voltage to much for correct usage
sparkfuns data sheet
regards john
 

AllyCat

Senior Member
Hi,

The C33725 appears to be a derivative of the BC337 NPN transistor, but you need to use a PNP transistor (with Emitter to the supply rail, Collector to the card/reader). The corresponding PNP transistor is the BC327, which is exactly what I would have recommended (without checking the exact drain current requirements of the card/reader). I'd start with a series Base resistor of about 1 kohms (from an output pin pulling to earth).

Beware that with such configurations you may need to be careful about "Phantom Powering" where voltage is supplied to an "OFF" device via the Data or Control Lines from an "ON" device).

Cheers, Alan.
 

johnlong

Senior Member
Thanks Alan
the device is set to T9600 where I pull the serial out high pause 100 before transmiting any data after which the write the line is held low
until the next write. Will that help with any possible phantom powering issues
regards john
 

AllyCat

Senior Member
Hi John,

Yes, that should be a good solution, and/or a series resistor of about 10k - 22k in the data line(s) could also avoid the issue. BTW a 1k series Base resistor (or a little higher) looks about right for a 3.3 volt supply, and almost any "Small-Signal/General Purpose" PNP transistor should be satisfactory.

IMHO, the Sparkfun data would be more "helpful" if the arrows indicated the actual direction signal flow, e.g. RXI <-- TXO (and maybe used the more conventional RXD and TXD names). But I think the direction of GRN --> DTR ("Data Terminal Ready") is correct (i.e. DTR is an output from the Card/Reader so shouldn't present a phantom power issue).

Cheers, Alan.
 

johnlong

Senior Member
Hi Alan
Have placed an order for the BC327 from fleebay so will give it ago when they arrive
thanks for the advice
regards john
 

johnlong

Senior Member
Hi Alan
the p-n-p arrived today breadboarded it up and connected the base to grn via 1k ressistor as sugested works a treat.
reading 0v at rest
Put a 5 minute delay in to see if I can remove the card and reinsert it without a hissy fit.
works a treat able to take it out open it up read the data put it back no issues
thanks for the solution so will attach it to the master code
ever upwards and onwards.
regerds john
 
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