Help ! I am stuck on stupid today..

[rshayes]

Can you use 2N3906 PNP transistors as emitter followers. No additional current required, since the emitter current is the output current.

[dacflyer]

gorgon >> the 7407 uses the 5v , but the led display will use 9v. i have no problem to use 5v to the ic..but the display will barely light at 5v.

stephen >> thanks,,but that would crowd the board way too much, that would be 24 transistors
for the total display

[positronicle]

--Edited by Positronicle--

[dacflyer]

positronicle >> the led display uses 6 volts min to light , so 7 volts max input to the ic should be safe. or will it ? i was looking at the PDF file for this ic .. http://www.fairchildsemi.com/ds/74%2F74ACT244.pdf

are you able to give me a partial schematic as far a hook up? i got a fair idea,,but i am not 100% sure... thanks

[dacflyer]

positronicle >> the led display uses 6 volts min to light , so 7 volts max input to the ic should be safe. or will it ? i was looking at the PDF file for this ic .. http://www.fairchildsemi.com/ds/74%2F74ACT244.pdf

are you able to give me a partial schematic as far a hook up? i got a fair idea,,but i am not 100% sure... thanks

[positronicle]

--Edited by Positronicle--

[L. Daniel Rosa]

positronicle, with a 57mm display it's likely that each segment is more than one LED in series. Three would explain the 6V requirement.

[dacflyer]

ok,, thanks..i think this will work fine,, i also found out that the leds are easier to see with a red filter..

and there seems to be 2 leds per segment of the display..

[dacflyer]

is there a difference between the 74AC244 and a 74LS244 or are they all the same ? i cannot find a difference..Hmmm
i am headed to the local shop to get a few,,,

[Gorgon]

Hi Dacflyer,
The AC is a cmos buffer with little input current, the LS is a low power TTL(TransistorTransistorLogic) and draws current on the input. They have both a voltage limit on the output equal to Vcc(5V).

If you use a led display with a forward voltage of >=4V you can use this buffer on a 9V supply. The led will offset the voltage. if not add some diodes in series with the led and make the offset big enough. This can be inserted at the common anode side of the led. You'll need to recalculate the current limiting resistor for the leds if you insert diodes.

TOK

[dacflyer]

GORGON >>> thanks,, thats what i did...
i have 9 volts going into the anode of the leds, then the cathodes to resistors, to the IC

all seems to be working so far on bench test. i just have to find time now to wire it to my temp display driver (ICL7107)

i will report later on as to how good it works..
thanks everyone...