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npn switch to ttl low

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gametec

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I have a cga to vga converter board that has a manual switch that grounds
pin 1 of a 74hc157 which selects the cga input if pin 1 is high its straight thru VGA.

I want to make it autodetect the cga signal and ground pin 1if cga signal is present.

I have found in the sync detection circuit where if signal present pin is 2.8-3 volts
and 0 volts if not.

My question is what can I use to ground pin1 of the 74hc157 when my sync pin is 2.8-3 volts. I have tried a 2n3904 NPN switch with emitter to GND collector to pin 1
of 74hct157 and base to the sync pin. It will turn off the vga but I guess pin 1 of 74hc157 doesnt go low enough to change to CGA. The voltage at pin 1 when cga signal is there trough the 2n3904 was about .9v.

Am I not using the correct resistor combination with the switch or am I barking up the wrong tree in general.
Can somebody give me a sample schematic that would point me in the right direction.

Any help greatly appreciated !!!!

Thanks,
gametec


54_1172388704.JPG
 

The transistor need a pull-up resistor on collector,

however this circuit could work with a 74LS,
but not 74HC
 

this circuit could work with a 74LS, but not 74HC.


Ok if it wont work with a 74hc then How would you recommend grounding
pin1 of the 74hc157. The circuit is allready there all I am trying to do is make
it switch automaticly instead of using Manual switch.


Thanks,
gametec
 

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