gametec
Newbie level 2
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
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