I was wondering what would be the best way to take a analog signal coming from a QRD1114 and turn it into a digital signal that can be processed by the 74HC165 serial in\serial out shift register. I have been looking around and read a little about Darlington transistors and seen a few analog to digital ICs. I have to do this 20 times so I am trying to reduce cost and was hoping to find a way to possibly use pull up resistors if at all possible. Any ideas?

 

See the attached. I just simulated the output of the sensor, not the input side, driving a 2N7002 N-channel enhanced MOSFET. It's in a SOT-23 package, which is surface mount. You could also use a 2N7000 MOSFET, which is in a TO-92 package.

 

Thank you for the help Wookie, you really do have all the answers lol.

 

Keep in mind that a 74HC14 Schmitt triggered inverter is a handy device for interfacing slowly changing, or noisy, analog signals to 74HC logic.

 

Gee, why didn't I think of that?

Another candidate is the 74HC7541: http://www.nxp.com/documents/data_sheet/74HC_HCT7541_CNV.pdf

Octal non-inverting Schmitt trigger 3-state buffer. Three of those connected to three parallel-in, serial-out 8-bit shift registers (or maybe a 74HC299 universal shift register) and you'd have a smaller board layout.

If you wanted to forgo the Schmitt-trigger inputs, you could use something like a '597 or '598 latching 8-bit PISO SR. Three 16- or 20-pin IC's for up to 24 inputs; that's not too shabby. But, the Schmitt triggers will help to avoid ambiguity.

 

TTL is very old logic with a very high input current.
But 74HCxxx logic is newer with no input current.
A HUGE DIFFERENCE.

 

Gee, why didn't I think of that?

Because you are not as lazy as I am.