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Let's say I have a photodiode, a shift register (74HC595) and a 5V source, and I connect these as:

5V source -- photodiode -- shift register data input.
(And other voltages to feed the register clock etc)

Do I need a resistor with the photodiode, or will it suffice to put the photodiode before the shift register?

What I am trying to do here is essentially use the photodiode as a switch which allows the register input to be set to 1 only when light falls on the PD.

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    \$\begingroup\$ Have a look at the datasheet for the photodiode and see what the output voltage is going to be. Then compare that with the logic threshold voltages of your shift register input. Then consider what the output voltage will be due to ambient light. Then link to the datasheet and fix the title! \$\endgroup\$ – Transistor Apr 5 at 19:27
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    \$\begingroup\$ A photo-transistor is far more suitable in this case than a photodiode which is among the most analog of all analog components and requires signal conditioning circuitry. You still might need some extra circuitry to ensure a clean edge even with a photo-transistor. \$\endgroup\$ – DKNguyen Apr 5 at 21:23
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Maybe on paper, but probably not in the real world. Photodiode current is very small, so it would take a very large resistor to produce enough voltage change to affect a CMOS gate input. You will get better performance and noise immunity with a phototransistor.

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  • \$\begingroup\$ Technically you COULD use a photodiode, but what you would be doing is basically emulating a photo-transistor with photodiode and a bunch of circuitry. But since that's complicated the only reason you would do this is if you needed very specific requirements from your that you could not find in an actual phototransistor. \$\endgroup\$ – DKNguyen Apr 5 at 21:27
  • \$\begingroup\$ Based on posts on another forum, I think he is asking about something more simple - a photodiode and one or two resistors to center the light-to-dark voltage change around the shift register input transition level. \$\endgroup\$ – AnalogKid Apr 5 at 23:34
  • \$\begingroup\$ What about about using a current mirror circuit that includes a photodiode and a normal transistor ? \$\endgroup\$ – Pryda Apr 6 at 13:42
  • \$\begingroup\$ Again using secret knowledge from another forum - times 800? \$\endgroup\$ – AnalogKid Apr 6 at 14:01

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