Am using a phototransistor (2 leads) to detect a photobeam break in an otherwise 24vdc system. Output signal from the photocell amp goes to the input of an ACCES digital DIO interface which requires 3-30 v ac or dc. For several reasons I use a dc for the input to the interface and want it to be negative.

Came up with the attached circuit which works and appears stable but is probably not optimal and would be pleased to receive feedback/suggestions/simple alternatives, etc.
Burt

 

Hello,

A comparator in the circuit will have a more stable switch level.
Here is an example that drives a relays:

This picture is linked from this page:
http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html

Bertus

 

Another approach for your consideration would be the use of a current mirror similar to the approach shown in this post. Of course you would replace the photodiode with your photo-transistor.

hgmjr

 

Are you saying you want the DIO interface input to be 0V when the phototransistor is not illuminated, and -24V when it is?

 

Are you saying you want the DIO interface input to be 0V when the phototransistor is not illuminated, and -24V when it is?

YES or at least less than 3v

 

Are you saying you want the DIO interface input to be 0V when the phototransistor is not illuminated, and -24V when it is?

Sorry, did not address both questions: Yes, less than 3 v and, when interrupted, any where between -4 and -24 v. The circuit I'm using works but I assume there is a better and (hopefully) equally simple solution.
Thanks,
Burt

 

Sorry, did not address both questions: Yes, less than 3 v and, when interrupted, any where between -4 and -24 v. The circuit I'm using works but I assume there is a better and (hopefully) equally simple solution.
Thanks,
Burt

How much current does your IO require? Can you link to a datasheet of the ACCES digital DIO interface?
Also, link to a datasheet of your phototransistor.