# Photodiode Teensy

To compute the latency of the system or for synchronization with a recording device, we have created an option to record a signal from the computer monitor using a photodiode via a teensy. This is a separate teensy from the one that is used for the water delivery. To do this the dlc processor script generates an oscillating binary signal that can be sent to the unity game and rendered as a black and white square in the top right hand corner of one of the computer monitors. This signal can be read using a photodiode that is attached to the screen via a teensy and is read on a separate thread within the dlc processor and time stamped. 

Here is a schematic of the circuit used in the MLAI lab:

```{image} ../../docs/images/photodiode_circuit_OPT101.png
:alt: pd
:class: bg-primary mb-1
:width: 400px
:align: center
```

Once the cicuit is built upload this [script](../../mouse_task/teensy/dual_water_valve/photodiode_reads.ino) onto it and update the `COM` port and `baudrate` in the `dlc_inference_w_pd` class in the `dlc_processor_socket_pd.py`. You can then set the Teensy argument to 1 and the teensy will be used.  


The model of the photodiode used by the Tolias lab and the circuit diagram for connecting it to the Teensy：

```{image} ../../docs/images/photodiode_teensy_ToliasLab.png
:alt: Photodiode TSL257
:class: bg-primary mb-1
:width: 400px
:align: center
```

To test the latency of the system run a session and then run this `notebook` found in the repo under this path: `/mouse_task/latency_tests/Latency_test_notebook/Latency_testing.ipynb` on the resulting `_PROC` file that gets saved by the DLClive-GUI. You may need to adjust the `flip_photodiode` and `std_above_mean`parameter depending on your photodiode.