Sure, I'd be happy to explain!

The light detection circuit you're referring to is known as a simple light-sensing switch. It uses a photoresistor or LDR (Light Dependent Resistor), two fixed resistors, and a NPN transistor. The designators for these components in your circuit are R_LDR for the LDR, R1 & R2 for the resistors, and Q1 for the NPN transistor.

Here's a quick rundown of how the circuit works:

This configuration forms a basic light-sensing circuit. You can adjust the sensitivity of the circuit by choosing different values for the fixed resistors (R1 and R2).

But in real applications, it is always good to have signal conditioning like a Schmitt trigger or a comparator after this circuit to have a clean digital off/on signal, as the transition between on/off in this configuration can be very soft and unpredictable, leading to multiple unwanted transitions.

The light that this will be intended to detect will be UV light - is this circuit and the selected components sufficient? Does the LDR need to be susceptible to UV wavelengths or will the visible light component of the UV LEDs be sufficient?

The light-sensitive component in this circuit is the LDR (A Light Dependent Resistor) represented as R_LDR in your design schematic.

Standard LDRs are usually sensitive to visible light. Their peak sensitivity is generally in the region of wavelengths where most room light sources emit, which is actually far from the UV spectrum range. Hence, their response to UV light could be significantly less and might not serve your purpose if you're specifically interested in detecting UV light.

For detecting UV light specifically, you would need a sensor that is sensitive to UV radiation. An example of this would be a UV photodiode, which works similar to an LDR but within the UV spectrum. Some phototransistors can also be sensitive to UV light.

It would also be advisable to understand what wavelength ranges your UV LEDs are emitting and select a UV detector that is optimally sensitive to those ranges.

If the UV LED also emits significant visible light and you are interested in those levels as well, then a conventional LDR might still be suitable. However, it could get triggered by any visible light source, not just your UV LEDs.

In summary, if the purpose is to specifically sense UV light, it's best to use a UV sensor. If sensing the visible light component is sufficient, then the current setup with the LDR might already serve the purpose. Always ensure that the LDR you choose has its sensitivity range in accordance with the light you want to sense (UV or visible).

How Can I replace this LDR circuit with the LTR-390UV-01

The Lite-On LTR-390UV-01 is an excellent choice for UV light detection. It is a photodiode that is specifically tailored to respond to UV light and has digital outputs which can be easily interfaced with a microcontroller.

Here is a general guideline on how to integrate the LTR-390UV-01 into your existing circuit design:

Now, you can read the UV light level directly within your code by communicating with the LTR-390UV-01 over the I2C interface. You will need the appropriate driver software in your Raspberry Pi to interface with the LTR-390UV-01, possibly provided by the manufacturer or a third-party.

Please validate these connections with the datasheets for the LTR-390UV-01 and your Raspberry Pi model. Note that not all Raspberry Pi GPIO pins are 5V tolerant so ensure that you're not overloading any pin by checking voltage levels.

Here below is the pin list for the component U1 (SN74LS151N) in a table format. Its functions have been derived based on the datasheet excerpt given.

Note: "Select Input" refers to the control inputs that determine which one of the data inputs will be selected.

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What do I connect to Strobe Input?

The strobe input (or enable input), denoted as '*G' on pin 7 of the SN74LS151N, is used to enable or disable the data selectors/multiplexers.

When the strobe input is at a low logic level, the device is enabled, and data from the selected input (determined by the select inputs A, B, and C) is passed to the output.

When the strobe input is at a high logic level, the device is disabled, forcing the W output high and the Y output low.

So, you should connect the strobe input to a signal that determines when you want the multiplexer to pass on the data. This signal could be from another section of your circuit or a GPIO pin of your microcontroller (such as your Raspberry Pi) if you want programmable control over the enabling/disabling of the multiplexer[1].

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