IR tester - suggestions?

[hippy]

What the signal is at 'X' seems to be the current unknown, and that is essential for testing. I expect it's at some voltage when the LED is disconnected, dropping to some other voltage but not reaching 0V when the LED is connected and lit.

Analogue and opto-diodes are not my thing, but looking at the basic principle of operation, which seems to be pull the supply voltage down when light is detected, it seems to me that pulling the supply down may cause the opto-diode to no longer detect light when it is present, that would therefore present a pulse or pulsing pull on the power supply.

I am guessing the capacitor and the bottom right of the circuit affects that behaviour in some way but I don't know how., and don't have the skills to figure it out. Hence my approach of looking at what it does present, not worrying over how it works. But the circuitry may have more importance. It may be that someone should analyse how it actually works.

 

[Man_in_uk]

But does it ? What you are testing is whether it works under the conditions of your testing which it seems are rather different to what the conditions will be when it is actually in service.

You are correct. I am not subjecting these PCB's to a real world scenario, so I cannot be 100% confident that they are perfect. The purpose of my device to give someone a visual indicator that they have not broken a cable or smashed a PCB while removing a large cumbersome assembly.

The only time these parts have failed in the past, is when someone damages them due to being heavy-handed in removing the assembly. In order to detect the objects they are looking for, they have to be mounted in a very exposed position and get damaged easily. Once the assembly has been removed, there is no way to know if any damage has been done.

Two options are available. 1. replace both PCB's & harness for new items before re-installing.
2. Use a visual indicator to check that the RX PCB can still detect IR sent from the TX PCB.

The design I have used is simple. I do not need to sync the TX pulse with the RX to identify a broken PCB. All I need is a visual indicator that the RX PCB is doing something, in this case pulling 5v down to 2v (hence the ADC). Each time this v-drop changes, my indicator changes state. If any part of this circuit has been damaged ( Cable harness - TX PCB - RX PCB) the indicator will not flash. Placing your hand in between the PCB's also stops the indicator flashing.

This device does not really test anything. It will not show where any fault might be but it does show someone that they have not killed it, just yet!
(They might just smash everything to bits while installing the assembly)