DIY T4 LDR Tester

OK, I am fed up with the T4 thing, except for Kubi.

So I am going to burn a board for a T4 LDR tester, supply the artwork, it takes a transformer from rat shack, some leds, some redulators, some trim pots, in the end, you will have a precision LDR calibrater and matcher so everybody in the world can build their own LA2.

Picking a world wide part number for a green LED so that we can say 200 microamps = 80 K on a good cell- is that possible?
We will find out.
The LED will have to be consistant and readiably available, so that an HMLP 4700 in US = Calibrated T4 in Germany.

This is an area for quick research.
Maybe even a Radio Shack LED, but they might be all over the place.
I could yuse some help in designing a precision current source, which can be trimmed out each session, for the leds.

Thinking LM317 adj with J202 or precision referance.

A green LED doesn't translate very well to 'real world' applications - the LA-2A and LA-3A use electroluminescent panels whose color spectrums are more in the light blue range (and vary depending on the frequencies fed into them); LA-4s and LA-5s use red LEDs. There is also no way to guarantee consistent intensity between LEDs of the same type based simply on voltage or current draw. You would need a calibrated meter such as a photographer's exposure meter to do that.

Also, the dynamic characteristics of the photocell determine the sound more than the simple 'on' resistance.

Not trying to 'poo-poo' the project, just trying to share some real-word experience after trying many ways to test T4's that failed miserably.

Here is an article with an interesting idea. Use the junction in the LED to measure its own temperature. Then you can adjust the measured light intensity reading by the tempco or control the current to get a constant known junction temperature.

http://cie2.nist.gov/LED Sympo 2001/Abstracts/Schanda_LED_Standards.pdf

Joe, were way ahead of you here, but I hear ya.

OK, I figured out a constant current mechanism good to 3 point acuracy, or microamps, precision regs and fets not good enuff, we need opamp servo circuit, so i took a 4-20 ma sensor circuit and scled down the current.

This is the correct way to supply the leds, without a doubt. temp stable, the whole nine yards.

Count me in for a couple of boards!!!

---Joe

I downlaoded AP's circuit maker, pretty cool, spits out a gerber,
see the sofware mets, much better than si metrix form Caetna orwhatever that company is.

just 24 volt single supply, 4 ea. 4-20 sensors heavily modified, (take out the 20 ma front end, tweak the 4 ma for lower current, and thats it.