[sdiy] THAT 2155 on sale

Dave Brown davebr at modularsynthesis.com
Sat Apr 11 07:24:31 CEST 2020

I wired up a comparator circuit today and tested a VTL5C3, VTL5C4 and a Silonex. The comparator turns on the LED and the drop in resistance is near instantaneous. The LED goes off fairly quickly and you can watch the decay of the resistance by monitoring the voltage divider on a scope. The resulting ramps are only about 200 mV in amplitude with the hysteresis I used.  The VTL5C3 oscillated at 222 Hz which is a 4.5 mS decay. The VTL5C4 oscillated at 83 Hz which is a 12 mS decay. The Silonex oscillated at 2 Hz which is a 500 mS delay. I tried decreasing the hysteresis resistor so I could move the trip points farther apart so I could cover a wider current range and also see the turn on delay but couldn’t get it to oscillate. I may play with it more tomorrow. My impression is this is a cumbersome way to test but may show variation in decay. I don’t know how consistent vactrols are in that regard. But they need to operate over a wider current range than what I did.


I have always matched my vactrols by calculating a series resistor to test at 5 mA. I wire up my DMM to resistance across the vactrol, turn on the voltage and take the first reading to 2 digits, so something like 4K3 or 2K5. The values will change the longer the vactrol is on so I always get the first initial reading. Then I group them together based if they are within 100R of each other, so I would group 3.5K with 3.4K and 3.6K. Some will have very low resistance like 83X R or 56X R. Again I only use two digits. I set aside the really low readings and the really high readings. Occasionally I will find some at 8K and above. Most are in the 1K – 5K range.


Then I look at the circuit to use them and decide if I can compensate with other resistor changes as to which ones I should use where. I’ve always had good results with this method.




From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Laughing Man
Sent: Friday, April 10, 2020 7:54 PM
To: Ullrich Peter
Cc: synth-diy at synth-diy.org DIY
Subject: Re: [sdiy] THAT 2155 on sale


I'll give the comparator circuit a shot, though I'm still curious what the datasheet is referring to in regards to some specific amount of current being pushed through the LDR. The Excelitas datasheet also mentions that currents lower than 1mA would result in excessively high resistances. 


That's the main thing that's stopping me from being sure I've tested them right: I'm not sure how I know I've got the right amount of current flowing through the LDR. If I don't, then the specs won't line up by a long shot. It seems as if I just have to get a 'golden vactrol' that I know will work in the situation I want, and just line them up, which is bugging my scientific side.

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