Homebrew your own Vactrols

By Johan Bodin, SM6LKM
K9AYloop list, August 18, 2000

I have successfully homebrewed a Vactrol from four clear ultra-bright narrow beam yellow LEDs pointed at an old ORP-12 LDR from Philips. Out of the five ORP-12s in my junk box, I was lucky to find one which was able to get down to 23 ohms with 50 mA through each of the LEDs. The other four LDRs had about 100-150 ohms under the same conditions.
In my first attempt to make a Vactrol, I used a 3 watt incandescent lightbulb. The resistance range was good, of course, but the heat from the lamp made the LDR resistance drift a lot so I decided to try LEDs.
CdS LDRs have a broad spectral response (much like the human eye) but the peak is around yellow so yellow LEDs are probably the best choice.
One source for good LDRs might be old junked oil burners (for house heating). They often contain a *big* CdS LDR for the "flame supervisor relay". The LDR "looks" at the flame and shuts down the blower/oil pump in case of flame-out. The LDRs I have seen in oil burners have a few sq. cm active area and are capable of quite a low resistance. There are different LDR shapes to be found in oil burners. One is ORP-90 which is housed in a 7-pin miniature tube/valve envelope!
Check the junk pile at your local plumber or junk yard ;-)

Australian way

By Richard Jary
K9AYloop list, August 18, 2000

I have had the problem here that Vactrols are not available in Australia - at least that I have found. To get a low enough resistance I have connected 3 sets of LDRs + high intensity amber LEDs in parallel, allows me to vary down to around 70 ohms (low enough for the application I was using). Cost was around A$2 per "unit" I think.