A very well regarded envelope filter that many think is the best sounding one of them all with this being based on RG's Neutron project files due to the unobtainium status of some of the original parts. I have modified this slightly from the original because I didn't like the idea of the Gain pot and first opamp stage being permanently in the signal path of your effect chain, and there's really no reason why it should be now DPDT and 3PDT stomps are available so cheaply. So I have made this one true bypass so just use your preferred method.
Some people have used two LEDs or vactrols to make this but I have opted for a single LED with the LDRs on either side, so you can just bend them to face the LED as you see in some Univibe type effects. To get the best out of it the LED and LDRs need to be in darkness which obviously isn't a problem when the effect is boxed, but for testing before being boxed you will want to put a cover of some sort like a small purpose made cardboard surround to keep out the unwanted light.
For LDRs Javi suggested the GL5539 which is available cheaply and in quantity on eBay and is 40-100k light, 5M dark resistance. But I'd suggest socketing them, and probably the LED as well so you can try a few in there and stick with the combination which sounds best for you.
The trimmer was included simply to allow you to fine tune the brightness of the LED which should allow you to tweak the best setting out of it.
The effect needs a +/-9V supply and so I have shown the usual voltage inverter board. You can add 5 rows to the main board if you prefer but I thought this would make more sense and allow people to get it in a 125B pretty comfortably with the supply board stuck on an inner wall somewhere out of the way. You can also omit that entirely and use a +/- 9V supply if you have one, or two batteries if you prefer.
The original included a rotary switch for the Mode selection, but there are only 3 selections and so this is easily accomplished with a DPDT on/on/on toggle which again saves lots of space. If you go with my choice here make sure the contacts change as shown in the diagram. Some of these switches have the opposite contacts in the middle position, and so if you get one like that you can still use it but you'll need to swap the poles. So lug 1 becomes 4, 2 becomes 5, 3 becomes 6 and vice versa. Then it will work fine. And don't forget the link you need between lugs 2 and 4.
I couldn't find any marketing blurb for a 1972 effect (the year I was born) so for fun here's the patent blurb instead:
A sound effects generator for modifying a sound generating signal is comprised of a voltage-controlled filter having a variable peak response frequency for transmitting at a maximum amplitude those frequency components included in the sound generating signal that correspond to the instantaneous filter peak response frequency. The peak response frequency is determinable by a control voltage applied to the voltage-controlled filter; such a control voltage being proportional to the envelope of the sound generating signal as extracted by a control voltage generator. The filter peak response frequency is thus varied in accordance with the varying envelope of the sound generating signal.
Eh? Start again please but slower.
And I have to say it really does sound fantastic in this vid.
And as per Javi's request this is as per the original with a 4 pole 3 way rotary switch for Mode, and with a simplified bypass that leaves the first gain stage in circuit. Just a DPDT stomp is required for bypass, or SPDT if you don't want LED indication. I left the charge pump board separate though because I just can't face adding any more links to it! :o)