A Simple LDR Envelope Filter that is just delightful

I designed  a low pass type envelope filter using a Fliege filter topology . The result was a really easy to build filter that provides1st order low pass ..all the way up to 2nd order low pass with extreme bandpass peaked low pass(high Q, resonance…etc). I really like this pedal.


Demo Sound Clips:

Short attack, max intensity:

Mid Attack, low intensity:

Long attack, high intensity:

This filter topology has some nice features, such as:

Fixed stable gain of two in the pass band, regardless of Q

Highly adjustable Q

Uses only two op amps

Low parts count in general

Well suited for single supply operation( in my case I just direct coupled the input to a half supply biased input stage)

It also has a couple of drawbacks:

It requires three LDR’s unlike  the Mutron for example which only uses two

I could not get a TL072 to work well in the circuit. I needed to use a higher bandwidth, rail to rail type op amp, but there are plenty of amps that work well.

The OP Amps I tried were the OP213(works fine but expensive), OPA1652, and the TS922. These are all low noise, high bandwidth devices. The TL072 broke into oscillation and distorted. Maybe someone else can get it to work? I don’t have time to figure out the problem but I assume there is some sort of undesired phase shift making an oscillator out of the thing.

The LDRs can be commercial units but I just built some  from scratch – they were easy and cheap to make. I used flat faced green LED’s and small CDS photo-resistors. I super-glued them together and then just cover with heat shrink to make them light tight. It took me about ten minutes.


LED glued to photocell:


Final device:

The  parts I used are: LED digikey part # VAOL-5701DE4-ND and photocell  digikey part # PDV-P9008-ND

Circuit description:

Pretty simple – A emitter follower buffers the input and provides 1/2 supply bias for the the filter op amps. The Filter uses two LDRs for the filter sweep and one more for the intensity or Q of the filter. When LDR3 is .707 the vaulue of  the other LDR’s (1 and 2), you get an ideal 2nd order low pass characteristic. When LDR3 is higher you get bandpass peaking and this can be quite extreme. As the filter sweeps, the ratio between the LDRs must be the same, that is why you need the third one and not just a variable resistor.

The LDRs are driven by a current mode amplifier and a simple diode peak detector. I used a LM358 for this but the the OP213, OPA1652 and the TS922 used in the filter section will also work. The design provides a sensitivity control, intensity(resonance) control and an attack control. The attack control really makes it easy to dial in the sweet spot of the filter and get very snappy or very slow plodding whah whah effects.

Circuit Schematic:

Minimu board

Populated surface mount circuit board:

Link to silkscreen:

Click to access minimussilkscreen.pdf

Link to Top layer surface mount layout:

Click to access minimur.pdf


7 thoughts on “A Simple LDR Envelope Filter that is just delightful

  1. Thank you for your hard work and posting this circuit. The application I’m using this in requires a circuit rearrangement with all thru-hole parts. The PCB mask AND schematic helped get everything connected. You rock my friend!

  2. I’m setting up to build your MiniMu in a few days. Do you have any parts substitutions for the MiniMu to be “tuned” or track the low end of a bass guitar? I play a six string bass and have a giant range. I’m looking for this circuit to warble the low notes and much as the top notes.
    Modifications in original design so far –
    input cap: 220nf, both IC’s: OPA2134, transistor: 2N3904, all LED’s are yellow. I removed the 2 resistors (1K & 2.2K) on the output and replaced it with an onboard 10K trim pot. Anything else I should switch out?
    Thanks for your help and I will share my point-to-point, through-hole version when verified.

  3. Built this on my own PCB using an NJM2068 as U2, and the suggested LEDs and LDRs. It definitely cops the classic Mutron sound I’m looking for, and is super low noise. A couple of observations, coming from a bass player.
    1) The sensitivity pot wants to be log taper.
    2) I found 10K as an attack pot to be way too slow for my needs, so I cut it in half to 5K, and even then, I have the attack set as fast as it will go. The 10K decay resistor feels just right.
    3) It plays nice with all my other pedals, especially my octave divider.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s