LDR Control of LM13700?

[mth5044]

I tried Frequency Central's Funky MF filter and I found it sounded great - awesome project Rick, if you ever read this.

http://www.diystompboxes.com/smfforum/index.php?topic=73021.0

I'd like to replace the envelope detector with an LDR in order to control the filter with other stuff - LFO's, sequencers, expression pedal and whatnot. Since the 13700 is current controlled, I figured Ohm's law would come into play here, but I wanted to check in with you all to see if this was heading in the right direction. We always want to be below 2mA according to the datasheet, but one assumption I'm making is that we want to get as close possible to get the full range of the effect.

http://www.ti.com.cn/cn/lit/ds/symlink/lm13700.pdf

The circuit could go as 9V > LDR > CLR > Iabc (pins 1 and 16). Assuming the LDR won't get any lower than 500 ohms, a CLR of 4k7 would get us a maximum current of 1.7mA. Even if a super fresh battery putting out 10V and the LED/LDR at it's brightest, the current would just be a hair under 2mA. 

I am perplexed as to what the voltage divider is in the MF schematic. It looks like it is biasing the control current, but I can't figure out why. I have a feeling it is going to send what I had previously outlined down the trash compactor.

Any insight is much appreciated. Thank you.

[PRR]

Rick's plan has a LM324 (can only go to -1.2V of B+) driving a 10K resistor into Iabc. So he wasn't planning for more than about 0.8mA.

The "voltage divider" can't be that. Iabc pin never comes above a Volt (0.6V for one type; 1.2V for the other; I ferget which this is). So the 100K does about nothing, the 220K bleeds around 0.036mA into Iabc so the OTA never goes full-off.

The total 0.8mA-0.036mA range is about 22:1 of frequency or 4.4 Octaves, a nice musical range.

[mth5044]

So to change the range, something like thiiiiis?

9V > LDR (10M - 500 ohms) > 10k >  Iabc
|____________> 220k >___________^

That would give 0.04mA to 0.85mA. Hurray?

[Keppy]

That looks fine if you're using the full range of the LDR. If it never goes full on you might need to reduce the 10k (or increase the 220k and change cap values) to maintain your range.

Remember that the current gets divided between two OTAs, so the 1.7mA in your first post is really .85mA per OTA. You've got a little wiggle room (not a ton, all the datasheet examples for that IC seem to aim at 1mA, well below the max rating). You're easily safe at 4.7k. Any lower and you better be sure about your power supply and LDR.

Incidentally, why the LDR? LED brightness depends on current, OTA gain depends on current... Why not feed whatever your control voltage is directly through the 10k resistor as you already have it?

[Seljer]

Incidentally, why the LDR? LED brightness depends on current, OTA gain depends on current... Why not feed whatever your control voltage is directly through the 10k resistor as you already have it?

This! You're unnecessarily combining two solutions to a problem that work perfectly fine on their own own (unless you want to recreate the slight lag LDRs have). Route in your CV at the positive input of the buffer opamp by the 2.2uF cap at the end of the envelope detector.

[mth5044]

The reasoning behind the LDR bit was because I have a ton of circuits that modulate an LED - LFO, envelope generator, sequencer and what not - that are hangin out, so I thought it would be interesting to apply some of them to an 13700 filter. That, and amps are scary and different.

So, in order to make these things work together, I'd have to convert the voltage that is driving the LED to current that drives the two OTAs as Keppy said.

For example, using the Electric Druid LFO, refering to the schematic on page 5:

http://www.electricdruid.net/datasheets/TAPLFO2Datasheet.pdf

There are two CV outputs, one 5V (0 to 5) at the filter output and one 10V (-5 to 5). Supposing I fed the 5V CV into the 10k resistors (with the 220k to 9V and 100k to GND, I guess), that would give a range of 0.5mA - 0.04, close to a 12:1, lower than what PRR has mentioned. Lowering that 10k to 4k7 gives 1.1mA - 0.04, 27:1. I suppose that I could put a 2k trim in series with the 4k7, giving a max mA range of 0.75 - 1.1 and set it to what sounds good. 

I guess that could make sense?