Trying to add an LDR sweep effect to a Park Autowah - is this possible?

[moid]

Hello everyone, hope you've had a good weekend!

Just when you thought I couldn't break something else... well I sort of did... On the good news side I built a Park Autowah and it worked first time I plugged it in (a rare experience for me). It sounds wonderful (I don't like Autowahs much, but this one can make lovely slow sweeping sounds which are great). I even successfully added some mods to change the sweep 'shape' so it can be asymmetric and that worked, and I increased the values of most of the pots and made a slower version of the autowah - everything is great. While testing it I discovered that if I turned the Tone pot back and forth slowly while playing guitar through it, the frequency the wah effects moves about and sounds amazing. I immediately remembered that many nice people here helped me add an automated version of this effect to a Comet Filter pedal I built a few years ago, so I thought, aha, add a flashing LED and LDR circuit and attach it to the Park Autowah with a switch to disable the Tone Pot and that will be awesome... except it of course does nothing I've got the LED / LDR circuit on breadboard at the moment, and it flashes happily, the cable leading from that circuit to the Autowah shows that as the LED flashes I get 0 - 29K resistance (the Tone Pot I was replacing is 10K) so although that is a higher value I would've expected to hear it do something... I also tried different resistors in the LED/LDR circuit to get 0 - 5K resistance and that did nothing either... do I need to hit exactly 0-10K resistance for this to work? Or does this sort of shoddy hack job not work with this type of circuit? Have I attached the LED/LDR to the wrong part of the circuit? When the LED / LDR circuit is switched into the Autowah the guitar still goes through the Autowah, but all the 'wah' is disabled - it just sounds clean.

Any thoughts / musings / muttered curses?

Here is a schematic of my LED / LDR circuit:


Here is a schematic of the Park Autowah (some of my pot values are different)


Here is the vero layout I built the effect from (some of my pot values are different)


Thanks very much!

[duck_arse]

moidy, here's how to do what you are saying. RV1 is the pot you are wiggling. Q4 is your transistor, as in your diagram, for the moment.

but a few things. you really want probably a phase shift oscillator, one transistor, like in the ea trem, because the 555 will square the led. the sweep will sound more like a bee baa siren. and that led - go easy! 10k series resistor [my R3] will restrain the blindings - ldr's have retinas, too.

and sweep range. youse best try it as shown, and then complain about how the swweep is too wide or too narrow or can't be shifted, because it will all those things.

[moid]

Thanks Duck, sorry for the slow reply, have had to do some DIY over the past couple of days (not the fun, soldering kind!). I hope to get some time tomorrow afternoon to look at this - although at a glance it your drawing seems to be very similar to what I did anyway? apart from needing a 10K resistor of course! I will try that. I'm confused (no surprise!) that this circuit will give a square wave effect - when I watch the LED it slowly fades up and then down; to my eyes in a very triangle type wave? The circuit will be on a separate piece of vero to the main board - aha, maybe that's what you mean about the 555 causing issues? If it was on the same PCB it causes issues? I think we avoided the problem a few years ago by placing the LED LDR on a separate board. Have no fear, I built the circuit on vero and had the genius modification idea after building it, so any extra fun has to be on a separate bit of vero.

The EA trem - is that this one?


Is the phase shift oscillator the part around Q3?

[duck_arse]

ahh, yes, you is 100% correct.

<rabbit_on>
re: your last circuit dia. it is your diagram and you can draw them any way you wish. and but however - done stricktly proper, your 555 would be drawn a little box with no mechanical information apart from pin numbers, and each pin would have its functional name writ upon. that way [the pins can be placed anywhere convenient on your little box and] observers don't then need to look at datasheets to find that something is connected to the discharge pin, which shows a charge/discharge triangle-y wave, and not to the output pin, which gives square on off.

and I could give you a list of IC's that seem to get drawn lacking the schematic information over and over and over - but I won't. LM386. 40106.
<rabbit_on_OFF>

so yes, your observations of the led waveshape is correct. but as you drew the ldr and switch/pot, you need a second glance. as for the ea trem pso, yes, from the line of the led and 10k and Q3 and to the right is the oscillator in question.

[moid]

Good point! I will try to label my ICs better in future OK I think I have some news. I added a 10K resistor as you said, also switched the 100R resistor that was between the LDR and 9V to be between the LDR and ground (because that made much more sense!) and there is a resistance of between 0.5K and 1.7K on the LDR and all the way to where this LDR circuit joins the Autowah and it's Depth pot. My son reckons he can hear some subtle modulation on only the low E of the guitar when it is played throught the pedal (I think I can hear it, but I'm not sure, my hearing is crap at both low end and high end these days, even with my hearing aids in). Presumably this circuit needs to be outputting 0 - 10K to substitute for the same sweep range as the original Tone 10K pot? Should I try a smaller resistor? I'm guessing yes, because that 470R gave me more resistance variety. I'll try that next!

[moid]

OK that was a false optimistic post We are definitely not hearing anything, when the autowah circuit is on it is removing some of the highs a bit - if I bypass it I get the same note, just clearer - and changing the LED pulse rate does nothing. I put a 1K resistor in place of the 10K and got the resistance range on the LDR to go from 0.5K to 7.3K, which should be enough to generate a pulsing effect in the Autowah, but it does nothing. Maybe this circuit just doesn't work when it is fed a constantly changing resistance going to the Depth pot (although it responds happily to the Tone pot being wiggled if the LDR is switched out). Very odd.

[duck_arse]

While testing it I discovered that if I turned the Tone pot back and forth slowly while playing guitar through it, the frequency the wah effects moves about and sounds amazing. I immediately remembered that many nice people here helped me add an automated version of this effect to a Comet Filter pedal I built a few years ago, so I thought, aha, add a flashing LED and LDR circuit and attach it to the Park Autowah with a switch to disable the Tone Pot and that will be awesome...

Here is a schematic of the Park Autowah (some of my pot values are different)

you say turning the tone pot gives you the wah sweep. so we want to emulate that knob turning with an oscillator driven something. turning the pot [RV3] only varies the resistance, TONE 2 and TONE 3. so you add your ldr across the tone pot, TONE 2 and TONE 3, and when the light shines, the ldr resistance in parallel with the pot drops, so it behaves as if Paul's little tremolo devil is a'twiddle yer knob. there is no connection to supply. there is no connection to ground. there is no added series resistor, of any value. just the switch to open the ldr circuit and the pot. see my diagram.




my RV1 is your tone pot. my "wherever" is your TONE 3, my "elsewhere" is your TONE 2. it is only the led/CLR/driver transistor that needs ground and supply connection.

as I've drawn will work. but, as you set the tone pot to less resistance, the ldr high resistance has little to no effect on the sweeping. and, because you so far have no means of controlling the brightness of the led, it will sweep the ldr from its darkest/high resistance to its brightest/low resistance all the time. full sweep across the tone pot at all setting of tone pot, so you can't have any subtle.

try it, let us know.

[moid]

Yippeeeeee! We have a (mostly) working Autowah effect You know it's amazing what happens when you connect a circuit correctly I'm just beginning to suspect you might be smart... or maybe I'm stupid... nah, you're just smart So - the good news, the LED LDR is now sweeping the Tone pot. The bad news is that it's range is not as wide as the tone pot without the LDR in it. This is true even with the Tone pot set to full CCW (if I set tone to full CW there is very little effect on the audio, so it acts as a blend knob to a certain extent which is nice!). So I started trying different resistors in the place of R3 on your schematic:
I tried a 470ohm resistor and got 0.4K and 1.6K (this one sounded worst, very muddy, no high end)
I tried a 1K resistor and output between 0.5K and 4.3K (sounded better than the 470ohm)
I tried a 2K2 and that got me 0.7K to 4.8K (sounded pretty similar to the 1K)
I tried a 10K and that got me 1.6K to 6.4K - the best so far - better at the high end of frequencies, but a lot less effect at the bass end where very little happens. It looks to me that increasing R3 would give me a higher end to the sweep, but cut the low end, whereas greedy sod that I am, I want both low and high of course! At the moment this is not as wide a range as the tone pot on its own (if I disconnect the LDR and twiddle the pot with my dainty fingers). So my guess is now that I need to modify other components in the circuit to get a wider range - so that would be the LED or the LDR? I assumed that the white ultrabright LED would be the brightest LED I have and going dimmer or to a coloured LED would reduce the resistance range rather than improve it, so presumably that means trying a different LDR... I guess I'll break into my mixed bag of weird LDRs and start auditioning a few! Please yell if I've got that the wrong way round and should try changing the LED. Thanks for your help!

[duck_arse]

measure. open the ldr switch, so you are in manual mode [I just made that up]. sweep the tone pot, find settings/extremes you like. power off. measure the pot resistance at your preffered settings. report them here.

don't bother with led colour changess, nor led sizes. the led is less than mm's from the poor ldr, thow much change do you think you might get? you can shift the range some with the led current, switched resistors would be easiest. but measure first.

[moid]

Thanks Duck! OK the results were different to what I expected - we like the pot through most of its travel, which turns out to be from full CW - 0K (the DMM says minimum is 0.02K) up to about 8.7K when the pot is three quarters CCW. The pot itself goes to almost 11K, but the final values from 8.7 to 11K are in the deep bass end and have no effect on guitar strings - in that we can't hear anything happening to the strings at that point, the modulation appears to be static.

As resistance falls the Wah cuts the bass more, and as resistance increases it cuts the high frequencies more.

I hope that makes sense?

I did try a few different LDR from different sources, but haven't yet found anything that has a significantly larger or noticeably different resistance range to the original LDR. Thank you for warning me about the LED; I mentioned it because years ago changing the LED on the Comet Filter I made had a huge difference in the strength of the modulation effect and I wasn't sure if it would be so with this circuit.

[amptramp]

The CdS photoconductor response is quite broad at 400 nm to 800 nm, so maybe some change in LED colour would change the sensitivity, but anything from red, orange, yellow, green and blue should be able to get full operation from a CdS cell.  There may be a slight reduction in sensitivity at the edges of the band.

You may be able to find CdSe cells which have a faster response (10 msec time constant rather than the 100 msec of CdS) but the resistance is temperature sensitive and these cells are a bit more rare.

[diffeq]

not to hijack the thread, but if phase shift oscillator is run on lower (5V) voltage, do I need to adjust resistor values??

[moid]

The CdS photoconductor response is quite broad at 400 nm to 800 nm, so maybe some change in LED colour would change the sensitivity, but anything from red, orange, yellow, green and blue should be able to get full operation from a CdS cell.  There may be a slight reduction in sensitivity at the edges of the band.

You may be able to find CdSe cells which have a faster response (10 msec time constant rather than the 100 msec of CdS) but the resistance is temperature sensitive and these cells are a bit more rare.

Thanks amptramp - so by using a white LED I should be hitting all of the colour spectrum unless the LDR is unusually sensitive to a specific frequency range of light?

not to hijack the thread, but if phase shift oscillator is run on lower (5V) voltage, do I need to adjust resistor values??

Go ahead and ask! I'm sorry I have no idea what the answer is though.


Duck (and for anyone else that likes pictures) I've redrawn the schematic for the LED LDR circuit below, hopefully with clearer labels

[moid]

While swearing at my non functioning tremolo circuit I thought I would try adjusting this Autowah a bit more (because who doesn't love an extra dose of failure to accompany misery?). I have changed the 10K resistor that connects the LED and the transistor into a 470R. That gives me a 3K - 13K sweep; which is too high and not also not low enough. It was the best option after trying a wide variety of resistors from 100R to 10K. So then I thought maybe adding a resistor elsewhere in the circuit might help? So in the spirit of Bob Hope, if you take an infinite number of monkeys and give them an infinite number of breadboards and components, one of them will eventually invent the Klon... well I started shoving resistors in places they probably weren't intended to go Putting a resistor (of any amount) between Tone Lug 3 and the LDR did nothing, or adjusted the values by tiny amounts. Then I stuck a resistor in parallel with the LDR. Low value resistors (100ohm) gave me a much lower bottom end range - down to 1K but at the same time wrecked the upper range to only 1.3K - so that was useless. A 10K resistor gave me a low range of 2.5K and a maximum of 7.2K, so at the higher end that is pretty good (near to the 8.7K I wanted) but I'd still like the lower value to drop to nearer to 0K. Logic dictates that as I increase the resistance value of this resistor both the upper and lower values will raise. Should I try something else? Adding a capacitor to things somewhere? Connecting something to ground? Invoking the dark lord with a sacrifice* and using the entrails to connect random parts of the circuit together? Any suggestions would be lovely! Thanks

Updated Schematic below:


*Apologies to any vegan satanists; I am happy to consider the use of vegetables of differing resistance and or capacitance values to summon the correct multi dimensional being while also staying within contemporary values of approved demon summoning guidelines.

[diffeq]

if you take an infinite number of monkeys and give them an infinite number of breadboards and components

you can do it. do you have a 10k pot? wire it like this:

- measure your 9v exactly
- sweep the pot until your LDR hits 8.7k
- measure Vf point voltage
- power off and measure pots resistance, between R1 and pot wiper.

using that you find out what lowest current your LED needs for high resistance limit. we can't slam LDR into 0 ohm, but 500 is realistic, so you repeat measurement above aiming to get LDR into 500-400R ballpark. report your findings.

7555 swings 1/3 to 2/3 of the supply, so roughly +3 to +6 volts. this will need to be scaled with an opamp to get the right currents into your LED.

[duck_arse]

what type number ldr are you using? it might be you want a different type.

[Baran Ismen]

This is one of the pedals in my to-do list, as there seems no other auto-wah working properly 

I wonder about your pot values and why did you change them from the original.

[moid]

Thanks everyone, sorry about the slow reply, work has started up again and my brain has been on that (and I was also finishing a modified Seppuku FX Memory-loss-a-like, which actually worked - after I finished blowing up an op amp and an LED! - and so I got excited about finishing that). Back to some updates:

what type number ldr are you using? it might be you want a different type.

I was using something labelled as 8 - 24K. I tried a load of different LDRs from my stash and found one that is labelled 9K - 20K - whether that is correct I don't know, it came in a mixed bag of LDRs I bought on ebay 10 years ago, but whatever it is, it outputs 230ohms to 9.3K which is near perfect Some guitar testing from my son later and it hits the full sweep of audio I need. Yay! I'm very happy. I should mention I removed the 10K resistor that was running in parallel with the LDR. With it in I got different settings 6ohms (!?) to 4.3K. So that has gone.

Diffeq - thanks for your suggestions - I was going to try them if my LDR selection hadn't helped, but I do appreciate you drawing that schematic for me - thanks!

This is one of the pedals in my to-do list, as there seems no other auto-wah working properly 

I wonder about your pot values and why did you change them from the original.

The pot value changes were suggestions I read on another site (maybe the dirtboxlayouts one, or tagboardfx?) where someone suggested larger values to give a greater depth and a wider frequency sweep - they suggested trying different values up to 20K; to me the 10K pots sounded the best and gave a much more pronounced effect than the original 5K pots which seemed a bit tame to me - but you might want to test these to see how you like them - I like pedals that go to extreme settings; not that I always use them at maximums, but I like having the option there if i want it. The original values still sound good - I liked them on a demo I saw on Youtube enough to want to build the circuit, but having a stronger effect really makes this cicuit come alive for me. The Speed pot  - I have two of them, a 100K and a 500K with a switch to go between them. This allows me to have speeds that go faster and also much slower than the original circuit. I will admit the first reason for changing it was purely practical though - I didn't have any 47K or 50 K lin pots, and the 50K log pots I did have did not really work well with this circuit. I need to buy more pots! The fast bit is not something I think I will use much (it can get to near ring mod) but the slower speeds are lovely; I intend to put this circuit after a long reverb I think to slowly modulate a very drawn out reverb sound. The idea sounds good in my head, we'll see if works!

I'm going to draw a vero layout of the LED LDR circuit next and build it, hopefully I can get that working.

By the way does anyone know if there is anywhere that sells 9K - 20K LDRs in the UK? I only have one of these resistors, and I prefer to have a backup of same parts in case I break something, and Bitsbox, Mouser, Amazon and JPRElectronics don't sell them; I found some 8K - 20K LDRs on ebay that are quite expensive, so I'll buy them if I can't find anything else, but it seems that the only LDRs that are easily available in the UK are the GL5XXX series ones.

[moid]

Updated schematic and vero layout - would anyone mind please checking my vero layout is OK so I can start building it? Thanks