NOTE ; Circuits using the LDR/LED modules will not work with the new "Superbright" LEDs .
These two projects , Wah and Fuzz, are the results of a modification to a Morley dual channel volume control pedal that one of my sons suggested I undertake as He had no use for the volume unit but thought I could modify the pedal into a Wah unit. I decided later to add a Fuzz circuit and combine the two into a single switchable unit as described further on.
The WAH Module
Not being one to re-invent the wheel, I downloaded several circuits from the Internet and after breadboarding severals I chose the original Morley circuit as it had the best sound and was using an LDR/LED control which simplified its construction.
I then proceeded to modify the circuit by adding RV2 to control the output level , RV1 to provide a range adjustment to the wah and R8,R9 to provide limits for the LED current range . I dug into my junk box for all the parts but the LDR ( CDS Photocell ) which I bought from RadioShack Cat # 276-116 .
The input signal is filtered and fed to pin 2 of the 741 op-amp and amplified by the feed back combination of R5,R6,C3,C5 and C6.The output is taken from C7 to the volume control RV2.
The circuit being used in an AC configuration needs a common ground level which is supplied to pin 3 by the voltage divider made up of R3,R4 and C2.
The Wah action is controlled by the activation of the LED shining on the LDR whose resistance varies depending on the LDR brightness which is controlled by RV1 . When the LED is off the LDR resistance one side of which is connected to the junction of C5-C6 has little effect on the feedback gain circuit but as soon as the LED is turned on with the foot switch SW1, the LDR resistance drops and shunt C5,C6 to ground through the LDR resistance boosting the gain and shifting the sound frequencies, thus the Wah effect.
The foot switch , SW1 , is a heavy duty push button or mcroswitch momentarely SPST switch mounted on top of the case .
Resistors R8 and R9 may be soldered directly to the center and right tab of RV1 and one side of SW1 is connected directly to the other end of R9 , the other side of the SW1 is connected to the positive supply .
I would like to mention that I always use a socket , nobody is perfect and sometime a screw-up need not be a nightmare of desoldering and reinstalling a suspicious IC.
I avoid making printed circuit for most of my projects, instead I have becomed skilled in using #26 or smaller bare wire in making connections as laid out on a piece of perforated board to the proper size. It looks good , it is rugged and can be repaired easily . Below is a suggested layout and tracing is also shown for those who whish to make Printed circuit board
All resistors are 1/4 W , capacitors miniature ,voltge rating @10 volts min. except for C8 which is rated 20volts.
Wah Update (May 2002 )
After using the wah that I originally published above for a while and hearing so much rave about the smooth performance of the inductor type , I decided to look into a way to to enable more flexibility in producing a Wah sound similar to the variable pedal and came up with the design shown below .
Using the phase shifting design with an op-amp sounds very close to the inductor type Wah circuit but with no up and down variation like the pedal allows .
I concentrated on the triggering aspect of the LED/LCD to try and produce a varying attack and decay that can closely resemble the pedal action . While retaining the Wah circuit but discarded the output volume control , I redesigned the triggering portion with another op-amp with a ramping function adjustable with RV2 .
The 741 op-amp is used in the inverted mode as a simple DC amplifier with a 100uF capacitor in parrallel with the 470 ohms resistor in series with RV2 ( 5K ) feed back resistors connected between pin 2 and 6 . By varying RV2 we can select the attack and decay time constant of the Wah from fast to slow ( or vice-versa ). With the 100 ohms resistor connected to the inverting input ( pin 2 ) and left floating ( not connected ) the ouput at pin 6 of the op-amp is about 2 volts , this voltage may be high enough in most cases to induce current through RV1 and RV1a resistors to illuminate the LED . I therefore inserted a diode (1N4148 or 1N914 ) in series to drop the voltage by half a volt to insure the LED is off when S1 is not closed.
RV1 is the main control to adjust the LED sensitivity from a hard to a soft wah and to compensate for different LEDs sensitivity , I added a miniature trimpot ( RV1a ) to be adjusted as required when testing the wah function . It can be replaced by a fixed resistor once the preferred value is found .
As soon as S1 is closed connecting the 100 ohms resistor to ground , the op-amp swings into its inverting mode and produces a positive voltage of 8 volts charging the feed back capacitor ( 100uF ) and depending on the setting of RV2 setting the delay time , turn on the LED gradually depending on RV1 setting which is used to control the current to the LED.
It will be noted that a fast repeated action of S1 on/off will reduce the delay time set by RV2 for an interesting effect depending on RV1 setting .
The LDR/LED capsule
NOTE: New superbright LEDs will not work
As mentionned I got the LDR in a package of five from RadioShack . No data is supplied . I needed to select the widest resistance range available of the five LDRs and used the schematic set-up to test each one. To prevent ambient light to reflect on the LDR I used a 1 1/2" length of black shrink tubing to accomodade the LDR in one end and the LED facing the LDR from the other end and connected the make-shift module as per the circuit details and proceeded to test for minimum and maximum resistance measured with my ohmmeter by varying RV1 and with power on/off. I then inserted the chosen one into the shrink tubing along with the LED. Make note which is the positive lead of the LED and rotate the LED so that the positive lead is situated in the tube as to match the future physical connection to R7 as shown on the PC module.
With the heat of a lighter or match slowly heat the shrink tubing to a tight fit making sure to pinch each end while still warm .
This section applies to all circuits altough some terms and descriptions may vary .
If you are going to use only the Wah system you are ready to insert the module into a strong enclosure either metal or plastic about 3" by 5" , large enought and deep enought ( about 1 1/2 " dept ) to accomodate the foot switches, SW1/S1 and the by - pass switch , the Wah module and a 9V battery ( use a battery clip ) and one of each input and output jacks on opposite sides . The small SPST power switch if used and controls could mounted oneach sides of the enclosure and the LED on top but well away from and protected from the foot action.
Use shielded wire for all input and output connections to prevent hum noise and make sure the shield is connected to the negative or ground side of the battery as well as the case ( if metal ) .
For more guitar effect two fuzz circuits are described on the next page followed by a complete description of the Wah and Fuzz system assembly on the switching page.
More Wah Continued