This is an intermediate to advanced project and I do not recommend it
as a first project if you are just getting started in synths or electronics.
Only the circuit and some explanation are shown here. A lot of project building
experience and electronics
knowledge and equipment ownership (scope, meters, etc.) is taken for granted.
If you are interested in building this project please read the entire page before
ordering PC boards to ensure that the information provided is thorough enough for
you to complete the project successfully.
Features
LED Voltage Level Meter Totally cool looking.
Two 3 input DC voltage mixers with offset adjust and normal and inverted outputs.
Linear Lag Processor (long and short time select).
RC Lag Processor (long and short time select).
Introduction
A great assortment of functions in one module. Two 3 input DC voltage mixers with normal and inverted output,
a linear lag processor, an RC lag processor and a LED voltage level meter.
The voltage meter looks cool and lets you have a graphical idea of where that voltage level you like to use to modulate
your filter or vca is so that later repeating it will be easier.
Although simple in nature the voltmeter is useful and has several input ranges (+/-1V, +/-5V and +/-10V). Whereas
an analog or even a digital meter takes time to find the level this circuit lights the LED immediately giving
you a rough idea of the voltage level you're looking at even when it is
changing rapidly. The DC mixers let you mix LFO outputs or any voltage sources. The lag processors both
give plenty of range to go from almost immediate voltage following to delays lasting many seconds before the output voltage arrives at the
level fed into the input.
Multi-Function Module Schematic Page 1 (LED Volt Meter) PDF
Page one of the schematic contains the voltage meter. Starting at the top of the schematic we see the voltage input buffer (U1-A) which is
used as a unity gain voltage follower. We also find the voltage range selection circuitry (U1-D and U1-C and associated components).
The 1N5239 9.1V zener is used for range selection reference generation.
9.1V is generated at the junction of current limiter R2 and the cathode of Z1. This is fed to the resistor divider R3 and R6. R6 must be
adjusted so that there is 1.00V on the output of U1-D (pin 14) when S1 is in the center position.
Unity gain followers have the gain formula (1 + R-feedback/R-Gainset). When
switch S1 is in the center position there is no R-Gainset in the circuit and the follower gives a gain of 1. With R9 in the circuit (S1 set to the
10V range setting) the gain goes to 10 (1 + 270K/30K) and with R10 in the circuit the gain is very close to 5 (actually 4.97) which is more
than close enough for this circuit.
Notice that the output of U1-D is fed into the input of unity gain inverter U1-C. U1-C's output will always
be U1-D's output multiplied by negative one. When U1-D's output is 1V, U1-C's output is -1V.
When U1-D's output is 5V, U1-C's output is -5V.
When U1-D's output is 10V, U1-C's output is -10V.
Thus S1 sets the voltage levels that are fed to both VHI and VLO between +/-10V, +/-1V and +/-5V.
The heart of the circuit is the group of comparators at the bottom of page 1 of the schematic. Notice that the VHI circuit point is the output
of U1-D and the VLO circuit point is the output of U1-C. VHI is dropped on a series of 10K resistors (R12, R18, R26, R27, R32)
which act as a voltage divider. So when VHI is 10V (S1 set to 10V range) there will be 10V at pin 2 of U2-A,
8V at pin 6 of U2-B,
6V at pin 9 of U2-C,
4V at pin 13 of U2-D,
and 2V at pin 2 of U3-A.
In general the resistor dividers will see to it that 1/5, 2/5, 3/5, 4/5, and 5/5 of VHI and VLO will be used as the comparator
threshold settings no matter what range you are set for.
These levels are fed to the inverting inputs of the respective op amps which are used as comparators. They comprise the
reference input voltages to the comparators.
In this configuration the output of each of these op-amps will be low (-11V) or high (+11V) depending on the voltage level fed to each ones
non-inverting input.
Circuit point VMS (V-Measured), the output of op amp U1-A unity gain follower is applied to all of the the non-inverting inputs of these
op amps (used as comparators)
U2-A,
U2-B,
U2-C,
U2-D,
and U3-A.
When VMS is at ground all of the outputs of
U2-A,
U2-B,
U2-C,
U2-D,
and U3-A are low. If VMS goes above 2V the output of U3-A goes high and current can flow through Q7 (PNP transistor),
LED10 and R33. Q7 conducts the current
from U3-A's output through the LED and R33 because U2-D's output is low (-11V) which keeps Q7 (PNP transistor) turned on.
If the voltage on VMS goes above 4V then the output of U2-D goes high. U2-D's output being high now turns on LED8 via current through Q8
and R33 but U2-D's output also turns off Q7 thus LED10 goes off. If the voltage on VMS goes above 6V guess what happens... OK, now LED5 glows
because of current through Q3 but both Q5 and Q7 are turned off because of the high levels on the outputs of U2-C and U2D. So the
basic idea is that as the voltage goes higher the next higher comparator will go high and thus the LEDs below that level are turned
off. This configuration ensures that only one LED is on at a time and thus conserves current and in my book looks cooler.
The right side of schematic page 1 shows the circuitry used to meaure the negative voltage. As you can see the principle is the same
however the inverting and non-inverting inputs of the comparators are switched and the references generated across the 10K resistors in series
(R15, R20, R24, R29, and R31)
are applied to the non-inverting inputs and
that VMS is applied to the inverting inputs. Now the voltage level on VMS has to go below the reference voltage in order for the outputs
of these comparators to go high. Remember that when VHI measures 10V volts that VLO measures -10V. What this means is that
there is
-10V at pin 12 of U4-D
-8V at pin 10 of U4-C
-6V at pin 5 of U4-B
-4V at pin 3 of U4-A
-2V at pin 5 of U3-B.
So if VMS goes below -2V then the output of U3-B goes high (+11V) and current can flow through Q8, LED11 and R33 thus lighting LED11. If the voltage
on VMS goes below -4V then U4-A's output goes high, turning on LED9 and turning off Q8 and in turn LED11. You can see that as the voltage goes
lower the comparators continue to go high in turn illuminating their respective LED and turning off the preceding LEDs by turning off the
preceding LED's control transistor.
LED6 is the "near ground" indicator and it lights if the voltage on VMS is between about +18.5% and -18.5% of the range setting.
U3-C and U3-D comprise a window comparator. In order for LED8 to light, both U3-C and U3-D outputs must be high. If either output is
low LED6 is robbed of current because either diode D1 or D2 is forward biased and sinking the current through R14 to -11V. When the voltage on
VMS is between the two reference voltages applied to U3-C pin 10 and U3-D pin 13 then both comparator outputs are high and
LED6 can light because current through R14 is not sucked into either D1 or D2.
Multi-Function Module Schematic Page 2 (Voltage Mixers) PDF
Page 2 shows the Voltage Mixers. I will only describe the operation of the one closest to the top of the page. The other
circuit functions identically.
U5-A is the main voltage summer. The 10K level pots (R44,R56, and R61) feed the 1M input summing resistors (R43, R49, and R57) respectively.
U5-A's feedback resistor is
also 1M and thus it has a gain of one. The output of U5-A is fed to both the inverted voltage out jack and to the input
of unity inverter U5-B which inverts the signal fed into it's inverting input and supplies it to the non-inverted outpus jack.
Two inversions make a non-inversion just like -1 * -1 = 1. The bias control R34 applies between -12V and +12V to the top of
the resistor divider made up of R36 (300K) and R41 (100K). Thus between +3V and -3V is applied to the non-inverting inputs
of U5-C and U5-A as the bias knob is rotated from end to end.
The DC offset applied to the non-inverting input of U5-A sees a gain of 2 because the formula for
non-inverting gain is (1 + R-feedback/R-input) thus as the bias knob is adjusted the output of U5-A swings between
-6V and +6V. Thus any signal at the output of U5-A can be offset by between -6V and +6V. Notice that buffer U5-C applies a
gain of 4 to the DC level applied to it's non-inverting output. This is necessary so that the offset voltage applied to
U5-B's non-inverting input is twice that applied to it's inverting input via U5-A's output. If the same bias was applied to
U5-B's non-inverting input as is applied to U5-A's non-inverting input the net effect would be that U5-B would not reflect the
desired DC offset but would remain at it's initial DC offset. This is because of common mode rejection. The same DC level applied to
both the inverting and non-inverting inputs of an op-amp will result in an output of 0. Since we apply twice the DC offset to
U5-B's non-inverting input we get the desired result. Both outputs move simultaneously up or down when we adjust the bias control.
For example when we set the bias pot to have 1V of offset on the output of U5-A we will have 2V at the non-inverting input of
U5-B. Since this is a difference of 1V, U5-B's output will have a DC offset of 1V, just what we want.
Multi-Function Module Schematic Page 3 (Lag Processors) PDF
Page three of the schematic shows the two lag processors. The one at the top of the page provides a linear ramp between changes in voltage
and the one at the bottom provides an RC curve between changes in voltage.
U7-A provides buffering for the voltage applied to the linear lag processor. U7-A's output is fed input U7-B's inverting input. U7-B provides
a very high gain block which is used to detect the difference between the output of the integrator made up of U7-D and the input voltage.
When the voltage on the input of the linear lag processor is higher than the voltage at the output of the integrator (U7-D) the output
of U7-B is basically pegged at the lowest possible output voltage of U7-B about -11V (when using +/-12V). U7-B's output is fed to the top of
a voltage divider made up of R68 and R75 (consider that S2 is open). The voltage divider and op-amp U7-C are used to lower the voltage
and thus the current that is fed via series combination of R71 and lag adjust pot R72 to the input of integrator U7-D (and cap C17). In this
situation (input voltage higher than integrator output) the low level fed to the integrator causes it to ramp up (at the rate determined by
both the Short/Long switch selection and adjustment of R72). The integrator continues to ramp up until the voltage on the non-inverting input
of U7-B equals the voltage on it's inverting input. At that point the op amp's output goes to 0 (remember common mode rejection). This zero (or ground)
level is reflected at the output of follower U7-C and thus at the inverting input of U7-D. Since now both inputs are at ground the voltage
on the output of the integrator is held since no current needs to flow in or out of U7-D's pin 13. The output just stays where it's at so as not
to disturb the delicate balance on it's inputs. However if the input goes lower than the voltage on the output of the integrator the output of
U7-B goes as high as it can (about 11V on +/-12V system) and drives the integrator down until the inputs balance again at which time the
integrator's output again "holds". C20 and R74 in the feedback loop of U7-B minimize the ringing on the output of U7-B during transitions
between either voltage extreme and ground and stabilize the op amp to avoid run away oscillation. When S2 is closed the 200 ohm resistor is
put in parallel with the divider resistor R75 thus lowering the voltage level available to provide current to the integrator. You can get some
VERY long lag times but the circuit follows the input very faithfully and eventually arrives at the input voltage plus or minus a millivolt or two.
The bottom of schematic page 3 contains the simple RC lag processor which is essentially an input voltage follower, a 1M pot, a selectable
slew cap and another follower. The only thing to remember here is to use the tantalum cap for C22 because leakage could prevent the
output from ever reaching the input voltage with long lag times and switch S3 closed. A nice low leakage film type cap for C21 is also a good idea.
Approx. Current Consumption
+12V
60mA
-12V
60mA
Multi-Function Module PCB Component Values (Parts Side Shown) PDF
Multi-Function Module PCB Component Designators (Parts Side Shown) PDF
Multi-Function Module PCB Bottom Copper (Parts Side Shown)
Multi-Function Module PCB Top Copper(Parts Side Shown)
This panel is just a convenient way to show the controls included in the module and how to wire the unit. You will probably want
to design your own. I used the side mounted LEDs in my panels and fashioned the panel and the
mounting brackets to accomodate them but you can run wires from the board to the LEDs if you want to. Follow the schematic carefully to
see how to wire up the anodes and cathodes of the LEDs. The RED led cathodes can all be wired together and then only one wire run to the
board for them. The anodes must be wired separately. Remember that the "near ground" green LED needs to be wired separately.
Multi-Function Module Front Panel and Wiring Rear View
PDF
This panel is just a convenient way to show the controls included in the module and how to wire the unit. You will probably want
to design your own. Remember that this is the rear view of the panel and thus the controls are mirrored from the panel layout.
