Hello again. I am still working away on a RP2040 toolchain--stuck on getting I2C to work--so I've turned the post over this time to Elton Glover from Otter Mods--they make really cool analog synth modules, and Elton was nice enough to share one of his designs--"GateKeeper" with me and you, faithful AudioDiWhy readers.
Take it away Elton!
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The core of this module uses a 555 timer to generate a gate of a desired length. If you are not familiar with this versatile chip, there are many tutorials available online. Here is a link to a tutorial and a calculator to help determine component values:
555 Calculator
https://ohmslawcalculator.com/555-monostable-calculator
555 Tutorial
https://www.electronics-tutorials.ws/waveforms/555_timer.html
Configuration
The 555 timer is set up in monostable mode, often referred to as “one-shot” meaning it will not self-oscillate. With the values I chose, the adjustable output gate can range anywhere from ~0.02 seconds up to around 5 seconds. Some of the resistor and capacitor values (specifically RV2 and C3) can be adjusted for longer or shorter duration, but I felt these gave me a good usable range for the typical patches I’m making.
This version of the module build uses through-hole components for easier assembly, but I have also made a double gatekeeper version using SMD components. You can tune the build to your own taste.
Functional Walkthrough
The clock input (J1) can accept nearly any type of signal pulse, which is then fed through the filter made up of C1-R6 and passed through D1 to produce a spike. This is buffered by U1A and the output turns on Q1, which is set up as an inverter.
This inverted pulse is fed into the Trigger (pin 2) of our 555 (U2). This initiates the charging cycle of C3. The combination of R5 with the adjustable value of RV1 sets how long this charge time takes. Once the charge level of C3 reaches the ⅔ supply threshold, the Discharge (pin 7) is opened and drains C3 until another Trigger event occurs. Concurrently, the Output (Q in this schematic, pin 3) turns on during the charge time of C3. The output is buffered by U1B and is the end result used to output the desired Gate signal. R1 and D2 provide an indicator LED to signify the gate output length. R2 is used as a simple means of circuit protection.
PCB Layout
Here is how I chose to layout the board. I am using the 3.5mm jacks as the mount for the faceplate, and thus wanted to space them out evenly across the board to give the module the most support.
This module is 3HP wide, and the standard 3U height for the Eurorack format:
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| You can get your own Gatekeeper PCB and its front panel on PCBWay's project page, here. |
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COOL! Thanks again to Elton Glover for letting me post this extremely useful module. Also for PCBWAY for fabbing the PCB's for the post (there are two--main board and PCB used for the front panel). Get gerbers for both here.
If you have questions or need more information, please contact Elton via his website--here.
I'll be back soon with more Euro modules, and maybe someday I'll have an RP2040 MCU working well enough on the bench to do useful things. Stay tuned!
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