I built a nonlinearcircuits Spasm, a.k.a. LDRJerk. It’s a Sprott jerk chaos circuit controlled by a DIY 7-way vactrol. This is the first SMD (Surface Mounted Device) project I’ve done in a while. It was easier than I thought it would be. The module is in two parts, a small board containing all the SMD components and the Eurorack power header, and a longer board that holds the pots, jacks and an LED. The two are connected by a group of male and female headers, four of which form a rectangle that cleverly closes up to make a light box where a single, small LED illuminates seven LDRs (Light Dependent Resistors or photocells).

The longer PCB holds the pots, jacks, LED, and the male pin headers

I would recommend soldering all ten headers (5 male, 5 female) before trying to fit the boards together. (In fact, putting the boards together is the very last step.) Just make sure that all the headers are standing perpendicular to their boards. I thought I was smart and soldered only a pin on each end, then tried fitting them together before soldering. Whoops. Pulling them apart was not easy. I had to work with a small screwdriver to carefully separate them all, gradually.

Parts Options

Depending on how fast you want it to run, you can choose different parts for the Spasm. Two different LDRs are given in the BOM. GL5516, which I used (and which is in the useful Synthcube BOM) is for faster, and GL5549 will go a lot slower but with wider frequency range and more unpredictability.

Three capacitors (must all be the same value) set the speed also, and vary from 0.01uf to 10uf, the larger, the slower result. I chose to use the slow 10uf caps together with the faster LDR, and this works fine, giving me an LFO range with occasional unpredictable high frequency bursts.

You also have to choose resistors for the LEDs. For RLbox I used a 1K, and for RL a 10K to go with a super bright orange LED I had around. Not having any idea what to use for the Box LED, I went with the part listed in the Synthcube BOM, Mouser 859-LTL-4213, which is a 3mm red.

All the SMD parts can be obtained from Mouser, and here the Synthcube BOM is useful again. I could not find the S1JL rectifiers, so I choose Mouser 821-HS1KL-R3G. These are just diodes in-line with the power rails for reverse power protection.

Here are some photos of the completed module.

A couple of scope shots showing two of the four concurrent outputs.