Monthly Archives: March 2014

The simplest, cheapest CD tray etchant agitator, with a 4096 and a transistor H-bridge

A broken CDrom is a perfect candidate for a simple and cheap etchant agitator. What’s an etchant agitator? It’s a device that makes the etchant (Ferric chloride, for example) move around the container when etching PCBs. Why? It greatly speeds up etching, by washing away the top layer of corroded copper, exposing clean copper to be etched.

I’ve seen many CD tray etchant agitators online, but all of them are too expensive and/or too complicated. Using integrated H-bridges and microcontrollers or decade counters for such a simple design is totaly overkill to me. Here’s one example, showing operation:

(operation at 3:57)

I had some 4096s (Schmitt trigger NAND gate) lying around (and they’re better than 555s when it comes to symmetrical duty cycle squarewaves), and I decided to design the cheapest, simplest, lowest-component-count CD tray etchant agitator. While some might wonder why I didn’t go with a 555, I’d have to use an inverter anyway, and 555s suck at symmetric-duty-cycle operation.

The circuit consists of a squarewave oscillator made with a single 4096 gate, a (logical) inverter made with another 4096 gate (needed by H-bridge) and a simple, common transistor H-bridge with some protection. Hookup is really simple, as you can see. The circuit can be easily modified to use a quad NOR gate or an inverter, as long as they have Schmitt trigger inputs.

You can replace R1 with a 100k linear pot (wired as a variable resistor) in series with a 20k resistor, for 170ms to 1s period adjustment.

The price should be under 2€.

Here’s the PDF, including a schematic, layout and PCB:

CD tray controller thumbnail

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