AutoFire

This definitely belongs in the ANCIENT category. Back in my early days of learning electronics, I could never get enough of my Forrest Mims’ books. In the 80’s, Radio Shack sold his “Getting Started in Electronics” book, and then later he created a whole line of “Engineer’s Mini-Notebooks”. I had practically every one. They were awesome and had superb explanations of virtually everything, including soldering, diodes, transformers, BJTs, MOSFETs, linear IC’s, op-amps, digital logic, TTL, CMOS, oscillators, optoelectronics, photovoltaics, you name it.

These books and Radio Shack’s “100-in-1” kits sealed the deal on me becoming an Electrical Engineer. I still have my copies of Mims’ books in the basement and highly recommend them for newcomers since the fundamentals don’t change. It looks like you can still pick up a copy of the “green book” although with a new cover!

Anyhow, once those books taught me how to use the 555 timer and I learned about oscillators, I figured I had a better application that just another LED blinker or delay circuit. I was in high school and my younger brothers we’re really enjoying playing games on the Commodore 64 (a few of which I wrote), so I decided to step it up a notch and make an “auto-fire” joystick adapter box.

For anyone familiar with the C=64, you’ll remember that it used the same DB9 connectors as the Atari 2600 for the joystick ports. Four momentary switches for up-down-left-right and a fifth for the fire button. 5V and ground were also available as well as pins for paddles (potentiometers). For this project, there was no need for the paddle connections.

To make an “auto-fire” device, I put together a box with an oscillator circuit that plugged in between the C=64 and the joystick. This had male and female DB9 connectors (from Radio Shack of course back then) and intercepted the fire button signal. A toggle switch allowed the user to select “pass-through” which was normal fire button operation, or “auto-fire”. I assume it should work with an Atari 2600 fine as well, but never tested that.

In “auto-fire” mode, the fire button enabled the oscillator whose square wave output went to the C=64 in place of the fire button signal. A potentiometer knob for dialing in the auto-fire frequency was the only other control. The user could then tune the pulse rate for whatever game they were playing.

This wasn’t by any means a brand new idea. Up-level joysticks were already on the market with auto-fire features, but I was too cheap to buy one and had a lot more fun learning to build my own.

I hadn’t seen my “auto-fire” box for decades until I found it in among some old stuff in my Mom’s basement. I definitely had to take it apart and see if my construction techniques were as good as I remembered – 30 AWG wire wrap wire (all the same color), masking tape, dull-looking solder joints, crooked bolts, and a hole for the DB9 apparently cut out with a file. Nothing to write home about here.

Anyhow, I was glad to still have it and be able to peek inside what was essentially a time capsule for me. My favorite part – discovering that I built it with 4011 CMOS NANDs! I had no idea it wasn’t a 555! The NAND gate oscillator circuit was definitely straight from Forrest Mims’ books.

My brothers loved it, especially once they broke all their records on Epyx Summer Games. 🙂

UPDATE 12/20: Wired it up and it still runs! Measured 4 Hz up to about 450 Hz as I adjusted the potentiometer. First time it had been powered up in decades!