Variable Speed Rapid Fire!

I had a look inside my Sega Mega Drive Arcade Power Stick with the intent of just copying it's "Megafire" circuit but found a custom IC doing all the work. Here's my attempt a functional equivalent - made from cheap parts. Maybe it would have been simpler to use a microcontroller...but not as interesting.

It has all the features you'd expect from a rapid fire circuit...such as:
- Shot rate variable from 2 to 20 Hz.
- Speed adjustment is linear LINEAR!
- Four buttons (or less, or even more with another 4066), each with its own switch.
- CMOS switch outputs - for interface with common ground or matrix wired joysticks.
- Blinky LED optical feedback.


This circuit is based on Voltage Controlled Oscillator which appears on the first page of the LM358 datasheet. It will run from a power supply of 3 to 6 volts and can be powered from a higher voltage (up to 15v) if the following modifications are made: addition of a 10k resistor between Q1's base and ground, addition of a 10k resistor between Q2's base and ground, change of R11 to prevent LED destruction.

Here's a simplified version, no CMOS switches, common ground only:

12/6/07
index