24 April, 2012

Incredible projects based on 7400 logic

Last fall there was a competition at DangerousPrototypes with many interesting entries based on the 7400 logic gate family. The ancestry of these circuits goes back to the 60's and early 70's, but they are still popular. I have picked out some of the ham radio related entries here.

They said "The Open 7400 Logic Competition brings awareness to open hardware and software, and fosters understanding of the building blocks of modern integrated circuits. What can you build with logic chips?"

I think the most interesting and inspiring entries were:
  • 80M direct conversion receiver. This circuit is a direct conversion (DC) receiver making use of a digital mixer. The intent was to build it entirely out of 74HC parts, but in the end there was not enough gain, so a dual op-amp was also needed.
  • AM radio transmitter. The  frequency is digitally tunable from 520kHz – 1.6MHz. The RF section is strong enough to broadcast across a room.
  • AM radio. This is an AM radio with a 74HCU04 inverter. The inverters are used both as RF amplifiers and as audio amplifiers.
  • FM modulation and demodulation system.  A Voltage Controlled Oscillator (VCO) is FM-modulated and the signal sent to an IR LED. It is then received by a photo-transistor and cleaned up to trigger a short-pulsed monostable which charges a capacitor where the voltage will be proportional to the trigger frequency, thus recovering the original waveform.
  • Capacitance meter. It works by using the unknown capacitor in an RC oscillator, which changes its output frequency proportionally to the capacitance. The frequency is then measured by comparison to a known stable clock and displayed on a 7-segment display
  • Capacitive Sensing, the Hard Way. In the author's own words this is an over-engineered 16 channel input capacitive sensor with RS232 output.
  • 74LS00 based full wave rectifier. Without powering the 7400 chip, 2 x 6 VAC input gives 10.4 VDC output. The key to this is the input protection zener diodes which are used here. 
Judging criteria and categories were:
  • Originality. A smart new design that inspires
  • Documentation. Schematics, theory, pictures and/or video
  • (Mis)use of 7400 logic. Show us what 7400 logic was (never) meant to do
  • Technical prowess
  • Build quality
  • Imagination and creativity
Winners? The 7400 Capacitive Touch Array (Capacitive touch sensing the hard way) won the grand prize of $100 and first pick of the prizes. The 7400-based Capacitance Meter won the first place and next pick from the prizes.

I think that one of the 74HC240 logic gate transmitter designs that float around in ham circles (originally coming from N7KSB in QST some time in the 90's) would have been a worthy entry which could have done well also. Amateur radio should probably try to connect better with e.g. the Open Source Hardware movement.

It is a little late, but I congratulate the winners and all the entries for their creativity and skill.


Image from Wikipedia Commons, composed by Audrius Meskauskas.