Digital Logic Sequencer

 This prototype circuit is a good example of how my design process works.  The point at which I built this prototype was in the middle of the design process, and not the starting point.  The customer already had an electronic design, and wanted to have a Printed Circuit Board made.  So, I replicated the electronic design with the circuit design tool I was using at the time (OrCAD).  Then generated the Printed Circuit Board layout.  Once we had released the Gerber files for fabrication is when I decided to prototype the design while we were waiting on the fabricated Printed Circuit Boards.

My main concern is that we got all the connections that needed to be made hooked up correctly.  There was one enable pin that needed to be hooked up in order for the circuit to work properly.  I was able to stop the PCB Fabrication House before they printed the PCB, and get the extra connection added.  The Printed Circuit Board worked the first time once we had received, and assembled it.

The operation of the circuit works like this.  First is the oscillator is a Quad Op Amp, LM324, which is configured as a relaxation oscillator.  The LM324 has four Op Amps in it, so I used one for the relaxation oscillator, and the other three as buffer amplifiers.  The clock signal is then fed to a digital counter, CD4024, a seven bit binary counter, which takes an incoming pulse train, the clock, and counts the number of pulses.  The output is in binary, and there are seven output pins for the seven bits.  The high order bits (4, 5, and 6) are fed to the sequencers address bits.  The sequencer is a MC14051 Analog Multiplexor.  The address input bits select which output channel get connected to the source.  So, when you input address bits in binary order they will switch the outputs in numeric order, the sequence.  This could also be an arbitrary selection if you were to use a processor to drive the address inputs.

This prototype is typical for small electronic prototypes.  I use single sided printed circuit boards with 100 mil hole spacing, and plated with a pad per hole layout.  All the connections are made with discrete wires, in this case 26 AWG solid core wire.  Not all the components have 100 mil spacing connections, and you wind up bending legs, and modifying the board as needed to accommodate the components.  I have made hundreds of prototype bards similar to this to work out circuits, and do proof of concept work.  This one is fun because it is visual with all the LEDs.