Instrumentation amp protoboard

OSH Park’s artistic rendering of the EKG blinky board. The battery clip (for 2 CR-2032 batteries) takes up most of the space.

After designing and ordering the PC board for my EKG blinky circuit, I decided to try designing a prototyping board for students to design their own EKG (or other instrumentation amplifier applications).

Eagle’s depiction of the protoboard. The blue layer is the routing on the bottom (which I made with very fat wires, since there was no need for thin ones).

I decided to leave off the battery clip, but add a barrel jack for a 5v wall wart.  I also added a 4-pin header (not just a 2-pin header) for connecting to an Arduino (allowing the Arduino to power the board and to read the Vbias signal as well as the output).  The header is not dedicated to any particular function—each pin is connected to 3 pads and nothing else.

The only parts of the circuit I prewired for the students were the power distribution and the Rgain resistor for the INA126P.  For the other inputs and outputs for the amplifiers, I’ve provided 4 empty pads (3 for the inputs of the instrumentation amp).  I’ve also provided 10 clusters of 3 pads, unconnected to the rest of the circuit.  Students can use these for nodes in the circuit that are not the inputs or outputs of the amplifiers.

The top left has screw terminals for the EKG wires. Bottom left is a power barrel and screw terminals for power input. Moving across are a 4-pin header output (male or female—I’ve not decided which yet), an LED with room for a series resistor, and a trimpot. There are spaces for six non-dedicated resistors (in addition to Rgain and a series resistor for the LED).

I’ve not put any dedicated places for capacitors, but the standard 0.1″ spacing allows them to be placed between any pair of adjacent conductors.  It will be necessary to use a few of the 3-hole clusters to connect capacitors.

I’m considering modifying the design some more, to be able to add a resistor into the power path before the 220μF capacitor, to get better low-pass filtering of wall wart power.  Students could always jumper it with a wire if they did not want the resistor.

Design image showing only the top-of-board layers, for students to design their wiring and component layout before soldering.

For student design efforts, I’ll distribute a file with the top layers show, so that students can print a full-page picture and sketch in the capacitors, resistors, and wires they want to add, before soldering anything up.

The protoboard is about 2″ on a side, so would cost me $20 for 3 copies from OSH Park.  Getting 40 boards for the class would drop the price to about $4/board from OSH Park.  We could get 40–60 boards from 4pcb.com for about $200, but we’d have to cut them apart ourselves. I think that 4pcb.com gets cheaper once we get up into the 1000s of boards, but we never will, as we only need 40–80 a year and will want to redesign every few years.