Read Rope / Electronic Nerve

A linear relationship between output voltage and the resistance at output is the most desired relationship.

This is because when reading from an ADC, ideally one would want to read voltage values with mostly equal differences between combinations that give different equivalent resistances at the output.

For example, if the relationship between output voltage and output resistance were logarithmic, then it would be difficult to distinguish one combination from another due to both having different equivalent resistances at output that produce nearly the same output voltage.

If the relationship between output voltage and output resistance were exponential, adding a small resistance would produce a negligible effect on the output voltage.

The total cost of making 5 pcb prototypes would be $142.15 USD not including shipping cost and not including tubing to contain pcb prototype and push button when bent.

Parts List + Total Cost

• 5x FlexPCB from PCBCart, $124.35
• 10x KTR18EZPF3004 3Meg SMT Resistor, $2.00
• 5x KTR18EZPF3004 2Meg SMT Resistor, $1.00
• 5x KTR18EZPF1004 1Meg SMT Resistor, $1.00
• 5x KTR18EZPF1004 750k SMT Resistor, $1.00
• 20x KSR221GNCLFS SPST-NC Tactile Switch, $12.80

Most Expensive to Least Expensive Items

1. FlexPCB
2. SPST-NC Tactile Switches
3. 3Meg, 2Meg, 1Meg, 750k SMT Resistors

URL for Parts

• KTR18EZPF3004
• KTR18EZPF2004
• KTR18EZPF1004
• KTR18EZPF7503
• KSR221GNCLFS-SMT-NCSPST

I tried to solder together through-hole resistors with surface-mount SPST NC push-button switches.

It failed. I managed to get all the resistors and switches soldered together, but the solder joints broke with little bit of force from pushing the read rope into the vinyl tubing.

This experience showed me that read rope will require an expensive prototype of a flexible pcb with only surface-mount components to accomplish the objective of creation of a thin device that outputs voltage depending on location of lightweight bends.

A through-hole prototype for this is not viable because small through-hole switches are relatively big and have a higher actuator force requirement than surface-mount SPST NC switches. This would not work well with the requirements of the read rope of being small and requiring little force from a bend to activate the switch.

A through-hole prototype for read rope is also not viable because the through-hole resistors are relatively big for the intended application and bring more noise than surface-mount chip resistors.

I managed to solder together SMT tactile switches in parallel with resistors.

Unfortunately, a solder joint breaks when trying to push the read rope into 0.170 inch inside diameter vinyl tubing.

From this experience, I have learned more considerations to make to turn this into a final product.

Switch

• The actuator force, force applied to switch to change its state(Closed to Open), needs to be light enough so that bending the tubing at the location of the switch will push the button.
• The switch needs to be small enough to have a resistor soldered underneath or next to it and still fit in tubing. Unfortunately, this leaves only SMT tactile switches available.

Tubing

• The tubing that will contain the read rope wires and switches and resistors will need to be very flexible. It also needs to contain a long wire segment that leads back to ground.
• It also must be thick enough to push the tactile switch button if bent at a location where there is a switch.
• Vinyl works well enough; however, more appropriate options for material need to be explored.

Flexible PCB

• Since SMT tactile switches need to be used, flexible printed circuit board will need to be considered for the version of the product that will be manufactured.
• The flexible PCB will need to be thin enough to fit into tubing.
• Fortunately, using SMT tactile switches and chip resistors will make read rope using flexible pcb to be very small due to size of the chip resistors and SMT tactile switches. 

One important consideration to take when developing read rope into a product is the precision of the output voltage as determined by precision of input.

1. Precision of Output Voltage. How well can output voltage be read.
   1. The resistors used will determine the ratio of resistance between output terminal and ground to total resistance in the circuit which determines the output voltage.
      1. The tolerance of these resistors needs to be considered for when output voltage will be fed into an analog-to-digital converter which expects a precise value for a certain bend. 1% tolerance in resistors is ideal.
      2. The difference between resistors will determine the difference between output voltages at certain bends. Ideally the resistors should be on the same order to avoid 1 bend at lower end not adding noticeable amount to output voltage.
2. Precision of Input. Where the bends can be made.
   1. Ideally for a very precise small read rope, the resistors will need to be placed very close to each other for bends to be interpreted that are very close to each other.
   2. Size of SPST switch and resistors will determine placement of resistors and size of bends.
   3. Ideally, for a large rope where only a few bends need to be read, resistors can be placed far apart from each other and connected through long wire. 

I realized that the project will make a rope that can only register multiple bends in one direction ( down ) with 1 output.

However, this is a good thing since it is one thing it can do well and its simplicity allows it to be used in a modular way.

For example, multiple read ropes in different orientations - different down directions for each rope - can be joined together to form a rope that can register multiple bends in multiple different directions. 

This rope will have an output for each direction since it is multiple 1 output read ropes joined together.

One challenge that will need to be addressed when developing this project into a product will be that finding appropriate pushbuttons.

Requirement for Push Button

• The push button needs to be able to small enough to fit inside small tubing that has a diameter of less than 0.25 inches. This is so that it can be portable and easily bendable.
• It needs to be normally-closed SPST switch of which there are few options available. It must be this type in order for the resistor to get shorted which is how the project works.
• It must have an acceptable life of at least 100k cycles for a durable consumer product.

I made a 4-bend prototype with resistors 3 Mega, 2 Mega,  1 Mega, 750k ohms. 

I put the wire and connected resistors in vinyl tubing of 1/4 inch outside diameter , 0.170 inch inside diameter.

It worked!

Unfortunately, it did not work in a consistent manner because the wire connections would also bend and get out of place.

The wire connection is not reliable; therefore, I will use very small normally-closed momentary push button switches since those achieve the same function as the 2 wires touching.