V3–Power Meter–The Complete How To

For the current visitors to the blog, it's finally here! For new visitors welcome to the complete how to. What I am going to explain start to finish how to make and advanced left and right measuring power meter. The quickest and easiest way to figure out how to put your own together or find out how other power meters work such as SRM, Quarq, Cycleops or Power2max is watch the videos in sequence below.
Left to Right V1, V2, V3 of my power meter. Questions? kwakeham@gmail.com

V3 Short Demo Video

The first thing I’m going to do is a cost breakdown. It’s likely what is on peoples minds most. Most power meters are minimum of $1500 dollars, ranging to almost $4000.

V3 Parts Cost	quantity	price	total
Crank	1	140	140
Strain Gage	2	12.5	25
Instrumentation Amp	2	4.1	8.2
Low power ADC	2	18	36
Micocontroller (Pro Micro)	1	25	25
ANT+  AP2 Module	1	24	24
Resistors	4	4.6	18.4
PCB (Rush Proto Service)	1	37.5	37.5
Misc (Caps, wire, pins, etc)	1	20	20
Build a PM for:			334.1
Garmin Edge 500	1	250	250
Strain Gauge Supplies	1	1000	1000
Gage Installation tester (used)	1	100	100
V2 boards + parts + spares	1	600	600
V3 Spares	1	400	400
Labour	1	500+ hours	Free
			2350
Development Total:		Total	2684.1
Development Total:		Total w/tax	3033.033

The cost to build is pretty cheap at less than 350 dollars, however gearing up, and V2 developmental costs means this is much much more expensive to cut your teeth on.
However, lets look at projected cost for V4 if I can develop it. This is does not include the huge amount of labour so don’t get excited and expect power meters on the market at 250 dollars. It’s not happening.

V4 Projected Parts Cost	quantity	price	total
Crank	1	140	140
Strain Gage	2	12.5	25
Low power ADC	1	13	13
Micocontroller (nRF51422)	1	2.3	2.3
PCB (Longer turnaround service)	1	20	20
Misc (Caps, wire, pins, etc)	1	40	40
			240.3

Videos

1. Introduction
2. What is power, torque, and angular velocity?
3. How is cadence and torque measured? What is a strain gauge? How do we measure such tiny resistance changes
4. How do I install that strain gauge? What’s important to know.
5. Let’s put it all together and see it working & double check the calibration and on Bike Demo.
6. Short Demo (Previously Posted)

How to Links

1. How to install strain gauges
1. The Power Meter Origin – The abbreviated version of this and this page. Those pages are now deprecated and will shortly be edited to say as much.
2. V3 Power Meter–Initial Instrumentation – How V3 was instrumented. Lots of pictures and explanation.
2. Power Meter PCB File (And Eagle) – NOTE: See posting below “Hacking your own PCB”. I replaced the LM4140 with a voltage divider as it’s more stable but the PCB was not updated. The Board also has a coin cell holder, temporarily replaced with a lithium pack as it draws to much current for the coin cells.
3. Code – Located here. Will be updated soon with better commenting.

Additional and Useful Links

1. Vishay Data Books -
1. Surface Preparation – PDF Link
2. Technical notes – Lots of tech notes on installation, errors, sensitivity, etc.
2. Intertechnology Ltd. -  Canadian distributor for Vishay products
3. Digikey – My main supplier for components, they also have some Vishay strain gauges.
4. Omega – Low cost instrumentation, sensors, and strain gauges + installation supplies. I’ve never used their supplies but the double shear gauge is used in V3
5. Binsfield Strain Gauge installation videos – Part 1 – Useful to understand installation basics.

Important Posts (chronological)

1. ANT+ For Embedded – Some history
2. AP2 Module and design changes - evolution of V2
3. Omega Strain Gauges – Receiving the new gauges for V3
4. V3 Power Meter Initial Instrumentation – Important about how I installed the strain gauges
5. V3 Reading sensors and transmitting power – The basics of getting the power meter working
6. V3 – Power Consumption – Measures and outlining current consumption. NOTE: This is still not solved. Right now V3 is a power hog and the coin cells DO NOT WORK. I’m using a 110mah Lithium Polymer Cell until I sort it
7. Hacking your own PCB – Critical, replace the LM4140 with a voltage divider, it will work better. Still works with LM4140 though.
8. V3 – Why it works now and Power Meter Code – Found a short, solved all my problems by fixing it. 1.5 months of messing around.

Some more pictures

Installed on my bike on my cycle trainer.


You can see the lithium Cell that replaced the coin cell holder.

The furthest most chips are AD623, the ones closest to the Pro Micro is AD8321. The top where the LM4140 would go is replaced with a hack for a voltage divider.

34 Gauge Wire Coated in polyurethane heading from the “torque tube” to the PCB.

Strain Gauge inside to measure left leg torque.

Double Shear gauge to measure drive side torque.