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Monday, November 12, 2012

A Dollar Store Voltmeter - $4 volt meter from a dollar store pedometer

This looks like a nice little project for those wanting a cheap way to monitor DC Voltage in a Solar System or any constant DC Voltage Measurements. But, if you shop around. You can find a Multimeter for about the same cost...

LCD Digital Ohm VOLT Meter AC DC Voltmeter Multimeter
From BrainyTrade
Price: $2.76

$2.76 + $4.99 shipping

In Stock. Sold by standbyon (ships from Hong Kong)

And for about $10, you can buy one, read made...
http://www.parts-express.com/cat/panel-meters/542
(and more links blow)

Still, this is a Cool Project and I learned allot, just reading about it...

Don

A Dollar Store Voltmeter

How to make a cheap panel voltmeter from a dollar-store pedometer.

The idea

After installing a small solar-panel in my shed (see DIY solar Charge Controller article) I realized that a nice addition to it would be a panel voltmeter. It's useful to keep an eye in the battery voltage to see if it is charged (or even overcharged). Of course one can buy these things online. This one from Amazon.com goes for $18 which is a bit more than I was willing to spend for something like this. Then it occurred to me that, for this particular application, I don't need a particularly fast refresh rate. The onset of winter and summer doesn't usually happen within seconds, so something that updates only every couple of seconds, or even minutes, is more than adequate to monitor a battery charge level. It occurred to me that I could use a cheap $1 PIC16F683 micro-controller to "measure" the voltage and display it using a dollar-store pedometer. The "dollar voltmeter" was born.

The 4-4-4 plan

My objective was to implement this voltmeter along my famous 4-4-4 plan. The voltmeter should cost less than $4, consume less than 4 mA and last more than 4 years. While the jury is still out on the "4 years" thing, I'm happy to report the other 4-4 objectives were met.

$4

This cost target was easy to meet since the main components (the PIC micro-controller and the pedometer) cost about $2 combined. The rest are small and inexpensive discrete components. If like me, you keep a collection of parts around in your "lab", then you probably will already have them. In any case, even if you don't here's a rough cost estimate:

BOM Cost (Jameco.com Prices)

- Dollar Pedometer: $1.00

- PIC12F683: $1.39

- 2 x BC547C: $0.16

- 78L05: $0.25

- 10 uF cap $0.10

- 2 x0.1 uF cap $0.20

- Red LED $0.12

- 5 Resistors $0.20

- Joy of making it: priceless

Total: $3.42

Hardware Design. How to Expose a Pedometer

This is the very fancy $1 pedometer I bought at my local Dollar-Tree store. Amazing how they can make these things so cheap...

The Pedometer

Opening the thing up reveals something about it's inner workings. The device is powered from a single 1.5V button cell, so this means I'll have to provide a 1.5V supply to it (more on this later). The step counting mechanism is quite simple. When the user steps, this causes a slight vertical motion that momentarily connects the small metal piece on the top of the figure to the spring on the top of the PCB. My idea is to replicate this contact using a transistor configured as a switch activated by the PIC micro-controller. The pedometer also has a single Reset button on the front that resets the count to zero. I'll use a second PIC output and a second transistor to control the Reset.

Exposed Pedometer

The white wire in the following photo is the "Reset" button connection. You will also need to connect the +1.5 V supply (yellow), the "Trigger" input (purple) and the ground (blue). These wires will connect to the small PIC12F683 circuit that is the core of the voltmeter (the pedometer serves only as a humble display device).