The Inner Circle

I’ve just got back off of holiday, so I haven’t had a chance to read comments and things until now. Andy commented on my previous post and wondered how much the power monitoring project was going to cost me all tolled. So I started to think about it, from the beginning I’d always excepted it was always going to be slightly more expensive then buying a off the shelf solution to just “monitor” my power… but didn’t want to just monitor… I want to record and trend.

So how much are all the other monitors out there? From “ethicalsuperstore.com” they appear as follows:
Efergy Energy Saving Meter - £39.95
OWL Wireless Energy Monitor - £29.95
Wattson Home Energy Meter - £99.95 (Unfair to include this one really, it’s more a style statement then a monitor
Eco-Eye Elite - £49.99 (Interestingly from the pictures appears to be using a CR Magnetics CR-3110 current transformer, hadn’t seen this one until today.)

So if I arrive inside of £29.95 to £49.99 I’m doing okey. Now until I decided to start on this project I had no tools or components in stock, when I lived with my parents I’d just use my fathers, but alas I haven’t lived there for three years and until this had the need for them. I wont include tools in this, they are things I should have had a long time ago.

The initial project was envisaged to have a current transformer, micro controller and a blue tooth dongle to provide remote telemetry. Of course as I continued to work on it I found I wanted more. So I added on an I2C RTC and an I2C EEPROM to store historical information when I couldn’t manually poll.

My project still isn’t fully built and is in various bits because I’m testing different aspects, it’s got components on it that it wont need in the long run (like TTL to RS232 line drivers). I’ve also done a few extra things like using DIP/DIL sockets wherever a chip is concerned so that I don’t ruin a chip while soldering.

So what’s it cost? Main components first:
PIC16F767 - Free (Microchip Direct Sample) - Closest on CPC is about £5
24AA102S (1mbit flash) - Free (Microchip Direct Sample) - Closest on CPC is about £3
OWL “Spare” Current Transformer - £6
Phillips Compat RTC - £3.50
Resistors (x8) - £0.10 (Brought in bulk from CPC)
Capacitors (x6) - £0.22 (Brought in bulk from CPC)
Diodes (x4) - £1.00 (Low voltage drop needed)
Veroboard - £1 (Brought and cut from CPC)
SparkFun Bluetooth Module - $59.95 or ~£32.50

Yes I know… it’s the last item you’ll gawp at. But really why bother doing it another way? A USB bluetooth dongle is a couple of dollars, and laptops have it built in. It’s range isn’t bad, and on most OS it emulates a serial port which makes writing software easy. Most of all, it’s two way communication. I did look at using a 433Mhz transmitter/receiver pair, but there’s to much in my house on 433Mhz already and it’s only one way.

So how else do a plan to push the total cost up for me? Well one thing you have to look at is power, you might be lucky and be able to feed the power to the board from the inside via a hole in the wall, that isn’t an option for me for a couple of reasons. I dislike throw away batteries and I don’t want to have to go out every few weeks/months.

A lead acid battery and a solar panel where my chosen (and currently untested solution), the 12V battery was £5 and the solar panel “designed” to trickle charge 12V batteries was around £12, both from eBay. From what I’ve worked out, I will still be okey to recharge used current from the night/day, even if during the day the solar panel is only working at 20% efficiency (which hey in the British summer we’ll be lucky ;).

Other things I’d like to do to make it more fun would be a digital potentiometer so that I could dynamically change the burden resistor as the current changes to get the best range at high current and most accuracy at low current. I’m having issues finding an I2C digital pot in DIP format, so I might have to resort to an SPI chip for that, which introduces more complexity in the I2C/SPI lines. I really want to use I2C though. That would be about £3, or free via samples.

So if you were going to pay for it all, I guess the entire project would be: ~ £71… youch we are over budget. But then again, I’d probably ask you to find one with a bluetooth interface for less, with historical, dynamic resolution and solar power. If you removed solar panel/battery it’d drop by £17, which is down to £54 which is closer, but not as easy for you and probably worse for the environment. I haven’t paid for my PIC or EEPROM so it’s more like £63 for me.

Let’s not even think how much my time writing the micro controller firmware would cost if I was charging eh? But I’m making this because it’s a lot of fun. So Andy, I hope that answers your question and doesn’t scare you off!

D.