Raspberry Pi Electricity Monitor

Lowering energy consumption is a great way to save money; and you can’t improve something without measuring it. An internet connected energy monitor would be great, but spending money to save money leaves a bit of a sour taste. How about dragging out that Raspberry Pi from the back of the garage; and using that to do it?

I think for some electricity meters you can wire up directly to the meter; but it’s a bit too close to 240V for my taste. Luckily many electricity meters also have a “pulse output”; a blinking light that indicates consumption – usually 1 pulse for a watt hour, (Wh) equivalent to using one watt for one hour; and 1000 pulses for a kilowatt hour, (kWh) which is about 10p worth of electricity at the moment. Perhaps my electricity meter is a bit unusual – an easy to find PDF online gives the “meter constant” as 800 pulses per kWh; so it’s best to check.

Once you know your meter constant, you can of course get various apps like this one to do it all for you; using the phone camera to watch for the flashing LED. However my wife refuses to hold the phone while i walk around the house turning all the light fittings on and off; and I couldn’t find a stand for my phone that would hold it in the right place. So i’m forced to do something more interesting.

Reading the LED using a Pi is pretty easy in principle. A “light dependent resistor” (LDR) is a component whose resistance decreases as it is exposed to more light. By measuring the resistance we can detect the change in light – we can tell when the LED is on or off. Getting these components and connecting them up to the Pi was probably the most difficult part for me – but the Adafruit website has a good guide, to summarise:

  • Get an LDR like this one; with a resistance range from 200KΩ to 10KΩ
  • Get a 1uF capacitor like this one, rated for greater than 5V
  • Use a Pi pin diagram to make sure we’re connecting one side of the LDR to the GPIO 18 pin, and one side to the 3.3V pin
  • attach the negative side of the capacitor (marked with a -) to the ground pin; and the positive side to the GPIO 18 pin
  • I got hold of some longish jumper wires – gives a bit of flexibility when connecting to the Pi. I cut the ends off and soldered it to the LDR and cap.


Note the pro touch of adding a little bit of electrical tape around the connection between the LDR and the cap.

Why do we need the capacitor? The Raspberry Pi’s general purpose input/output (GPIO) pins are digital – they can only be either “high” or “low” and this depends on the voltage passing through. (approx 2V is considered “high”) The LDR is analogue; so we need to use a capacitor to quantise the values. This article explains it well.

Hardware done – on to the software.

The Adafruit guide also includes a bit of Python for measuring the resistance; so that can be used as the basis for the software side. The function RCtime does all the work of reading a value off the Pi’s general purpose input/output (GPIO) pin and gives us back a nice integer value.

So we first of all need to determine whether the light is on or off; based on some threshold value – i.e. what resistance from the LDR means the light is on.

threshold = 7000
while True:
     reading = RCtime(18)
     signal = True
     if reading > threshold:
         signal = False

It took me a while to get the right threshold value – I also ended up “masking off” the LDR by putting it inside a bit of wood with a hole drilled through it, and blue tacking it to the meter! Doing this made it much easier to find the threshold.


Then, if the LED is on, and it was previously off, we can work out how long it was off for, and use that to calculate an instantaneous reading in watts of the power that is being used:

if lastSignal == False and signal == True:
    newTime = time.time()
    difference = newTime - lastTime
    power = seconds_in_an_hour / (difference * meter_constant)
    lastTime = newTime
lastSignal = signal

Finally, we can write it all out to stdout. When we run it, we can always redirect stdout so we can e.g. save the data for later analysis.

python monitor.py > power.csv

Now; it would be great to get this data up to a cloud service like Azure or maybe just plot.ly and have a graph I can obsess over day and night… but i’ll leave that for a later project.

You can find the code on Github.


Markov chains are kind of like state machines; with a probability attached to each transition. Each state has no memory of previous states. They have plenty of applications but a very common one is generating realistic text – for example, fooling Bayesian spam filters.

I’ve had a long standing desire to make a Twitter bot using Markov chains; perhaps to make up for the lack of my own tweets! The plan is pretty easy, we need to build our model, produce some output; and use the Twitter API to post it.

The theory behind building the model is simple. If we take a sample corpus; for example the first paragraph of this blog post; we can analyse the text to see that if the current letter is a then the probability of the next letter being an r is 0.15; the probability of it being a t is 0.2; the probability of it being an i is 0.1 and so on.

This can then be extended to pairs of letters; or even words. Then; by walking the resulting Markov chain, we can mimic the style of the writer. From this description; it’s easy to see how the size of the corpus is going to change the probabilities and affect the final result.

First things first – we need a reasonably large corpus from which to generate the text – I picked John Keats; hopeless romantic and lover of nightingales; which fits with a theme of tweeting. We can get hold of a few bits of Keat’s work from Project Gutenburg. Some clean up of the text is required to remove unwanted words – the preamble; line numbers and headings – otherwise these will “pollute” our corpus.

As for the code; this being the 21st Century we don’t have to do much of this ourselves. We can quickly Google a bit of Python that’ll generate the Markov chain model and use it to output some text; all courtesy of Shabda Raaj.

A first run leaves a bit to be desired; so we’ll make a few minor adjustments – we make everything lower case; and add a back off to prevent stop words like “and” or “of” appearing at the end of our sentences:

stopwords = ['and', 'of', 'with', 'the', 'a', 'which']
# backoff until no stopwords
while gen_words[-1].lower() in stopwords:

For now, i’ve decided against stripping punctuation from the corpus; and lower casing words before they went into our Markov model. Without doing this; “day.”, “day” and “Day” are all treated as separate words; so our output has a bit less variety – often Keatsbot will lift whole sentences from the underlying corpus. What a fraud. But I think on balance it gets us closer to Keat’s style; since punctuation is of course part of that style.

Finally; we want to tweet it. Ricky Rosario helps us with this, pointing us to the excellent Python Twitter Tools. We just need to pip install twitter to download the package; then it’s as easy as:

twitter = Twitter(auth=OAuth(token, token_key, con_secret_key, con_secret))

So – set up a Twitter account; add an application from the developer console to get the various OAuth keys; and we can sing of summer in full-throated ease!

You can find the full code on Github.