## Sunday, 8 March 2015

### IR LED Glasses

A friend at the hackspace helps run a community centre and from time to time they run electronics workshops.  One of the proposed workshops focused around preventing a person's face being visible to monitoring video cameras which appear to have become incredibly prevalent in the UK.

One such method of obscuring ones face is to swamp the video camera CCA (charge-coupled Array) with infra-red light.  This can be achieved with some bright infra-red LEDS which appear clear to the human eye.  What is needed is a simple electronic circuit which flashes some IR (infra-red) LEDS very quickly and can be easily mounted on a hat or a pair of glasses.  This was directly inspired by an article on Hackaday:

anti-paparazzi-sunglasses

The circuit will have to be battery powered, easily constructed, simple and cheap!  With those design constraints I decided to use one of the most popular integrated circuits in the world....the 555 timer!

555 timer IC wikipedia entry

555 Datasheet

We are going to use the 555 timer IC to output a constant square wave at high frequency.  This really means we are going to turn an electronic signal on and off very quickly...we are then going to use this constant on off signal to control a transistor which will turn the LEDS on and off very quickly.  We are doing this for a good reason - it means we can run the LEDS at full power and save battery life.  It makes it more efficient.

The circuit operation for the 555 circuit constantly outputting a square wave is known as "Astable" mode.  There are lots of calculator programs and circuits available for creating 555 circuits because it is so popular.

555 Astable Calculator

If you click the link it provides a webpage based calculator which if we type in a few component values it will tell us what frequency the square wave output will be.  I often use circuit simulators and calculators when designing circuits as it makes things much easier.  I also don't see why I should always start from scratch when designing simple circuits - why reinvent the wheel?  As long as the circuit theory is understood there are no issues.

Here is the circuit we are going to use:

The circuit works as follows:

The 555 timer IC is configured in astable mode to constantly output a sqaure wave signal at pin three. This sqaure wave is used to drive the gate of an N-type field effect transistor which in turn pulses the infra-red LEDS on and off.  The frequency of the square wave output from the 555 timer is preset by the 10k resistor, 680k resistor and 4.7pF capacitor.  The 10nF capacitor on pin 5 (control voltage pin) is to stabilise the 555 timer operation.  The circuit can be powered from any voltage between 4.5V and 16V.

Here is a short video showing the circuit being simulated:

Constructing the circuit on stripboard or veroboard is probably the quickest method to prototype the circuit but I like designing printed circuit boards so here is my PCB layout:

 IR Glasses PCB Top Layer

 IR Glasses PCB Bottom Layer

Here is the parts list with the corresponding Farnell ordering codes and costs:

The farnell parts cost in total is £1.53 for a single unit.  Most of the parts are bought in lots of 50 pieces so the actual costs will be higher.

If we were going to make lots of this circuit I would get professional printed circuit boards manufactured.  I tend to use Elecrow Bazaar as they give great service for the price.

Elecrow Bazaar

As the PCB is less than 5 cm x 5 cm we can get ten printed circuit boards made for \$9.90 or £6.58 so a single PCB will be £0.66 - that's really quite cheap!  I haven't included shipping in that cost but it's around another £10 so that brings the cost of the PCB up to £1.66

All that's needed is to register with Elecrow and then upload the Gerber files for the design and pay for the service and ten to fifteen days later some professionally made printed circuit boards arrive in the post.  Be aware that in order to do this I would have to share my original design files and you would then need to export what is known as the gerber files from Cadsoft Eagle - a topic for another post.

Just for fun here is the PCB rendered in 3D so we can see how the circuit would look if it was completed.

 3D render plan View
 3D render ISO view
 3D render bottom side of PCB
Just for those who are keeping a tally I reckon the final cost of getting this circuit operating with a pair of glasses is:

Components - £1.53
PCB - £1.66
9V battery - £1.74
LED Glasses - £1.63
Cable ties - £0.90
Wire - £0.50

Total - £7.96 per kit

I did some development work on this circuit using a breadboard.  Here are some pictures showing the circuit in operation.  The operation of the bright IR leds does obscure black and white cameras. Modern colour cameras unfortunately have a very good infra-red filter which means this method of obfuscation won't work particularly well.

Here are some photos showing the circuit in operation...

 In Colour!
 In Black and White!
 At an Angle
Well...that's about it.  If you want to make IR LEDS flash quickly without using a microcontroller then this is the circuit for you.  The costs for making this into a pair of anti camera glasses is £8.00 which I think it pretty reasonable.  That's all for now - take care!