Friday, 26 July 2013

Edge-lit Display Decoder Test

The display is being illuminated using 10 white LEDs salvaged from a £1 string of battery-powered Christmas lights.  This is a very cheap way to get white LEDs with flat tops and I found 30 more LEDs on strings like this so that's enough for 3 more digits.

The decoder takes a 4-bit binary-coded decimal (BCD) value and uses a 74HC138 to decode 0-7 and a 74HC00 to decode the remaining 8 and 9 digits (values above 9 are meaningless in this set-up).  To test the circuit I connected a rotary BCD-encoded switch I've had in the spares box for almost 20 years.  The switch goes from 0-15 (decimal) coded in 4-bits and the flickers to 0 you can see in the video are caused by the switch breaking before make.

This is the hard work done.  Using 2 more HC chips and a ULN2803 I can build a simple 2-4 line decoder with increased current handling. This will allow a PIC or Arduino to multiplex drive 4 displays using just 6 output lines. I might even use an old 16F84 I have as an I2C slave to control the displays.

Clock, thermometer or rev counter?

Video -

Wednesday, 24 July 2013

Arduino DS18S20 With LCD

Arduino lite ATMEGA-16 test circuit with a DS18S20 temperature sensor and a 2x16 character LCD display. Using the Arduino libraries this is very straightforward. The green LED is lit when the temperature is below 25 C and the red LED comes on if the temperature goes above 25 C.

The small LCD panel was salvaged from an old specialist printer and as well as a 'Power' LED there is another green LED and a red LED on the panel along with the required resistors.


The DS18B20 sensor IC mounted on the breadboard.  The big blue potentiometer is for setting the contrast on the LCD display.

Video of this working -

The next steps are to get 3 sensors working with this circuit and then to see if I can get the 2-wire option working, which would be a handy improvement.

Thursday, 18 July 2013

Skoda come, Skoda go.

Well, we bought a much newer Skoda Fabia a few weeks ago and after much soul-searching I decided to give the Renault RX4 a chance and get rid of the old Fabia (MCZ).  This was no easy decision because the Fabia has been incredibly reliable and is still going strong at just short of 160,000 miles with no engine problems.  The problem is that it is handy to have one larger car and having spent £750 on a new turbo for the RX4 it seemed daft to sell it, so there you go.  I switched the insurance over to the Renault yesterday and the old Fabia will be off to the vehicle dismantlers (i.e. scrappie) tomorrow.

I will almost certainly have a tear in my eye and I definitely do not want to be around when they start pulling her apart!

I quite fancy one of these though, once I've saved enough up to buy a second-hand one.

Tuesday, 16 July 2013

Experimental electromagnet valve

Mains water valve salvaged from an old washing machine that was fly-tipped at a beauty spot about 3 years ago. I stripped it and took the carcass to the dump. Civic-minded and eco-friendly all rolled into one.

Experimental 12-volt version using a quickly (i.e. badly) wound coil using wire taken from an old home brew motor that was never finished.  It works after a fashion, but would probably need a proper tightly-wound coil to be effective. The valve opens when 12 volts is applied, but not fully.  The resistance across the coil is about 8.5 ohms which I think is a bit low for this type of coil.

It was an interesting wee experiment and I might develop it sometime if I get more suitable wire.

Interesting Display

This is just a taster. This is an interesting edge-lit numeric display from the 1960s, a successor to the Nixie tubes. The arrival of 7-segment displays in the late 60s probably made these ones obsolete so they are quite rare.

Each digit 0-9 is inscribed on a thin piece of transparent acrylic and these are sandwiched together. Each digit can be illuminated by its own 28-volt bulb.

Nowadays bright white LEDs are the alternative to small bulbs so here we have the digit '8' being lit up by an LED driven from 5 volts.

More to follow.

Wednesday, 10 July 2013

MR2 Dashboard tinkering

Being a 1980s car, the MR2 doesn't have a DIN radio slot or ISO connections for car audio. The surround had been butchered at one point years ago to try and make a CD player fit and looked grim. On top of that the lid for the ash tray had fallen off and as nobody is going to smoke in the car I took the ash tray and lighter out and stuck the lid down permanently with epoxy.

So I needed a 12-volt socket to charge my 'phone and to neaten up the radio surround.

I got this lightweight radio in Lidl on offer. It has no CD, but plays back from SD cards and USB devices, which suits me for a summer car. Power output is low, but I'm putting a power amp in my centre speaker cabinet along with a pair of better speakers, so not a problem. All 4 outputs can drive the MR2's own speakers.

The red panel will be covered in black vinyl from a big old handbag I got for 50p in a charity shop and the 12-volt sockets are a £1 adaptor from Poundland. Only drawback at present is the USB hole is too deep for a normal plug, but I'm working on that.

The panel itself is made from my new favourite stuff, PVC foamboard. This stuff is rigid, pretty tough and can be cut with a craft knife. Great for prototyping and light fabrication.