Last month (Feb 2003) we introduced the innovative PICAXE08 IC and a simple application, a flashing LED, all built into a solderless "PICNIK box".

If you don't happen to own a shop, you can always find something else to count - just remember to take into account four legs and a tail . . .

If you didn't see last month's issue, it will pay you to get a copy and read it because some of the concepts might be a little different to what you're used to, such as BASIC Stamps, etc.

By now, readers should have familiarised themselves with basic operations and the commands high, low, pause and goto.

PICAXE instructions are much the same as used in classic BASIC programming. There are a lot of BASIC texts around (no pun intended!), such as Greg Perry's "QBASIC by Example" (QUE 1993) or from the Parallax BASIC Stamp Programming Manuals. There's also a huge amount of information available on the 'net.

Given that you don't need to be a programming guru to use PICAXE instructions, initial applications here will be educational rather than exotic.

PICAXEs have only enough RAM for about 40 program lines, so there's some incentive to code efficiently! Our simple application here uses about half the memory available, so room remains for your own tweaking.

Shop Door Minder

This reveals just how useful such PC feedback is-you can note all the variables ticking away nicely!

A shop door minder usually consists of a light source which creates a beam, a light sensor which detects that beam and a buzzer circuit which sounds when the beam is broken. Sometimes there is a delay circuit to make the buzzer sound long enough to be noticed; sometimes the buzzer "locks on" when it is actuated to behave more like an alarm.

This month's circuit acts as a "shop door minder with attitude", since it not only sounds a buzzer when the light beam is broken but also provides a simple "traffic" indication by flashing a LED at an increasingly faster rate with each event.

Now that could be handy - if you don't happen to own a shop and wish to count customers, this project could, at a glance, tell you the number of times your cat door is opened! Don't own a cat? You'll think of something to count!

The flash rate increases are not linear but counts per minute can be noted and related to visitor numbers via a simple lookup table. A further LED could even be added (pin 0?) to indicate a count of 10 had been reached, at which point the main counter could drop back to a slow rate again. By this means, up to perhaps 30 events can be quite reliably "read off ".

If a PC remains attached, the exact count (to 255 events) can be just read off using debug. Mmm - 255 cats visited last night? What IS in your pantry!

The circuit

Aside from the core parts mentioned last month, just a few extra resistors are used, one of which is a light dependent resistor, or LDR.

Your setup values may vary depending on your application but 100Ω in the voltage divider, plus similar as a LED dropping resistor, should do.

For initial trials a small bright light (perhaps a white LED) is also needed. More practical setups could see the light path extended with lenses, shrouds or even a mirror. Only small currents pass along the wires so leads to the LDR and light can be conveniently placed. Things really depend on the stray background light seen by the LDR, although this can be allowed for to some extent by altering readadc threshold values.

If this looks somewhat similar to last month's project, it just might be! The basic differences are the LED and LDR which provide an "analog" input to the PICAXE. The PICAXE programming does the rest.

For a longer light path the lens from a pocket torch may help to concentrate the beam, while the LDR could even be recessed into the torch base to give shading from stray light. The program's readadc threshold value, initially 160, may need lowering to suit of course.

To prevent false triggering from shadows, reflections or visitor legs (or tails!), a small delay, perhaps "pause 500", may be needed in the readadc loop. Obviously this delay may cause the unit to become unresponsive if events occur too rapidly.

Having a light on to hold the alarm off may be wasteful of battery power of course. Perhaps you could modify the program so that darkness holds the sensor off and room light alone is enough to trigger things - much as would occur when a medicine cupboard door was opened.

The point of these PICAXE circuits is to stimulate your interest, to see what can be done very cheaply and to help you come up with your own variations. Of course, we will help you along the way with more applications to keep those creative juices flowing!

Incidentally, if you come up with a scathingly brilliant PICAXE application that you would like to share with the world (and perhaps feature on these pages), please feel free to email me - s.t.swan@massey.ac.nz

Where to get the PICAXE-08

Currently there is no Australian distributor. Order direct from the UK agents (Revolution Education) on line shop www.techsupplies.co.uk - they are very prompt.

There are a few differences between the photo above and the Protoboard layout at right. Follow the layout and you shouldn't go wrong!