When Straylight (www.wargamerau.com) sent me this article he stressed that it was primarily concerned with the creation of the flashing landing light feature (an often requested topic here at TerraGenesis). I have therefore decided to publish it in the Reference section.
This is also a fine building though, and Straylight has included some construction pictures which I will include at the end.
The landing pad by Gary James is a great looking piece and I thought about how to add some lighting effects to it. Rather than do a straight copy, I started a new design that would give the impression of a runway type segment where I would be able to use a LED 'chaser' to guide the landspeeder pilots into the actual pad. It was to be more than just a single flashing light, which is a no-brainer, they sell them! I wanted to have a play with some lighting effects of my own choosing.
I made the building before starting work on the electronics, leaving the bottom open so I could do the wiring. The images below illustrate the design of the top and the way that the rest of the building was then constructed around it. Note the little weight, from a knitting machine my son found up at the dump, being used to hold it down while the glue dries. Geez it's been useful.
This project isn't so much about the terrain piece, but about the electronics behind it. I have assumed a little background knowledge about powers of 10 (milli, kilo etc).
To do all of this good stuff you will need access to an electronics shop to buy the bits and pieces, some skills with a soldering iron, some clippers and pliers, access to data sheets (read internet) and a lot of patience. Iï¿½m writing it with the idea of showing readers the way to incorporate some digital electronics into a scenery piece. Simple electronics. I won't pretend that you will be able to follow this article to the letter and build this circuit, there will be many missing bits and puzzles along the way, but this is the overview of what is needed.
Electronics stuff is quite cheap...for this circuit I used:
1 4022 IC $2
1 555 IC $0.80
around 2 dozen resistors @4c each
2 capacitors @ around 10c each
5 BC547 transistors maybe 30c each
1 piece of veroboard at around $2.00
16 or so LEDs at 20c each max.
a grand total of around $10.00AU
I already had the LEDs, the transistors and the capacitors, so only bought the ICs, reducing the cost even more. If you are keen, you can salvage components.
The project would make an excellent school project and impress the pants off most science teachers as it involves digital circuits and some design work. I'm not an electronics whiz by any stretch of the imagination, so had a lot of legwork to do before I could get it all together.
I have made a few pieces by ripping apart toy mobile phones, but for this one I wanted a specific effect. A Light Emitting Diode (LED) chaser is a circuit that flashes one LED after another in a sequence. Chaser circuits can be bought as kits from most major companies (Jaycar and Dick Smith's here in Australia) but they are quite costly for what they are ($20AU) and only light up one LED at a time. My idea was to have three LEDs light at a time, chasing in to the centre of the pad down the runway. 4 LED arrays was my initial aim, with each array made of three or four LEDS.
LEDs are great because they use so little current compared to normal incandescent lights (eg, torch lights). 15 to 25mA is the normal current required to turn them on to full brightness. Turning three on at a time will need three times the current, or 75mA being generous. Current is a direct measure of the power of a circuit, more current means more power.
Hooking up an LED to a battery requires that you add a resistor, to limit the current flowing though it. For reasons that will be explained later, my power source was to be a 6V battery, and since I had a pile of 562 Ohm resistors lying around, I did the calculations (see this article to find out how) to check that they would be okay. (They are in fact quite a bit higher than needed which results in dimmer lights).
There are many ways to build a chaser. I have chosen to use digital electronics to achieve the task. The idea will be to use a shift register (SR) to shift the output from one LED array to another, in sequence. Shift registers come in many flavours, but the basic idea is that a clock pulse causes the register to transfer the state of one output to the next in line. Better still, there are registers made with 4 and 8 outputs and they cost under $2. Even better, there is a device called a 4022 which not only acts as a shift register (it is called a 1 of 8 octal decoder, and is not really a shift register), but you don't have to worry about setting it up with the first output on, it automatically configures itself to send a single "on" through its 8 outputs in sequence.
The next major problem is the output current that the CMOS SR can deliver. Since it is a CMOS device, it is good for very little. It struggles to light even one LED, and since we need to light 3 or 4, we are going to have to boost the output.
To do this, we send the output of the SR through a transistor. The transistor acts like a little switch and when it detects a voltage (around 0.6V) from the SR at the base, it opens the channel between the collector and the emitter. We collect the 6V at the collector and send it to our LEDs through the emitter. To make sure we don't overload the SR by drawing too much current, we place a 470 ohm resistor between the SR output and the base of the transistor.
Easy, with 4 LED arrays to light up, we need four transistors. Since a cheap transistor is capable of delivering 100mA, my 3 LEDs at 75mA is achievable. You can use a general purpose transistor, I used an NPN type BC547 and recommend you follow in my footsteps, although another NPN can be substituted, such as a BC558. As an alternative, to keep the circuit looking a bit neater, you can also buy an IC (uln 2803) with 8 transistor driven outputs in a 'Darlington Pair' configuration (one transistor drives another, so that a decent output current can be obtained). These are around $4.00 each and can deliver 500mA, a bit of an overkill when I only need 75mA, so I opted for the cheaper transistors.
Now the next problem...how to drive clock pulses into the SR to make it work. Again, this can be achieved a number of ways, but most simply using a 555 timer. The clock pulses are quite important. The edges of the transition from 0V to 6V and back again need to be very clean and straight. They must also occur in a short amount of time (in the order of microseconds). Trying to make clock pulses by a hand operated switch is not possible, due to a bouncing effect and horrible looking edges. The beauty of the 555 is that it provides clean edges on the clock pulses for the 4022 at whatever frequency we want.
At this point I should mention a few things about the ICs. The 555 timer is built around a technology called TTL and is capable of delivering 25mA or so. TTL devices are quite robust and are not effected by static electricity. Unfortunately it needs 5V minimum to make it work. The 4022 is a CMOS device, which runs with a supply between 3 and 14V and uses very little current to run, but in turn delivers a piddling amount of current and is also very sensitive to
electrostatic. The 555 can drive the 4022 easily. The 4022 must also be handled with care, don't run around the room holding it while scuffing your feet on the carpet. Ideally it should be kept in a conductive bag and installed only when needed. Once inserted into the circuit it is quite resistant to damage. There is a 555 built using CMOS technology, but it is quite expensive, so I opted for a TTL instead.
ICs come in packages named after the number of 'legs', or pins they have. The 555 is an 8 pin device, while the 4022 is a 16 pin device. Datasheets tell us what pins do what job, and the pins are numbered from the top left, anti clockwise, to the top right, if the identifying notch on the IC is at the top, or 12 o'clock position. (See the image of the 555 IC shown below).
The ICs need good quality DC voltage to work. They hate ripples from powerpacks, although most plug packs work well. Since the 555 TTL requires 5V, I was forced to use either a 9V battery, or put 4 AA cells together and use 6V. I figured the thing would last a long time on battery power so gave away the idea of using a plugpack. Four AA cells last longer than a 9V battery, so I went for these.
The 555 is a common device, whole books have been written in its honour. The schematic for the 555 is shown below.
Again, Dick Smith provides the additional circuitry to turn a 555 into an astable and similar tutorials can be found on the net. National semiconductor provide datasheets on all of the ICs they make, the 555 datasheet can be found at:
<a href="http://www.national.com/ds/LM/LM555.pdf" target="_new">http://www.national.com/ds/LM/LM555.pdf</a>
What I needed to do was make an astable (or a multivibrator):
a bit of pen and paper work and I figured that a frequency of 3Hz would be about right, so selecting C as 4.7uF meant RA would be 1.8k and RB would be 56k.
Once the 555 was generating clock pulses, it is a simple matter to feed them into the 4022 SR to make it tick over. Since the 555 output is capable of driving the 4022, no additional work was needed there.
The 4022 schematic is shown to the right. Note that unlike the schematic for the 555 the pins on this scematic are not show in the same order as on the actual device.
The 4022 is a 16 pin CMOS4000 series device that it is also called a divider, since a clock pulse frequency would be divided by 8 if we look at any single output. I got the pinouts from a databook originally, but scrounged the diagram shown from the internet. A search of "CMOS IC pinouts" in google will turn up the information on the hundreds of ICs you are looking for. As discussed above, the outputs (0 to 7, all good electronics and computing starts counting at zero) need to be boosted using transistors. Confusingly, Vdd refers to the supply voltage, while Vss refers to the ground or earth. Another consideration when using CMOS devices such as the 4022, is that all unused inputs and outputs need to be tied to ground, in other words, don't leave an input or an output flapping in the breeze. The normal way to do this is to link them to the ground rail with a high value resistor, of the order of 10MOhm will do it. Leaving the pins "floating" runs the risk that the IC will try and drive them high and the chip gets destroyed. A lesson I learnt the hard way.
The final circuit diagram is:
Sorry about the faint diagram, it was a scan. Click <a href="http://www.terragenesis.co.uk/tgpix/infopages/2/174i.jpg">here</a> to get a larger version.
In short, the 555 configured as an astable feeds the clock pulse to the 4022 at pin 14. Pins 2, 1, 3 and 7 provide the outputs we want and these are fed into the bases of the transistor jungle on the right. A 470 Ohm resistor protects each output from the 4022 in case the transistor demands too much current. The transistor switches on the current into the LED array. I have only shown the first array, but it is easy to see where the other arrays fit in.
Initially I started with the 555, hooked up to drive a LED so I could see the flash rate (or clock pulse frequency) for myself. I used a BC547 transistor to boost the output of the 555 and made sure I could drive 4 LEDs easily. At this point the LEDs were flashing on and off and I adjusted my two frequency resistors to get about 3 flashes per second. Not very exciting, but I always start small and get more complicated. Using a breadboard makes this task very easy as they are designed to take ICs and electronic components. Each IC needs to be connected to a voltage source (Vcc) and ground, as well as the pins which provide ins and outs.
The image to the right shows the 555 on the breadboard, being used to flash four LEDs simultaneously. No chasing is being done yet. Note the single transistor being used to drive the 4 LEDs.
The next step was to hook up the 555 to the 4022 and test the outputs. The bread board was used to crank up some test LEDs through the BC547 transistors and I was very pleased to see it work first time. The clock frequency was great, and using 4 LED arrays would give me a neat effect....flash 1, flash 2, flash 3, flash 4....wait, wait, wait, wait...down the runway since the 4 spare outputs from the 8 stage 4022 were not being used. I later grabbed the fifth output (Q4) and used it to flash a central green LED in the landing pad itself.
In the next part and final part of this article we'll construct the actual circuit board and finish the model.
Copyright & Credits
TerraGenesis was created in 1997 by Gary James and is currently owned, edited and maintained by Andy Slater, however the ideas and opinions expressed are those of the individual contributors. TerraGenesis and its content are © Andy Slater, unless otherwise stated, and should not be reproduced without permission.
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