Week 5 : Completing the project
|Working with acrylic|
|Connecting the switch to the pcb||W|
|Testing the circuit||P|
|Enclosure||What have you learned?|
|Back to Games timer|
|This week's session consists of finishing off and testing the games timer circuit. When it works correctly, you will want to put it in a case. Instructions are given for working with acrylic and you can follow plans for making one particular design of enclosure. Alternatively, you can design and make your own enclosure.|
From the previous session, you should have a 2-pole 6-way rotary switch with lots of wires attached to it and a printed circuit board with all the electronic components mounted correctly. The diagram below shows how these should be wired together:
Cut the wires from the switch to the timing resistors to the same length, probably around 10 cm. Next STRIP, TWIST, and TIN the ends of the wires before soldering them into the appropriate holes on the printed circuit board.
Look carefully at the wiring to the battery clip. At the printed circuit board end, pass the wires up through the larger hole from the underside of the board and then insert the free end into the small hole, ready for soldering. This provides 'strain relief' for the wires, making it much less likely that the wires will break and fall off the board.
The circuit is finished, but does it work? To find out, you need to put the integrated circuits into their sockets. Remember that 'beasties' care about which leg is which and make sure that they are in the correct sockets. You need a 4060B astable/divider, a 4017B decade counter and a 4093B Schmitt trigger NAND gate integrated circuit.
Next, connect a PP3 battery to the battery clip. Turn the spindle of the rotary switch so that it is fully anti-clockwise, then click one position clockwise, connecting pole A to way 2 and pole B to way 8. The power supply should be connected to the pcb. If the circuit is working, one of the LEDs should illuminate. Turn the circuit board so that the tilt switch operates and the circuit is RESET. Turn the board the other way up, and note whether the LEDs are counting:
Does the 'bleep' 'bleep' circuit operate when the final LED is reached? How accurate is the timing of the 30 second interval?
If you have built your circuit carefully, it is quite likely that it will function first time. If it doesn't. don't despair! Most fault are easy to find and quick to repair.
If there are problems, work through the circuit in a logical way. Use an oscilloscope or a multimeter set to work as a voltmeter to investigate.
|You will need to refer to the CIRCUIT DIAGRAM and the PRINTED CIRCUIT BOARD layout for the games timer.|
|These two LINKS open information windows which expand to fill the screen. Click each link just once. You can switch between the circuit diagram, the pcb layout and the main text by clicking the icons on the Start bar.|
|When you close the main text window or move to a different page, you will need to close each of the information windows by clicking their close buttons.|
Connect the 0 V of the oscilloscope/voltmeter to the negative terminal of the battery using a lead with a crocodile clip. Touch the other probe to the positive terminal of the battery. Is the battery flat? If not, probe the circuit board at point where there should be a positive voltage. Is the rotary switch set to the correct position?
You should find +9 V at pin 16 of the 4060 and 4017 integrated circuits, and at pin 14 of the 4093. Equally important, you should find 0 V at pin 8 of the 4060 and 4017, and at pin 7 of the 4093. Check these points on the circuit board. If you can't detect the correct voltages, look for dry joints on the power supply tracks.
Even with a voltmeter, you should be able to detect pulses at pin 3 of the 4060. If not, check the circuit carefully around the 4060 for dry joints or solder bridges. If the terminals of the tilt switch have been bridged, the 4060 will be held permanently RESET. Have you used the correct components, for example 470 kW, not 470 W? If all else fails, try replacing the 4060 with a new one.
Again, check that the power supply to the 4017 is correct. Follow the track from the 4060, pin 3, to the 4017, pin 14. Are pulses from the 4060 actually reaching the 4017? Check that the RESET, pin 15, and the ENABLE, pin 13, are LOW.
The most likely fault with this section of the circuit is that one or more LEDs will fail to illuminate. Have you put them into the circuit the right way round? Are there any dry joints? Occasionally, an LED is faulty and needs to be replaced.
Check the power supply to the 4093. Nothing much will happen until the final LED is illuminated. Does the HIGH from pins 11/13 of the 4017 reach the 4093, pin 2? When it does, does the first Schmitt trigger astable start to pulse? Follow the signal from point to point through the circuit. Check soldering and component values around the 4093 and try replacing the 4093. Pins 8/9, and pins 12/13 of the 4093 are supposed to be connected.
The pulses at pin 12 of the 4093 should be 9 V in amplitude. At the base of the transistor, at the other end of the 33 kW resistor, the amplitude should be just 0.7 V. If this is true, check the collector connection of the transistor and the soldering around the piezo transducer. If not, check that you have used a transistor of the correct type and that it is connected the right way round.
Working through the circuit in way, you will be able to locate and correct any faults.
The diagram shows a simple design for an enclosure for the games timer:
As you can see, you need two L-shaped pieces of acrylic. The top of the enclosure is made from tranparent arcylic so that the LEDs will show through, while the base is made from opaque acrylic to hide the soldering!
The 30 mm M3 screws pass all the way through the base, the 20 mm hexagonal spacers and the printed circuit board, so that the 12 mm spacers can be screwed on to hold the pcb in place. You can then attach the top of the enclosure using 6 mm screws which screw into the top of the 12 mm spacers. The battery and the rotary switch will fit in the space under the printed circuit board.
Although you can easily think of more elegant enclosures, this design has the merit of simplicity and ease of construction. Mounting the LEDs on the surface of the case instead of directly on the printed circuit board makes it much more difficult to wire the LEDs into the circuit board.