BICYCLE REAR
FLASHING LIGHT
INSTRUCTION SHEET
Schematic Diagram
PCB Layout
The flashing warning lights for the rear of your bicycle uses SUPER-BRIGHT Light Emitting Diodes (LED) to produce a very visible safety feature for cyclists. The circuit is a variation of the "FLIP FLOP" Binary counter circuit that formed the basis of early computing. This circuit uses the principle of charging capacitors through metering resistors. A time lapse occurs while the capacitors charge. Changing the value of the capacitor and also varying the resistor value will give longer or shorter time lapse or delay. In this case the capacitors and their resistors are of different values to give unequal times in the ON and OFF modes. The flashing lights will flash about four times per second with the selected capacitors and resistors. You could experiment with different values to vary the frequency of flashes / second. As the capacitors are charged and reach the required voltage, they turn the transistors on in turn, turning them off again as the capacitors discharge. This crossover system of charging using two capacitors and two transistors gives an automatic free-running flip-flop called an astable flip-flop. It will keep going until switched off at the power supply.
Construction Method
1. Check your P.C.Board carefully for damage to the tracks. An Electronic Circuit Tester or a Multimeter will help check that there are no broken tracks.
2. Identify the resistors by their colour bands or with a multimeter. Bend the legs with long-nose pliers to match their holes and push them into place against the surface of the board. Remember that the drawing is looking at the PLAIN side with the tracks away from you. Bend the legs sideways a bit to stop the resistors from falling out.
3. The Electrolytic Capacitors are polarised. Look at the body of the cylinders to find the values and check their locations from the drawing. On the cylindrical body there is also an arrow pointing down one leg of the capacitor. This is the K (Negative) leg. This leg must go to the hole marked K on the drawing. They won't work if they are in the wrong way round.
4. The two transistors have a flat on the body. Mount them with the flats as the drawing shows.
5. The LEDs are also polarised. There is a flat on the small flange at the base of the globe. This is above the K leg, which is also the shorter leg. Put the flats as the drawing shows.
6. Soldering can be done but don't cook the transistors. Apply heat on the track first beside the leg. When the solder takes then apply solder and heat to form a small cone up to the leg, finally touching the leg with the soldering bit to make sure it is bonded. Don't try soldering the leg before you heat the track because cooling solder will run to the track, sit on top of it , but may not bond to it giving a "dry joint" which will not give proper contact - very hard to troubleshoot!
7. Solder the two pins in the holes for the switch connections, and solder wires to the pins and to the switch. Strip the insulation and solder the wires to centre and one end terminals on ONE side of the switch.
8. The legs of the battery holder are pushed through the holes in the Board from the TRACK side. Before putting it into the holes, tin the legs with a blob of solder right up near the plastic, and also put a blob on the pads where the legs will go. Keep the holes clear so the legs will fit in. Push the holder down almost to the board and touch the blobs of solder with the soldering bit to fuse the blobs of solder together.
9. Push in a battery and switch on. If your project doesn't work simply check locations, polarity of capacitors, transistors and LEDs, and re-solder all joints with a hot bit. Next you will need to design a case.
Have fun and go safely.