xample Circuits

3. Blinkie

A pulse generator, or astable, circuit can be made using four resistors, two capacitors, two LEDs and two transistors. 'Blinkie' uses these components to form a face, with the flashing LEDs representing the eyes.

Resistors: Resistors limit the flow of electric current. The value of a resistor is measured in ohms, W. The stripes on a resistor make up a colour code from which the value of a resistor can be worked out: orange, white, brown means 390 W, brown, black, orange means 10 000 W, or 10 kW. You will need two of each of these values to build 'Blinkie'.

Capacitors: Capacitors can store electric charge. They can be filled up and emptied like 'buckets'. Resistors can control how quickly this happens and, together, resistors and capacitors can be used to introduce time delays into circuits. The value of the capacitors needed is measured in microfarads, F. Suitable time delays will be obtained with 10 F, 22 F and 47 F capacitors. These have separate + and - connections and must be put into the circuit the right way round. The longer leg is the positive connection.

LEDs: An LED, or light-emitting diode, illuminates when an electric current flows through it. Like any diode, an LED will only allow current to flow easily in one direction, so these too will have to be inserted the right way round. The longer leg is positive.

Transistors: Transistors work like switches. A small current at one connection, the base, controls the flow of a much larger current flowing between the other two connections, called the collector, and emitter. In the 'Blinkie' circuit, the two transistors switch the LEDs ON and OFF at a rate which is determined by the filling and emptying of the two capacitors.

This is the 'Blinkie' circuit drawn with crocodile clips:

Download .CKT file

You will notice that the downloaded circuit is not quite identical to the circuit above. To get the simulation to work ...

crocodile clips simulates components with perfect accuracy. Real components are not perfect. For instance, a 10 kW resistor with a gold tolerance band may have a value 5% less or 5% more than its marked value. Polarised capacitor usually have tolerances of 20%. As a result of these variations in component values, one transistor will switch on more quickly than the other when power is first connected and oscillation will start. With the simulated circuit, both transistors switch on simultaneously and the circuit becomes 'stuck'. This will not happen with your real 'Blinkie' circuit.

The first diagram below shows the 'Blinkie' printed circuit board (PCB) from the component side. The track view is drawn as if the board was transparent, allowing you to see through to the pattern of tracks underneath. (This is the best way to design a printed circuit board and lets you check the arrangement of components against the circuit diagram.)

Print out this page and transfer the PCB design to your own PCB design software. Alternatively, you may be able to download a QUICKROUTE version of the PCB. This should work if you save the downloaded file to disc and open it from QUICKROUTE later.

Download .PCB file

Soldering: You must wear safety spectacles. Soldering irons are hot! Begin by finding the two 390 W resistors and fit them to the board (eyebrows), as illustrated above. Touch the soldering iron to the copper track on the bottom of the printed circuit board just at the pouint where the wire leg of the resistor comes through. This heats up the track and the leg ready for the solder. Push a little solder wire onto the tip of the iron until it melts and flows out onto the copper track. Let everything cool and finally cut off the extra wire from the leg. Soldering requires a little practice!

Next fit in the two 10 kW resistors (nose). The capacitors and the LEDs need to be soldered in the right way round. You can choose which colours of LED you would like. The transistors* must be fitted exactly as shown in the diagram if the circuit is to work. Lastly, solder in the battery clip. Check everything and then test your completed circuit with a 9 V PP3 battery.

* to match the PCB, transistors should have a TO92C pin arrangement, with the base connection in the middle. BC108 and other npn transistors with TO18 pin arrangement can also be used.

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