Part 3 - Oscillators
Oscillators have many uses in electronics and they range from simple to highly complex depending on the application requirement and here we will look at a couple of basic oscillator designs that can be used as building blocks in future designs.
A bit about stability
Oscillator stability relates to how well the oscillator can be set to a designed frequency, and stay running at the frequency without drifting. Heat, slight voltage variations, even stray capacitance from your hand being too close can all affect the stability. In expensive specialised equipment it's not uncommon to have to let the equipment warm up before you can be certain that the oscillator frequency has stabilised. In fact, some pieces of equipment have small heater ovens that make sure the oscillator boards and components are held at a certain temperature. For most hobbies applications stability is much less of an issue but it can cause problems if it gets out of hand.
The NE555 timer IC
The NE555 timer IC is probably one of the most used timer ICs of all time. It's small, simple to use, very versatile and cheap to purchase. It also has a wide operating voltage range making it suitable for use in mains or battery powered devices and has excellent stability over a wide voltage and temperature range.
The above circuit uses a handful of components and when connected to a power supply will produce a good clean square wave on pin 3 of the IC. With this circuit the frequency can be adjusted from around 600 Hz to over 7 KHz.
The breadboard layout reflects the circuit diagram shown above though adds a loudspeaker to the output of the IC.
Switching on the power and slowly adjusting the variable resistor R2 with a suitable screwdriver will change the output frequency.
Switching on the power and slowly adjusting the variable resistor R2 with a suitable screwdriver will change the output frequency.
|
If you also connect the output from the IC to an oscilloscope you should be able to see graphical representation of the signal being produced.
It should look something like the image on the left. There is a lot of information on the internet relating to the NE555 so I'm not going to reproduce it here. The circuit provided produces a square wave with a 50% duty cycle. This means that on/off period of the wave is identical. It is possible to modify the circuit to allow for an adjustable duty cycle and this has application in some specific instances, however for our on-going experimentation, this 50% duty cycle is ideal. |
I've deliberately limited the amount of information supplied on the breadboard layout so you will have to get used to reading the circuit diagrams to get component values. You will of course have to connect the breadboard up to a suitable power supply. Set the PSU output voltage to around 5v.
Also don't forget you should have read Breadboard initial setup, else you may have problems getting this circuit to work.
As mentioned the oscillator speed can be varied from around 600 Hz to 7 KHz. The minimum / maximum range of the oscillator is controlled by the values of R2+R3 and C1.
Also don't forget you should have read Breadboard initial setup, else you may have problems getting this circuit to work.
As mentioned the oscillator speed can be varied from around 600 Hz to 7 KHz. The minimum / maximum range of the oscillator is controlled by the values of R2+R3 and C1.
|
Switch off and change the value of C1 from 10nF (0.01uF) to 10uF. Make sure that the negative of the 10uF capacitor (which is usually marked on the component with a line running down the side), is connected to the negative power rail. Electrolytic capacitors are typically polarity sensitive (there are exceptions) and they will go boom if you connect them the wrong way around.
10uF has 1,000 times the capacitance of the original 10nF capacitor, so you can expect the oscillator to run approximately 1,0000 times slower. So, if the lowest frequency originally was 600 Hz, the oscillator should now be able to run down from around 0.6 Hz to around 7 Hz. This is pretty slow and if you switch on the power, probably the best you will hear are some feint clicks. Replace the loudspeaker with a LED and 470 ohm resistor as shown in the diagram on the left and then switch on. You should see the LED flashing on and off. You can vary the flash rate by adjusting the variable resistor. Read-up on the NE555 and study the datasheet then experiment with different values for C1 and R3. You can use your oscilloscope to try and calculate the frequency that is being produced. When you've finished leave the NE555 on the board as you will need it again in the next part. |
