Step 1: The Plan
This is how I’ll make the repeller.
The frequency of the output of the chip in this setup is given by the formula
Frequency = 1.44/((R1 + 2R2)C)
From experience I know that 12,000 Hz is a good frequency for the job, but feel free to try out others with different values of R1, R2 and C (if you have a breadboard I recommend trying a 100kR variable resistor as R1 and finding a suitable frequency). Do bear in mind that R1 should be at least 1kR.
12,000 = 1.44/((R1 + 2R2)C)
1.44/12,000 = (R1+2R2)C
1.44/(12,000 x 0.0000001) = R1 + 2R2 = 1200
I’ve used 100nF as my value for the capacitor, and I’ll use a 1kR resistor R1 and a 100R for R2.
As I’ll be using a 9V PP3 battery for +Vs, and my red LED has a voltage drop of 2V and draws 30mA of current, I ideally need R3 to be 233.3R. I have 220R resistors, but as they have 1% tolerance (i.e. they go up to 222.2R) they aren’t suitable to protect the LED from excessive current. The next highest value of resistor in my supply is 270R, which would limit the LED to drawing 25.9mA – perfectly acceptable. Check the ratings for your LED – don’t complain to me if its ratings are different to mine’s and it blows!
Ohm’s law: Current = Voltage/Resistance
Therefore Resistance = Voltage/Current
(9-2)V/(30 x 10-3)A = 233.3R
R4 is to increase the impedance (read resistance) of the output into the speaker. As with the LED’s resistor, less resistance means more current, and as power (in Watts) is given by current x voltage, less resistance means more power. “Nice!” I hear you remark in your best Jeremy Clarkson voice, but not so when you try to pass 2W into a speaker designed to take a maximum of 0.25W and you’re left with an expensive magnet. Read the specs for your speaker – do the maths and aim for the nominal power and don’t go over the maximum power. The nominal power of my speaker is 1W with a maximum power of 1.5W. A 47R resistor for R4 in series with my 8R speaker will give 1.02W of power (having measured the output of the 555 as 7.5V with a 9V supply).