The problem is mostly likely to do with the tolerance of the resistor. The resistors I used had gold bands and that means they have a tolerance of 5%. So, for a 100 ohm resistor, the resistance could be anywhere between 95 ohms and 105 ohms. In many electronic applications this amount of tolerance usually won’t have any effect on the circuit, but when trying to hit 440 Hz as precisely as possible, it can and did have a significant effect—my circuit generated F# (369.994Hz) instead of a 440A. If I tried to tune my guitar to F# I would be badly out of tune with other instruments!
It may turn out that adding the potentiometer is the best solution. Nonetheless, it might be worthwhile to take a look at other components that are available.
There is, in fact, a special kind of 555 timer that might be useful in this situation. It’s called the CSS555C. This 555 Timer IC has some additional circuitry built in and is programmable. You can also get a demonstration kit that makes it really easy to program and test the timer IC.
CSS555C Demonstration Kit Software and Hardware Installation
The software installation is very easy. Put the CD in CD drive and copy the CSS555C folder to your computer’s disk drive. I put the folder on my windows desktop.
Connect the USB cable to the USB connector on the demo board and the other end to an open USB port on your computer.
The “Found New Hardware Wizard” will run. The hardware installation is fairly straightforward but if you run into any problems installing the hardware, see the Installing Software and Hardware section of the CSS555C Demonstration Kit Operating Manual.
Working with the demo kit software
Open the CSS555C folder you saved to your computer and run CSS555C_DemoKit.exe. In the Device Power box, select the ON radio button.
First, locate the Trig-B Pin box, and select the “LO (Active)” radio button. Next, locate the “Counter Setting” box and select the radio button labeled “100.” Finally, click the "Program Device" button. Notice the blue LED on the demo board. The LED should begin flashing on and off.
Click the button labeled “Read Device” and the window should change to the following:
You can select different values in the “Counter Setting” box, and then click the “Program Device” button to see how it affects the flashing of the blue LED.
When you change the value in the “Counter Setting” box it’s like swapping out different capacitors. Swapping capacitors will also change the frequency of the square wave that is output on pin 3.
Preinstalled on the demo board is a .1uf capacitor on pin 2, a 1 mega ohm resistor on R1 and a 1 mega ohm resistor on R2. Selecting the “1 (Standard 555)” radio button should give a standard 555 timer square wave frequency output on pin 3.
If we punch these numbers into a 555 timer calculator:
Capacitor C (labeled C1 on the demo board) = .1uf
R1 (labeled RA on the demo board) = 1M
R2 (labeled RB on the demo board) = 1M
The square wave frequency output on pin 3 should = 4.81Hz as expected.
Now select the “10” radio button in the Counter Setting box.
The square wave frequency output on pin 3 will be .48 Hz. That would be like swapping out the .1uf capacitor and replacing it with a 1uf capacitor. So, you puch the following into the 555 timer calculator:
Capacitor C (labeled C1 on the demo board) = 1uf
R1 (labeled RA on the demo board) = 1M
R2 (labeled RB on the demo board) = 1M
The square wave frequency output on pin 3 should = .48 Hz
When you select the select the “100” radio button in the Counter Setting box, it’s like inserting a 10uf capacitor and frequency changes to .048 Hz.
As you select higher and her values, the frequency gets lower and lower--too low, in fact, to be useful in my guitar tuner application. Nonetheless, the advantage of the CSS555C is you don’t have to install more than one capacitor.
To build my guitar tuner circuit I will have to install the CSS555C with a .01uf capacitor on pin 2, a 100K resistor on R1 and about 113.96K on R2. The other advantage of the CS555C is an internal programmable “trim capacitor.”
Image Source
If the trim capacitor is set to 0 the internal trim capacitor will be set to 100pf (Pico farads) and the settings can be programmed between -127 and +127. Each incremental change by one will change the internal trim capacitor by 1/8 pf. Source
The internal trim capacitor was included in the CSS555C to compensate for the “errors in the values” of resistors R1 and R2. Source
If all goes well, the trim capacitor should account for the 5% tolerance of the resistors and allow fine tuning of the circuit to 440 Hz.
Before I can use the CSS555C in my zinc tablets box I will need to build a 3 volt battery pack since the maximum voltage for the CSS555C is 6 volts. I don’t want to use my 9 volt battery and accidentally fry it.
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