A low cost 'commercial' tuner that gives rapid accurate tuning for electric and acoustic guitars in a new simple way.

Many guitarists, especially beginners, experience difficulty in tuning their instruments. Even when using traditional 'pitch pipes' this can still be a problem if the string is out of tune by more than a semitone or two. I know from teaching basic guitar at evening classes that, with ten or twelve people trying to 'tune up' at the same time, the task is made almost impossible. There are of course electronic guitar tuners available and many are extremely good. Unfortunately, they also carry a rather high price tag. For this reason the Guitar Tuner described here was designed, the objectives being to produce a guitar tuner as quick and simple as possible to use, with a price closer to the old faithful 'pitch pipes' than to its commercially produced, electronic counterpart.

The system is extremely simple, using just two LED's, one for 'sharp' and one for 'flat'. To tune the selected string simply adjust both LED's to the same brightness and you're in tune. The accuracy of the device is quite sufficient to tune your guitar as well as can be done by ear alone, and remains at the set pitch with the battery voltage down as far as 8 volts (the unit being powered by only one PP3).

Circuit

Before looking at the circuit operation it is best to consider the requirements of the unit i.e. the frequencies we have to deal with. These are shown, for the six strings of the guitar, in Table 1. By the use of a frequency to voltage converting stage these are changed to DC voltage levels, which can be compared with reference voltages produced from a chain of high stability, close tolerance resistors, which are fed from a stabilised voltage supply. The frequency-to-voltage conversion is achieved by the single LM2917 chip (IC2).

The output of this device follows the expression: V_out = f_in x V_cc x R₁₁ x C₅ which is a linear relationship between the input frequency (f_in) and the output voltage (V_out), see Figure 1. Vcc is the internally stabilised voltage, between 7.3 and 7.5V, produced by the internal Zener of IC2, see circuit diagram, Figure 2. From the above expression, values of R11 and C5 are chosen to provide suitable output voltage changes for the guitar frequencies. The calculated voltages are also shown in Table 1.

Now to the circuit itself. As we only need to deal with frequencies between 82.4 and 329.6Hz, the input is filtered by the high-pass combination of C5, R11, which attenuates frequencies below 72Hz and thus helps eliminate any mains hum pick-up. The low pass combination of C2, R2 attentuates frequencies above 338Hz, reducing the effect of harmonics and spurious noise etc. above this frequency. The chosen values of R1, R2 set the gain of the first stage at 200. This high gain is tolerable because the effects of clipping of the signal are relatively unimportant in this circuit.

The next stage produces a gain of 20, such high gain being required to produce a strong signal even though the decay of the string is fairly rapid. This stage is followed by a Schmitt trigger to give a clean square wave at the input frequency; a square wave that will remain virtually constant until the guitar string has ceased to vibrate. This square wave is coupled to the input of IC2, the frequency-to-voltage converting stage, the fourth op-amp in IC1 being used solely to supply a midpoint voltage for biasing the rest of the circuit.

IC2 generates the linear conversion from frequency to voltage which remains linear over a fairly wide temperature range. Use is also made of its internally stabilised voltage supply (pin 9). Any variation in the stabilised voltage, whether caused by supply variation or temperature change, will effect the IC's output slope and the reference slope equally, and thus produces no noticeable effect on the unit's performance.

The reference chain comprises a series of resistors, the values of which have been chosen to give a voltage division equal to the intervals between the guitar strings. The resistors must be high stability, close tolerance types, as the accuracy of the Tuner is entirely dependent upon the accuracy of the reference chain. Most of the resistance values in the chain are fairly small and any slight variations from their actual values will not significantly affect the overall accuracy.

There are however two 1k6 resistors (R16, R26) in the chain; 1% resistors could vary by as much as ±16 ohms from their required values (though they probably won't vary by more than 2 or 3 ohms) and this would adversely affect the accuracy, so it is preferable to select these, or at least check their actual values before connecting them in circuit. The voltages derived from this chain are selected by the six way switch (S1) and compared with the voltage produced by IC2. This comparison is performed by IC3, with its own output directly driving D1, the LED which indicates that the note is flat. The 'sharp' LED (D2) is driven by a separate transistor (TR1).

Construction

Figure 3 shows the PCB and component overlay. Fit all the resistors to the PCB taking care to get the 1% ones in the correct places. Next, fit the capacitors, observing the polarity of the electrolytic types. The preset (RV1) and the three IC's can now be fitted, noting the position of pin 1 in each case. The LEDs stand up above the PCB, the base of each LED body being ½in. (13mm) above the board. It may help here to cut two pieces of insulation from some suitable gauge wire, each half an inch in length, and slide these over the legs of the LEDs to give the correct lead lengths. LED polarity is indicated by a flat on the body, next to the cathode connection, and this must be carefully observed. The last thing to fit to the PCB is the switch and for this the connecting lugs have to be cut off the switch contact tags, just below the circular end-pieces to allow them to be inserted through the board and soldered into position. The switch body then serves as the PCB support when it is finally fitted into the box.

Mark out and drill the holes in the box section, as shown in Figure 4. If this is done accurately, the switch bush and LEDs should fit neatly through the box front. Fit leads, about two inches in length, from the jack-socket to the board input connections and bend the tags of the jack-socket flat against its body. Connect the battery negative lead to the centre contact of the jack-socket and the positive lead to the board connection. With the battery connected, and before fitting the board into the box, the unit can be set-up as detailed below. Finally the unit can be completed by fitting the board, jack-socket and battery into the box, as shown in Figure 5.

Even though the unit will remain at the set pitch for battery voltages as low as 8V, it is advisable to use the Duracell type battery, as its full voltage is maintained for 90% of its useful life, thus ensuring good results for some considerable time. An added advantage is, should you run it flat, it will not leak.

Setting-up

After construction the unit will have to be adjusted to concert pitch i.e. A = 440Hz. This has to be done using a pitch source of known accuracy, be it a keyboard, audio-oscillator or whatever. It is best to set the chain from the top 'E' by selecting this on the switch, injecting the note of E (329.6Hz) at the input and adjusting the preset until both LEDs are at the same brightness, which will automatically ensure that all six notes are set at the correct pitch.

An added feature can be incorporated by bringing the preset control out to the front panel. The unit could then be used fortuning to pitches other than A = 440Hz, which would be useful if you are playing on a keyboard that is slightly sharp or flat. This would be done by feeding in a note from the keyboard, adjusting the tuner to this and in turn using it to tune your guitar.

Operation

As explained earlier, for correct tuning of the selected string both LEDs will appear to be at the same brightness level. We now want to find the best way of doing this. First, when the guitar lead is plugged into the unit, and before a string is struck, the LED indicating string 'flat' will be illuminated. Now select the appropriate string note (e.g. bottom E) on the switch and, with the guitar's volume control at maximum, strike the string; then either LED may light, showing 'flat' or 'sharp', (this will be true however far the string is out of tune). If the 'flat' LED stays lit, then wind the string up as quickly as you like until just the 'sharp' LED is illuminated indicating that the required note has just been passed.

Now detune the string (i.e. 'flat' LED lights) and gradually bring the string back up until the two LEDs come to the same brightness; the string is now in tune. It is always better to come up to the required note as this makes sure that any slack in the gearing of the machine-head is taken up and avoids any chance of the string slipping down out of tune again.

No doubt while carrying out the above operation the string will probably have to be picked more than once to maintain an input to the Tuner. This is more applicable to the higher pitched strings, as their vibrations die away more quickly, although once used to using the unit, you will find that all strings can be brought into tune with just one pick. On the initial pick of the string you will probably notice that the sharp LED will first light momentarily and then go out. This is due to the rich harmonic content of a plucked string and can be minimised by picking the string on or around the twelth fret, thus ensuring a strong fundamental vibration along the string's length, giving a quicker and longer-lasting reading on the LEDs. Incidentally, this is also true when using any other guitar tuning device.

It is not necessarily ideal to have a strong signal output from your guitar pick-up. Some guitars will give a clearer indication on the LEDs for several seconds when the string is hardly audible. An added advantage is that you will quickly detect an old string, that will be prone to slipping out of tune, by the Tuner's inability to hold the brightness of both LEDs without wavering.

It is also possible to use the Tuner for correct pitching of acoustic guitar strings, by plugging a microphone into its input. However, a small pre-amp may be needed to boost the microphone output in order to obtain a sufficiently good reading.