Wiring Circuit

• The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion.
• Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one.
• Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuits oscillation.
• Set R5 to read 1V RMS on an Audio Millivoltmeter connected to the output with R7 rotated fully clockwise, or to view a sinewave of 2.828V Peak-to-Peak amplitude on the oscilloscope.
• With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10KHz but R5 requires adjustment.
• High gain transistors are preferred for better performance.

Parts:

R1____________5K6  1/4W Resistor

R2____________1K8  1/4W Resistor

R3,R4________15K   1/4W Resistors

R5__________500R   1/2W Trimmer Cermet

R6__________330R   1/4W Resistor

R7__________470R   Linear Potentiometer

C1,C2________10nF  63V Polyester Capacitors

C3__________100µF  25V Electrolytic Capacitor

C4__________470nF  63V Polyester Capacitor

Q1,Q2_______BC238  25V 100mA NPN Transistors

LP1___________12V  40mA Filament Lamp Bulb (See Notes)

J1__________Phono chassis Socket

SW1__________SPST  Slider Switch

B1_____________9V  PP3

Clip for 9V PP3 Battery

Notes:

• Q1 & Q2 provide linear frequency operation of IC1 following P1 resistance variation.
• Q3 was added in order to obtain a louder click, similar to clockwork metronomes.
• A 12V micro battery was used to obtain a higher output power and more compactness.
• Rotate P1 fully towards R2, then set R1 to obtain 40 beats per minute (compare with another metronome).
• Rotate P1 fully towards R3, then set R4 to obtain 208 beats per minute.
• Finally mark the entire scale with the usual metronome steps, as follows:
  40 - 42 - 44 - 46 - 48 - 50 - 52 - 54 - 58 - 60 - 63 - 66 - 69 - 72 - 76 - 80 - 84 - 88 - 92 - 96 - 100 - 104 - 108 - 112 - 116 - 120 - 126 - 132 - 138 - 144 - 152 - 160 - 168 - 176 - 184 - 192 - 200 - 208.

Parts:

P1______100K   Linear Potentiometer

R1_______10K   1/2W Trimmer Cermet

R2_______10K   1/4W Resistor

R3______330K   1/4W Resistor

R4_______50K   1/2W Trimmer Cermet

R5______100K   1/4W Resistor

R6,R7_____1K   1/4W Resistor

C1________1µF   63V Polyester Capacitor

C2_______10nF   63V Polyester Capacitor

C3_______47µF   25V Electrolytic Capacitor

IC1_____NE555   General purpose timer IC

Q1,Q2___BC560C  45V 100mA Low noise High gain PNP Transistors

Q3_____ZTX753  100V 2A PNP Transistor

SW1______SPST Switch (Ganged with P1)

SPK______8 Ohm 40mm. Loudspeaker 

B1_____12V Battery (MN21, GP23A or VR22 type)