Disconnect the power supply and insert U2 (LM556). Do not insert U3 through U8. Use
a “bypass lead” to connect between pin 7 (GND) and pin 12 (Q-bar) of U3 socket. A
scrap pin lead from a resistor or capacitor is ideal for this purpose. Use another bypass
lead to connect between pin 8 and pin 13 of U8 socket. This will turn on the buzzer
oscillator. The setup is shown in Figure 3.
Figure 3. Use two “bypass leads” to turn on the buzzer oscillator in Test 2A.
Now apply battery power to the power jack, you should be able to hear the buzz tone.
Adjust VR2 to change the buzz tone and sound volume to the desired level. A piezo
buzzer has resonance frequency ranges depending on its physical construction. You
will notice that certain frequencies near resonance produce louder sound output.
If the oscillator is not working, check VR2, R13, R14, C7, C8 and Q3. Also check the
voltage at pin 10 of U2. It should be near 5.0V for the buzzer oscillator to work. If it is
less than 3.0V, check the bypass leads, Q3, R8, R9, R10, R11 and R12.
Test 2B: Counter oscillator
In this test, we want to verify that the oscillator of U2A (LM556) is working. Disconnect
the power and install a bypass lead between pin 2 and pin 14 of U3 socket. Do not
install U3 through U8. The test setup is shown in Figure 4.
